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'[OT] Transistor H-Bridge'
2000\04\07@025825 by Andrew Sempere

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I am attempting to build a transistor h-bridge to replace the DPDT relay
I'm using at the moment to switch the direction of a motor. (this is for
a project run by a 16f84).

I've seen this circuit (see schematic below) in several places.
Switching on a pair of transistors (transistor 1 and 4) should feed the
load ("M"). Switching off the first pair and switching on the second
pair (2 and 3) should do the same with reverse polarity.

This seems straightforward, but it simply won't work for me. At first I
was using lightweight transistors and I was concerned the current for
the motor toasted them, so I rebuilt using TIP122s. I've tried replacing
the motor with various other loads, including a lightbulb, leds, and a
huge 10 watt resistor. Still no luck.

What happens is... nothing. The motor won't turn, lights won't light,
etc.

According to the power supply meter there seems to be an awful lot of
current going somewhere, but not through the load (enough so that I
don't want to leave it on long enough to take readings).

However, if I put a meter across the motor connections I get a very
slight reading that is consistent with what I expect to see (the
polarity switches when I switch base pairs). This reading is really tiny
and, again, the load doesn't seem to be receiving any power.

I'm probably missing something very basic but it's been a few days and I
can't figure this out... any suggestions?

Beautiful Ascii schematic
(without the current limiting resistors for the base):

M)otor
C)ollector
B)ase
E)mitter

      +5v
C1-----|------C2
B1              B2
E1-|       |- E2
   |---M---|
C3-|       |- C4
B3              B4
E3-----|------E4
       |
      gnd

---------
Andrew Sempere
spam_OUTandrewTakeThisOuTspamevolve.net

2000\04\07@032014 by Mark Hull

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<DIV><FONT color=#000000 size=2>Hi Andrew</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT color=#000000 size=2>Try using PNP transistors in the &quot;top&quot;
legs of the H, and NPN in the &quot;bottom&quot; legs.</FONT></DIV>
<DIV><FONT color=#000000 size=2>Also use biasing resistors to switch the
transistors off (base to GND on the NPN </FONT></DIV>
<DIV><FONT color=#000000 size=2>and base to + on the PNP)</FONT></DIV>
<DIV><FONT size=2>You have to invert the drive signals (transistor 1 = PNP drive
base low, and transistor 4</FONT></DIV>
<DIV><FONT size=2>= NPN driven high)</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>Thats the way the bridge normatty operates :-)</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>I think your problem is that both transistors in a leg are
being switched on together, thus </FONT></DIV>
<DIV><FONT size=2>shorting the supply and cooking transistors.</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>Cheers</FONT></DIV>
<DIV><FONT size=2>Mark</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV></BODY></HTML>

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2000\04\07@045932 by paulb

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Andrew Sempere wrote:

> I am attempting to build a transistor h-bridge to replace the DPDT
> relay I'm using at the moment to switch the direction of a motor.

>        +5v
>  C1-----|------C2
> B1              B2
>  E1-|       |- E2
>     |---M---|
>  C3-|       |- C4
> B3              B4
>  E3-----|------E4
>         |
>        gnd

 OK, your circuit is fine, but there are a few things you should know
about it, which are addressed in the published designs such as the
classic: http://www.bobblick.com/bob/projects/hbridge/index.html

 For *your* design, we presume you are using all NPN transistors
(sorry, don't recognise the types you quoted).  Now, the "low-side"
transistors must be switched on through resistors, that is B3 and B4,
while you need no resistors for B1 and B2. If you connect the same drive
to both diametrically opposite bases together, the "low" transistor will
be biassed at the expense of the "high" transistor.

 Also, if you use all NPN transistors, you really need to switch on
each of the upper pair with a voltage significantly greater than 5V (in
which case they also will need resistors), as there is an emitter-base
voltage drop of 0.7 to 1.2V even when the transistor is fully switched
on and only 0.2V drop between emitter and collector.

