Post by thread:Driving a high voltage H-bridge from a PIC

> I'm trying to use a PIC to drive several 24 volt DC motors, using an > H-bridge so I can get forward and reverse with speed control. I'm just > wondering what the easiest way is to drive the MOSFETS. > The easiest way, especially for one-offs, is by using half-bridge/full-bridge driver ICs. [snip] > So far, the only suitable driver I have found is the IR2110 from > International Rectifier. Is this a good way to drive the H-bridge, or is > there a better, simpler way to make this work? > A few companies make MOSFET driver circuits. If you don't need the bridge to be fully on (100% duty cycle), and the supply voltage is 30V or less, then you have a choice amongst a few different vendors. DigiKey carries Linear Technology's LT1162. Harris makes (made?) them as well (I have used the HIP2500, but it has been obsoleted since then). Those are the only ones if have played with, but there are many others. It isn't really hard to roll your own. IR especially has lots of good app notes on this subject. But it's definitely easier to buy a driver IC.

{Quote hidden}> > Nicholas Irias wrote: > > > > I'm trying to use a PIC to drive several 24 volt DC motors, using an > > H-bridge so I can get forward and reverse with speed control. I'm just > > wondering what the easiest way is to drive the MOSFETS. > > > > It's easy to come up with a way to drive the low-side MOSFETS, since they > > only need 10 volts, but the high side MOSFETs need 34 volts at the gate. > > And at least the Maxim gate driver chips will only do 18 volts. > > > > So far, the only suitable driver I have found is the IR2110 from > > International Rectifier. Is this a good way to drive the H-bridge, or is > > there a better, simpler way to make this work? > > Check out Bob Blick's H-Bridge circuit. He uses Darlingtons and BJT's > instead of MOSFETs. > > http://www.bobblick.com/bob/projects/hbridge/index.html > > We're implementing a variation of this design here. We're changing the > Darlingtons to BJT's and adding flyback protection diodes. The thing that concerned my about that circuit is the 4V voltage drop. I was hoping to get a circuit that has a minimal drop of the supply voltage because I'm driving 12V motors with a 12V battery. Any suggestions? BAJ

> > http://www.bobblick.com/bob/projects/hbridge/index.html > > The thing that concerned my about that circuit is the 4V voltage drop. > > I was hoping to get a circuit that has a minimal drop of the supply voltage > because I'm driving 12V motors with a 12V battery. Hi Byron, It only drops 4 volts when it's at full current. At lower current your drop is more like 2 volts. If you think about how a DC motor works, you'll see that there is no way to operate a 12 volt motor with 12 volts and 5 amps at high speed. If you had a motor like this, it would draw much more than 5 amps at low speed, and destroy the H-bridge. At low speeds, it doesn't matter how much voltage gets to the motor, since you are torqueing around and the current is what matters. So in fact, dropping 4 volts at full current makes the bridge a little sloppy, but limits the current. Be sure to heatsink it well, though :-) It is easy enough to build one and test it, you really should since it does work pretty well and is tougher than a mosfet one. I would not consider building a mosfet hbridge for use with motors unless there was fast current limiting. Cheerful regards, Bob

Bob Blick wrote: {Quote hidden}> > > > www.bobblick.com/bob/projects/hbridge/index.html > > > > The thing that concerned my about that circuit is the 4V voltage drop. > > > > I was hoping to get a circuit that has a minimal drop of the supply voltage > > because I'm driving 12V motors with a 12V battery. > > Hi Byron, > > It only drops 4 volts when it's at full current. At lower current your > drop is more like 2 volts. If you think about how a DC motor works, you'll > see that there is no way to operate a 12 volt motor with 12 volts and 5 > amps at high speed. If you had a motor like this, it would draw much more > than 5 amps at low speed, and destroy the H-bridge. At low speeds, it > doesn't matter how much voltage gets to the motor, since you are torqueing > around and the current is what matters. So in fact, dropping 4 volts at > full current makes the bridge a little sloppy, but limits the current. > > Be sure to heatsink it well, though :-) > > It is easy enough to build one and test it, you really should since it > does work pretty well and is tougher than a mosfet one. I would not > consider building a mosfet hbridge for use with motors unless there was > fast current limiting. > > Cheerful regards, > Bob You can also change the Darlingtons to BJT's. I'm doing this primarily because there are far more choices in surface mount packages for BJT's than there are for darlingtons. This has the added benefit of a lower voltage drop. OTOH, you'll require a larger base drive... Sorta typical when you exchange BJT's and Darlingtons. If you choose MOSFET's you always are confronted with driving the high side. I'm not sure if this is on the web or not, but Motorola's AN-913 "Designing with TMOS Power MOSFETs" describes several ways to drive MOSFETs. In a recent design that had similar switching characteristics to a PWM H-bridge servo thingy, I chose to implement an isolated gate drive: I used a "gate-drive transformer" as a power supply and an Hewlitt-Packard HCPL-3100, an optically-isloated gate drive IC, as the driver. The primary side of the transformer was driven by a simple 74AC04 relaxation oscillator while the secondary was full-wave rectified and filtered to provide the power for the HCPL-3100. This works well for frequencies upto 20khz or so. Beyond that you might as well just use the gate drive transformer directly.

