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'Design Challenge - A better H Bridge MK2'
2005\07\16@082243 by Vasile Surducan

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On 7/16/05, Russell McMahon <> wrote:
{Quote hidden}

2005\07\17@002303 by Russell McMahon

Re shunting the motor to achieve more rapid braking - presently done
with a 22R parallel resistor.

> Thar 22 ohm resistor may be connected in a different
> way with just one transistor which will be ON only
> when the H command stops.

Apart from the motor being able to be driven with either polarity, the
bridge needs to draw about zero current when inactive. This makes the
use of series opposed polarity MOSFETs as the switching elements a far
easier solution than using bipolar transistors. The fact that the
shunt is applied only when the motor is not being powered has the
advantage of allowing a much lower shunt resistor value to be used, or
even none at all.

Or, a reed relay would work wonders :-).


'Design Challenge - A better H Bridge MK2'
2005\08\16@103559 by Russell McMahon
Criticism sought.

A month or so ago I was talking about H bridges.
Object was to drive a motor from 4 x C cells at about 300 mA run current.
Motor ideally needed 4v+, less still worked within reason. Motor was unhappy
with more than say 6 volts and supply CAN rise to 8v or even 10v when a
mains adaptor is added.. Existing bridge dropped 2v plus across bridge. Also
had no voltage limiting.
Low component cost was an object. Operation on VERY flat batteries
important. Off current should be under say 10 uA.
Commercial H bridge ICs tended to suffer from higher cost, poor saturation
voltage, too high minimum operating voltage.

This is what I came up with.
It's more complex than may be expected but not as bad as it feels at 1st
Original had 6 transistors. This has 9. Plus 3 more in optional brake which
wasn't part of original.

It works superbly.

Resistor values may not be final values.

Circuit (14 kB gif) is at

Motor Brake at top right is an optional extra. It works very well. R42 is

H bridge proper is at bottom.
Its job is to provide two saturated transistors for the motor drive when
driven on.
It does.

The "voltage limiter" is in fact a voltage regulator. It's job is to prevent
motor voltage from rising too high when supply is above spec. It does NOT
limit motor current except in its role as a voltage limiter. Rather than
grounding Q33 and Q32 emitter they are taken to effective ground by D31 or
D32 when the bridge is driven on. When the bridge is off the emitters float
and this reduces idle current to zero. The regulated voltage need not be
precise - just not overly high ever.

The motor brake has two MOSFETS in opposing series. When the bridge is off
the FETs are driven on by R41. Q43 is off. When the bridge is driven on Q43
is on and the FETs are off. The result is that the motor is shunted by R45
when the bridge is off and is not shunted at all when the bridge is on. R45
can be selected to suit required brake severity.

Q12, Q13, Q22, Q23, Q31 are something like BC337 / 2N2222.
Q31 dissipation needs to be watched.
Q41, Q42 need to be logic FETs to allow low battery voltages. The FETs are
the most expensive components.

Overall quiescent current is about zero. (Sub uA).

Q12/13/21/22 are NOT provided with base pulldowns as they have no voltage
applied when the bridge is off.

R33 is essential for correct voltage regulator action.
R35 is a good idea.

R42 is needed for good braking in both directions. Without it the floating
load makes FET clamping suspect in one motor direction.

It would be nice to think that an IC would do this job more easily and
cheaply. It would have to provide low Vsat (under 0.5 v total at say 300
mA), < 3mA input drive, very low off current (< 10 uA, motor voltage
limiting. Brake is an option which would be nice in an IC. `

Criticism invited. How would I do it better?

       Russell McMahon

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