 If you can't provide the "high-side drive", you may need to use PNP
transistors as suggested, and as Bob uses in the design above.  He uses
darlingtons, but you pay the same voltage drop penalty for using those
as you do for simply using NPN transistors on the "high side.
--
 Cheers,
       Paul B.

2000\04\07@075119 by Jinx

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Andrew, how does what you have compare to this?

http://www.scooby.flysaturn.com/cktlib/motordrv/

2000\04\07@124828 by rottosen

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Andrew Sempere wrote:
{Quote hidden}

Keep in mind that the TIP122's are darlingtons with internal resistors
from base to emitter on each transistor of the darlington. They are 5k
for the input transistor and 200 ohm for the output transistor. We may
need to know what your base resistors values are because of this. The
resistor should allow at least 10ma to flow into the base.

Second, you do not need (or want) a base resistor for transistors 1 and
2 since this is an emitter follower configuration and the resistor will
decrease the drive by dropping voltage from any base current flowing
through it.

Third. Since the transistors are darlingtons, they will drop a lot of
voltage when on.

For transistors 3 and 4 the on voltage is about equal to the base to
emitter drop of the input transistor plus the saturation voltage for the
output transistor. This will be about 0.8 volts minimum increasing with
the load current.

For transistors 1 and 2 the on voltage will be equal to the sum of the
base to emitter drops of the input and output transistors of the
darlington. This will be about 1.3 volts minimum.

If you add things up you will find that the motor will see a maximum of
about 5 volts minus 0.8 volts minus 1.3 volts which is only about 2.9
volts. This voltage decreases with heavier loads.

Do you really intend to deliver 10 watts to the motor? If so the voltage
will be a lot lower. Maybe less than a volt.


-- Rich

2000\04\07@135041 by CUTTLER!

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>Try using PNP transistors in the "top" legs of the H, and NPN in the "bottom" legs.
>Also use biasing resistors to switch the transistors off (base to GND on the NPN
>and base to + on the PNP)
>You have to invert the drive signals (transistor 1 = PNP drive base low, and
transistor 4
>= NPN driven high)

True, very true,
If possible and I reccomed that you change over to mosfet transistors.  Much lower on
resistance and heat buildup especially if this is a production prototype.  Also
protect your transistors and/ or mosfets with snubbers or at least protection diodes.

There are a few H bridge IC chips out there that do all of this for you and may
reduce your overall cost.

>
>I think your problem is that both transistors in a leg are being switched on
together, thus
>shorting the supply and cooking transistors.

Yes, I agree, or in a worst case senerio the transistors were zapped by inductive
kick when the switches were turned off thus possible massive short.

You may have seen this as arcs/sparks if your relays had a clear case.

Please, put the emitters on the supply rail side, not the "load" side.  Much better
for switching applacations, otherwise the transistors act as (at best) a voltage
drop.

I think...

Carl Bright
Communications Mfg. Co.
carlspamKILLspammmcable.com
http://home.mmcable.com/cuttler/

2000\04\07@174423 by Andrew Sempere

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Thanks to all for the very helpful replies and links to schematics. I'm
still sorting through the advice, but I'm pretty sure the problem lies
with the biasing of the opposed bases. (I had the same value resistors
connected to all bases).

I also am not providing a higher voltage for the upper pair of
transistors...

As far as mosfets I'm not really sure how to do this, I'm relatively new
to digital electronics and largely self taught (could you tell?)...
never worked with anything but NPNs and PNPs.

To answer a few questions, I'm using four NPNs.  And no I don't intend
to really deliver 10watts, although I implied that - I just happened to
have a big resistor around and tried it as the load.

The motors I'm controlling are actually quite small (yanked from small
toy RC cars). And the 122s are definitely overkill (they will be
replaced with much lighter weight transistors - not sure which yet).

I do know of off the shelf h-bridges but the cost I've seen per unit is
too much. I only intend to build a few of these.

Thanks again for the help,

Andrew

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