I am a bit confused here: why would the FETs require more precautions? The only thing I can think of would be that the power dissipation for a FET goes as the second power of current,and for a darlington/BJT, as the first power of current. Granted,this makes a big difference as current goes to infinity, but in certain cases, you might burn up the BJTs way before the FETs if the RDSon of the FETs is really low. Sean At 03:22 PM 7/23/99 -0700, you wrote: >It is easy enough to build one and test it, you really should since it >does work pretty well and is tougher than a mosfet one. I would not >consider building a mosfet hbridge for use with motors unless there was >fast current limiting. > >Cheerful regards, >Bob > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 shb7KILLspamcornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html

I wish I had seen your speed controllers before I wasted money on a couple of the 212's. The remaining ESCs I need for my project drive track motors and must be reversible and capable of EMF braking. My project is a scale amphibious vehicle, which can be seen at http://www.technogap.com/aavp7a1.htm. Of course, it will use an onboard PIC based computer to monitor and control track speed, synchronise operation of water jet throttles and reversing diverters, etc. -Nicholas {Quote hidden}>On my ESC (forward only) I run 5 FETs in parallel (one more than a 212D) >and use a better FET now than Astro uses, cuts the losses even farther. > >Check out my web page - http://www.rc-hydros.com - to see what I do with them. > >>I just ordered a few of the Micrel parts as Andy suggested ( MIC5022 ). > >Glad to know I was of help! > >Andy > >================================================================== >Andy Kunz Life is what we do to prepare for Eternity >------------------------------------------------------------------ >andyspam_OUTrc-hydros.com http://www.rc-hydros.com - Race Boats >@spam@andyKILLspammontanadesign.com http://www.montanadesign.com - Electronics >================================================================== >

Hi, i was just thinking, instaed of using a semiconductor switch to control the motors, why not use a switch mode power supply to directly drive the motors? If your circuit doesn't require reverse motor drive, wouldn't it be ideal to cut out the switches and just use the SMPS's drivers? Battery powered controllers would benefit even more from the SMPS's ability to step up the battery voltage and squeeze out every last drop from the batteries, (don't over squeeze to the point of battery breakdown tho..) Current sense in the SMPS could double as overload protection too. If response time is somewhat more critical, use a smaller filter cap for the outputs. What say you folks? Viable? Terry At 10:24 PM 7/24/99 -0700, you wrote: {Quote hidden}>At 01:55 AM 7/24/99 -0400, you wrote: >>I am a bit confused here: why would the FETs require more precautions? The >>only thing I can think of would be that the power dissipation for a FET >>goes as the second power of current,and for a darlington/BJT, as the first >>power of current. Granted,this makes a big difference as current goes to >>infinity, but in certain cases, you might burn up the BJTs way before the >>FETs if the RDSon of the FETs is really low. > >It's actually even worse than that. The R of the MOSFET increases with >temperature and current, so unless you go overkill or have protection you >have a big problem. With comparable devices, the bipolar will usually take >a lot more abuse(assuming it has enough drive current). Generally in a >small application it's easy enough to use a larger MOSFET, but I've >designed 1200 amp motor controllers and it makes sense to put in good >protection so you can use fewer MOSFETs. > >Cheers, >Bob >

