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'Driving a high voltage H-bridge from a PIC'
1999\07\23@135837 by Nicholas Irias

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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?

-Nicholas

1999\07\23@140632 by Andy Kunz

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>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 Micrel.  They have everything you need, maybe even in a single chip.

Andy

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1999\07\23@142326 by Craig Lee

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Nicholas,

Try ZETEX part number ZHB6790CT.

2.5A continuous up to 40VDC.

Digikey even stocks them.

Craig

> {Original Message removed}

1999\07\23@143330 by Eisermann, Phil [Ridg/CO]

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> 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.

1999\07\23@143534 by John Mitchell

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On Fri, 23 Jul 1999, 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.

If you're a cheapie, then you might check out the L298 Dual Full-Bridge
Driver -- up to 46V, 4A current. Our friends at http://www.bgmicro.com/
have them for $3.49 US (page 5.)


- j

1999\07\23@145132 by Scott Dattalo

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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.

Scott

1999\07\23@175718 by Byron A Jeff

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{Quote hidden}

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

1999\07\23@182430 by Bob Blick

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> > 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

1999\07\23@185931 by Scott Dattalo

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Bob Blick wrote:
{Quote hidden}

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.

1999\07\24@015652 by Sean H. Breheny

face picon face
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
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1999\07\24@111636 by Nicholas Irias

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Thanks everyone for all the suggestions.  I neglected to explain why I had
to drive FETs - I'm controlling astroflight cobalt motors that draw up to 30
amps, so most single chip solutions wont work unless several are run in
parallel.

I just ordered a few of the Micrel parts as Andy suggested ( MIC5022 ).
These half bridge drivers meet the gate drive requirements and as a bonus,
have current sense and overcurrent disconnect capability built in.


-Nicholas


{Original Message removed}

1999\07\24@113330 by Andy Kunz

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>I'm controlling astroflight cobalt motors that draw up to 30 amps,

30A is nothing.  I run 60A through modified 05 can motors all the time.
The Astro motors are much cooler at it due to their bigger brushes.

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

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1999\07\24@115452 by Nicholas Irias

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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}

1999\07\24@125817 by Andy Kunz

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At 08:57 AM 7/24/99 -0700, you wrote:
>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.

What was the problem(s) with the 212?

>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

WOW!  How do you have time to program??

Andy


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1999\07\24@134020 by Nicholas Irias

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>>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.
>
>What was the problem(s) with the 212?

I typed the wrong part number.  My astroflight ESCs are 207s.  And the only
problem was price of $120 each, since I could have made my own for less $$.
I just hadnt done enough reading on home rolled ESCs at the time I bought
those units.

-Nicholas

1999\07\25@012440 by Bob Blick

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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

1999\07\25@153928 by Terry

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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}

1999\07\25@162204 by Sean H. Breheny

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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}

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1999\07\25@173345 by paulb

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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.

1999\07\25@175906 by Terry

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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}

1999\07\26@044940 by g.daniel.invent.design

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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}

--
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.

1999\07\26@115809 by Sean H. Breheny

face picon face
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
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1999\07\26@123110 by Andy Kunz

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>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

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1999\07\27@022034 by Dave Bell

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Andy Kunz <RemoveMEsupportspam_OUTspamKILLspamMONTANADESIGN.COM> wrote:

>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:

....

>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.

I know DC motors that small exist, but for racing boats?

Dave

1999\07\27@030550 by Sean H. Breheny

face picon face
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}

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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1999\07\27@030756 by Sean H. Breheny

face picon face
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 Breheny
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| Electrical Engineering Student
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1999\07\27@101015 by Andy Kunz

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>>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
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'Driving a high voltage H-bridge from a PIC'
1999\09\04@090652 by Martin Nilsson
picon face
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

1999\09\05@175834 by Sean H. Breheny

face picon face
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
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