Hi Terry, >From the experimenting with PWM that I have done recently, I think your idea is excellent. The only problem is complexity. Here is how I understand PWM (someone please correct me if I am wrong): When you feed PWM to a DC motor, you might think that you are saving power as you turn down the motor speed. It is true that less mechanical work is being done,but more power is being wasted in the winding resistance. This is because the same average current needs to flow (if the frictional torque remains the same). If we have a motor that generates 10v of back emf at full speed and has 1 ohm of winding resistance, then with 12v PWM at 100% duty cycle, we have a continuous 2 amps flowing, we are using 24 watts of electrical power,and doing 20 watts of mechanical work (approximately, assuming that the transfer of power due to back EMF is purely mechanical). The efficiency is 20/24 = 83% If we now drop to 50% duty cycle, we still need an average current of 2 amps to oppose the friction,so we have 4 amps flowing during the on time. That's 48 watts of power for 50% of the time,or,again,24 watts of average electrical power. However, the back emf is now only 8v ( 12-8=4, 4/1ohm = 4 amps),so we are only doing 8*4=32 watts for half the time, or 16 watts mechanical work. The efficiency is now only 67%,and it reaches 0% when the duty cycle gets so low that the resistance doesn't allow enough current to flow to oppose the friction (in this case, we couldn't operate the motor below 16% duty cycle). As we decrease the duty cycle, the speed vs. duty cycle curve is nonlinear and drops expecially fast as we near 16%. With a switcher,however,we could forget PWM and just supply a continuous variable voltage to the motor. The current would stay the same and the RPM would linearly follow the voltage. The efficiency would not change much over the whole RPM range. Sean At 03:39 AM 7/26/99 +0800, you wrote: {Quote hidden}>Hi, i was just thinking, instaed of using a semiconductor switch to control >the motors, why not use a switch mode power supply to directly drive the >motors? > >If your circuit doesn't require reverse motor drive, wouldn't it be ideal >to cut out the switches and just use the SMPS's drivers? > >Battery powered controllers would benefit even more from the SMPS's ability >to step up the battery voltage and squeeze out every last drop from the >batteries, (don't over squeeze to the point of battery breakdown tho..) > >Current sense in the SMPS could double as overload protection too. If >response time is somewhat more critical, use a smaller filter cap for the >outputs. > >What say you folks? Viable? > >Terry > > > > >At 10:24 PM 7/24/99 -0700, you wrote: >>At 01:55 AM 7/24/99 -0400, you wrote: >>>I am a bit confused here: why would the FETs require more precautions? The >>>only thing I can think of would be that the power dissipation for a FET >>>goes as the second power of current,and for a darlington/BJT, as the first >>>power of current. Granted,this makes a big difference as current goes to >>>infinity, but in certain cases, you might burn up the BJTs way before the >>>FETs if the RDSon of the FETs is really low. >> >>It's actually even worse than that. The R of the MOSFET increases with >>temperature and current, so unless you go overkill or have protection you >>have a big problem. With comparable devices, the bipolar will usually take >>a lot more abuse(assuming it has enough drive current). Generally in a >>small application it's easy enough to use a larger MOSFET, but I've >>designed 1200 amp motor controllers and it makes sense to put in good >>protection so you can use fewer MOSFETs. >> >>Cheers, >>Bob >> > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 spamBeGoneshb7spamBeGonecornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html

Hi Sean, the disadvantages are that reversing the motor would still require a H bridge, response time would be slower and you won't be able to drive the motor at very slow speeds. (You could still pulse the SMPS output like a H bridge but response will be slower) Other then that, u'd have the best of 3 worlds, variable voltage, PWM motor controller and efficient use of batteries! Should be great for RC and other portable products. The easiest way to implement this would be to use a digital pot on the voltage sense/feedback divider available in some switch mode controllers. Either that or a bipolar/fet across the voltage set resistor. Normal digital pots won't go higher then 5V so a discretes solution is required. I guess rolling your own 8 bit, 256 step resistor array is the only way to go for better precision... Terry At 04:20 PM 7/25/99 -0400, you wrote: {Quote hidden}>Hi Terry, > >From the experimenting with PWM that I have done recently, I think your >idea is excellent. The only problem is complexity. > >Here is how I understand PWM (someone please correct me if I am wrong): > >When you feed PWM to a DC motor, you might think that you are saving power >as you turn down the motor speed. It is true that less mechanical work is >being done,but more power is being wasted in the winding resistance. This >is because the same average current needs to flow (if the frictional torque >remains the same). If we have a motor that generates 10v of back emf at >full speed and has 1 ohm of winding resistance, then with 12v PWM at 100% >duty cycle, we have a continuous 2 amps flowing, we are using 24 watts of >electrical power,and doing 20 watts of mechanical work (approximately, >assuming that the transfer of power due to back EMF is purely mechanical). >The efficiency is 20/24 = 83% > >If we now drop to 50% duty cycle, we still need an average current of 2 >amps to oppose the friction,so we have 4 amps flowing during the on time. >That's 48 watts of power for 50% of the time,or,again,24 watts of average >electrical power. However, the back emf is now only 8v ( 12-8=4, 4/1ohm = 4 >amps),so we are only doing 8*4=32 watts for half the time, or 16 watts >mechanical work. The efficiency is now only 67%,and it reaches 0% when the >duty cycle gets so low that the resistance doesn't allow enough current to >flow to oppose the friction (in this case, we couldn't operate the motor >below 16% duty cycle). As we decrease the duty cycle, the speed vs. duty >cycle curve is nonlinear and drops expecially fast as we near 16%. > >With a switcher,however,we could forget PWM and just supply a continuous >variable voltage to the motor. The current would stay the same and the RPM >would linearly follow the voltage. The efficiency would not change much >over the whole RPM range. > >Sean > > >At 03:39 AM 7/26/99 +0800, you wrote: >>Hi, i was just thinking, instaed of using a semiconductor switch to control >>the motors, why not use a switch mode power supply to directly drive the >>motors? >> >>If your circuit doesn't require reverse motor drive, wouldn't it be ideal >>to cut out the switches and just use the SMPS's drivers? >> >>Battery powered controllers would benefit even more from the SMPS's ability >>to step up the battery voltage and squeeze out every last drop from the >>batteries, (don't over squeeze to the point of battery breakdown tho..) >> >>Current sense in the SMPS could double as overload protection too. If >>response time is somewhat more critical, use a smaller filter cap for the >>outputs. >> >>What say you folks? Viable? >> >>Terry >> >> >> >> >>At 10:24 PM 7/24/99 -0700, you wrote: >>>At 01:55 AM 7/24/99 -0400, you wrote: >>>>I am a bit confused here: why would the FETs require more precautions? The >>>>only thing I can think of would be that the power dissipation for a FET >>>>goes as the second power of current,and for a darlington/BJT, as the first >>>>power of current. Granted,this makes a big difference as current goes to >>>>infinity, but in certain cases, you might burn up the BJTs way before the >>>>FETs if the RDSon of the FETs is really low. >>> >>>It's actually even worse than that. The R of the MOSFET increases with >>>temperature and current, so unless you go overkill or have protection you >>>have a big problem. With comparable devices, the bipolar will usually take >>>a lot more abuse(assuming it has enough drive current). Generally in a >>>small application it's easy enough to use a larger MOSFET, but I've >>>designed 1200 amp motor controllers and it makes sense to put in good >>>protection so you can use fewer MOSFETs. >>> >>>Cheers, >>>Bob >>> >> >| >| Sean Breheny >| Amateur Radio Callsign: KA3YXM >| Electrical Engineering Student >\--------------=---------------- >Save lives, please look at http://www.all.org >Personal page: http://www.people.cornell.edu/pages/shb7 >TakeThisOuTshb7EraseMEspam_OUTcornell.edu ICQ #: 3329174 >________________________________________________________ >NetZero - We believe in a FREE Internet. Shouldn't you? >Get your FREE Internet Access and Email at >http://www.netzero.net/download/index.html >

Hi Sean, I believe that you have forgotten that frictional loading will be largely proportional to speed, therefore reduction in speed gives a reduction in frictional loading. Work = force x distance, less distance (rotations) = less work There are of course many other factors, -Magneto striction, -Air flow drag, -Viscosity of lubricant, -Temperature of lubricant, -etc. Sometimes it *really* helps to try the real thing out before developing general theories ! regards, Graham Daniel. Sean H. Breheny wrote: {Quote hidden}> Here is how I understand PWM (someone please correct me if I am wrong): > > When you feed PWM to a DC motor, you might think that you are saving power > as you turn down the motor speed. It is true that less mechanical work is > being done,but more power is being wasted in the winding resistance. This > is because the same average current needs to flow (if the frictional torque > remains the same). If we have a motor that generates 10v of back emf at > full speed and has 1 ohm of winding resistance, then with 12v PWM at 100% > duty cycle, we have a continuous 2 amps flowing, we are using 24 watts of > electrical power,and doing 20 watts of mechanical work (approximately, > assuming that the transfer of power due to back EMF is purely mechanical). > The efficiency is 20/24 = 83% > > If we now drop to 50% duty cycle, we still need an average current of 2 > amps to oppose the friction,so we have 4 amps flowing during the on time. > That's 48 watts of power for 50% of the time,or,again,24 watts of average > electrical power. However, the back emf is now only 8v ( 12-8=4, 4/1ohm = 4 > amps),so we are only doing 8*4=32 watts for half the time, or 16 watts > mechanical work. The efficiency is now only 67%,and it reaches 0% when the > duty cycle gets so low that the resistance doesn't allow enough current to > flow to oppose the friction (in this case, we couldn't operate the motor > below 16% duty cycle). As we decrease the duty cycle, the speed vs. duty > cycle curve is nonlinear and drops expecially fast as we near 16%. > > With a switcher,however,we could forget PWM and just supply a continuous > variable voltage to the motor. The current would stay the same and the RPM > would linearly follow the voltage. The efficiency would not change much > over the whole RPM range. > > Sean > > At 03:39 AM 7/26/99 +0800, you wrote: > >Hi, i was just thinking, instaed of using a semiconductor switch to control > >the motors, why not use a switch mode power supply to directly drive the > >motors? > > > >If your circuit doesn't require reverse motor drive, wouldn't it be ideal > >to cut out the switches and just use the SMPS's drivers? > > > >Battery powered controllers would benefit even more from the SMPS's ability > >to step up the battery voltage and squeeze out every last drop from the > >batteries, (don't over squeeze to the point of battery breakdown tho..) > > > >Current sense in the SMPS could double as overload protection too. If > >response time is somewhat more critical, use a smaller filter cap for the > >outputs. > > > >What say you folks? Viable? > > > >Terry > > > > > > > > > >At 10:24 PM 7/24/99 -0700, you wrote: > >>At 01:55 AM 7/24/99 -0400, you wrote: > >>>I am a bit confused here: why would the FETs require more precautions? The > >>>only thing I can think of would be that the power dissipation for a FET > >>>goes as the second power of current,and for a darlington/BJT, as the first > >>>power of current. Granted,this makes a big difference as current goes to > >>>infinity, but in certain cases, you might burn up the BJTs way before the > >>>FETs if the RDSon of the FETs is really low. > >> > >>It's actually even worse than that. The R of the MOSFET increases with > >>temperature and current, so unless you go overkill or have protection you > >>have a big problem. With comparable devices, the bipolar will usually take > >>a lot more abuse(assuming it has enough drive current). Generally in a > >>small application it's easy enough to use a larger MOSFET, but I've > >>designed 1200 amp motor controllers and it makes sense to put in good > >>protection so you can use fewer MOSFETs. > >> > >>Cheers, > >>Bob > >> > > > | > | Sean Breheny > | Amateur Radio Callsign: KA3YXM > | Electrical Engineering Student > \--------------=---------------- > Save lives, please look at http://www.all.org > Personal page: http://www.people.cornell.edu/pages/shb7 > RemoveMEshb7TakeThisOuTcornell.edu ICQ #: 3329174 > ________________________________________________________ > NetZero - We believe in a FREE Internet. Shouldn't you? > Get your FREE Internet Access and Email at > http://www.netzero.net/download/index.html -- Steam engines may be out of fashion, but when you consider that an internal combustion engine would require recovery of waste heat by transfer just before top dead centre then fashion becomes rather redundant, USE STRATIFIED HEAT EXCHANGERS ! and external combustion. You heard it first from: Graham Daniel, managing director of Electronic Product Enhancements. Phone NZ 04 387 4347, Fax NZ 04 3874348, Cellular NZ 021 954 196.

Hi Graham, At 08:45 PM 7/26/99 +1200, you wrote: >Hi Sean, >I believe that you have forgotten that frictional loading will be >largely proportional to speed, therefore reduction in speed gives a >reduction in frictional loading. Work = force x distance, less >distance (rotations) = less work Are you saying that the work done is proporional to speed,or that the force (torque) is proportional to speed? In a recent project where I had to figure all this out, I found that the current drawn by a small DC motor attached to a gear/crank system was almost constant (varying from only 0.240 A to 0.3 A) over a wide range of voltage (from 2v to 6v), causing a large change in RPM. This lead me to believe that the major frictional torque was caused by surface to surface kinetic friction, and therefore the torque required to maintain speed was constant,and the work needed was proportional to speed. This is what I said in my previous email. >There are of course many other factors, >-Magneto striction, >-Air flow drag, >-Viscosity of lubricant, >-Temperature of lubricant, >-etc. I realize that there are many things involved (hence the 60mA change in current shown above), however, I am trying to create a first order model, so I can see the major factors involved. I do think, though, that the kinetic friction will really dominate in a small,cheap DC motor,and that's what I feel my experiment bore out. That being said, it WOULD be interesting,as you say, to do a more detailed study and see what other factors play a role in the friction. I'm sure that the ones you mention would top the list. > >Sometimes it *really* helps to try the real thing out before developing >general theories ! Well, I realize that I am no expert on motors, that's why I prefaced by discussion with "someone please correct me if I'm wrong". BUT, I wasn't simply talking with NO experience. I had just finished a month long project in which I controlled a pair of small DC motors with attached gear/crank systems using a PIC (gotta get the PIC in there <G>), did a computer simulation of how the PWM affected the motors, thought about it quite a bit,and used my scope and other tools to test my theories. >regards, >Graham Daniel. Thanks, Sean | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 shb7EraseME.....cornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html

>figure all this out, I found that the current drawn by a small DC motor >attached to a gear/crank system was almost constant (varying from only >0.240 A to 0.3 A) over a wide range of voltage (from 2v to 6v), causing a Having played around with "small DC motors" at large input currents (up to 60A sustained - model boat racing, of course), let me give you a little of the rules of thumb: A no-load motor will pull a relatively constant current across the RPM range for a small motor. When you get into the performance category, things change. As RPM increases, it isn't uncommon to see the current rise significantly as "other things" come into play. Other things include (but not limited to): Brush drag - related to square of linear speed (this is why racers usual ly cut the commutator down - it reduces diameter --> linear speed). Air drag - at very high speed (60K RPM is often attained in these sucker s no-load, 30K loaded), the aerodynamics of the armature start to come into play. FWIW, I've found that epoxy-balanced armatures tend to do better, I believe because they smooth out the rotating airflow. The outer edge of a loaded armature may be moving 25 MPH linear speed. Bearing drag - that's a big one! Improperly lubricated bearings (wrong oil, too much oil, too little oil) affect performance drastically. This is related to RPM^2 also. Heating - as the motor warms up (they DO get warm!) the heating of the windings reduces their efficiency as conductors. My motors are often hotter than 100 C after a 90-second run. Knowing this, the typical way to help rate a motor is to find the no-load current (it's easier to measure) at a particular RPM and use that as the basis for efficiency. That way we can work out the approximate efficiency under load, knowing the loaded RPM and the loaded current. I usually set my motors up for 9A no-load current. 9A vs. 240mA! Anybody wanting to get deep into this should check out "The Motor Handbook" by Bob Boucher. It's available from http://www.astroflight.com online. He goes into _practical_ motor stuff in an easy-to-understand way. Check out his bio while you're there. Andy ================================================================== Andy Kunz Life is what we do to prepare for Eternity ------------------------------------------------------------------ EraseMEandyrc-hydros.com http://www.rc-hydros.com - Race Boats RemoveMEandyEraseMEEraseMEmontanadesign.com http://www.montanadesign.com - Electronics ==================================================================

Thanks for the explanation, Bob. I am still a bit curious as to why a bipolar will usually take more abuse, though. Sean At 10:24 PM 7/24/99 -0700, you wrote: {Quote hidden}>At 01:55 AM 7/24/99 -0400, you wrote: >>I am a bit confused here: why would the FETs require more precautions? The >>only thing I can think of would be that the power dissipation for a FET >>goes as the second power of current,and for a darlington/BJT, as the first >>power of current. Granted,this makes a big difference as current goes to >>infinity, but in certain cases, you might burn up the BJTs way before the >>FETs if the RDSon of the FETs is really low. > >It's actually even worse than that. The R of the MOSFET increases with >temperature and current, so unless you go overkill or have protection you >have a big problem. With comparable devices, the bipolar will usually take >a lot more abuse(assuming it has enough drive current). Generally in a >small application it's easy enough to use a larger MOSFET, but I've >designed 1200 amp motor controllers and it makes sense to put in good >protection so you can use fewer MOSFETs. > >Cheers, >Bob > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 RemoveMEshb7TakeThisOuTspamcornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html

Doh! I should have realized it was that simple. I did try modeling the commutation diodes before, but I guess I either did something wrong or didn't have a high enough motor inductance,because I didn't notice the effect of keeping the current going during the PWM "off" time. Thanks, Paul, Sean At 07:33 AM 7/26/99 +1000, you wrote: {Quote hidden}>Sean H. Breheny wrote: > >> With a switcher,however,we could forget PWM and just supply a >> continuous variable voltage to the motor. The current would stay the >> same and the RPM would linearly follow the voltage. The efficiency >> would not change much over the whole RPM range. > > Your analysis regarding motor performance is correct. Implementation >of switchmode control as you describe essentially consists of using PWM >whilst providing a series inductor and commutation diode. The >commutation diode(s) may already be part of an H-bridge or if the FETs >may be operated "backwards", they may function as such. >-- > Cheers, > Paul B. > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 EraseMEshb7spamspamBeGonecornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html

>>Air drag - at very high speed (60K RPM is often attained in these suckers >>no-load, 30K loaded), the aerodynamics of the armature start to come into >>play. FWIW, I've found that epoxy-balanced armatures tend to do better, >>I believe because they smooth out the rotating airflow. The outer edge >>of a loaded armature may be moving 25 MPH linear speed. > >Eh? > >Did I drop a decimal, or did you, Andy? > >25 MPH = 2200 Feet per minute, divided by > 30000 revolutions per minute equals > 0.07333 feet per revolution, equals > 0.88 inches circumference, equals > 0.14 inch diameter. Yep, I blew it. I started with circumference when I meant diameter. So multiply the 25 by 3.14. 75 MPH is more reasonable. Andy ================================================================== Andy Kunz Life is what we do to prepare for Eternity ------------------------------------------------------------------ RemoveMEandyKILLspamrc-hydros.com http://www.rc-hydros.com - Race Boats andySTOPspamspam_OUTmontanadesign.com http://www.montanadesign.com - Electronics ==================================================================

Hi Martin, What an excellent discussion! PWMing a DC motor is one of those things (at least,in my newbie level of experience with it) which at first seems trivial,but you quickly reealize that there are many design choices to make (e.g.: what PWM frequency do I use? How much power will my switching elements need to dissipate? What voltage should I use? What overcurrent protection scheme? How do I prevent overheating of the driver transistors due to low thermal time constant of the die?,etc.) I came up against all of this for the first time when I designed a small wall-following robot for an app note written for Wirz Electronics. Luckily, Ben Wirz was able to share his wisdom about motor driver protection with me,and I got the general hang of it after doing a few simulations using a program I whipped up in QBASIC (somebody on this list once called QBASIC the best all-purpose math analysis package - I agree!!). I will be re-reading your explanation several times to get the full value of it. Sean At 03:05 PM 9/4/99 +0200, you wrote: >Oops, I accidentally mailed my message with the title > "Re: PICLIST Digest - 2 Sep 1999 to 3 Sep 1999 " >instead of > "Re: Driving a high voltage H-bridge from a PIC" >Sorry! In order to save bandwidth I won't remail it >with the right title. > > -- Martin Nilsson > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 spamBeGoneshb7STOPspamEraseMEcornell.edu ICQ #: 3329174