Subject:
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Re: it works!
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Mon, 22 Apr 1996 06:18:52 GMT
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Original-From:
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Chuck McManis <cmcmanis@netcom.comSTOPSPAM>
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Reply-To:
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cmcmanis@netcom./NoMoreSpam/com
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Viewed:
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2082 times
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Jeff Keyzer wrote:
>
> Hey, it's time to celebrate! :-) I managed to get the h-bridge
> circuit working, after thinking about it for a while, tinkering a bit, and
> reading a couple chapters on transistors... :-) I can run a motor forward
> and backward, with no problem. The thing I'd like to make sure of, though,
> is that all the transistors are saturated, but not oversaturated.
Great Jeff, I've been planning on doing the definitive breadboard/diagram
and document here but haven't gotten to it.
Don't worry overmuch about "oversaturation", you are using the transistors
as switches and as long as you don't violate the max base current your fine
(max for 2907 is probably 10mA, the max for the TIP120/125 is 120mA)
The 2907 has a fairly high beta (typically in the neighborhood of 50 or so)
so with a 3.5mA base current it will conduct 150mA+ if sourced that much.
Did you say which TIP transistors you ended up using? Did you use the TIP125
and TIP 120s ? If so they have a gain of over 2000 so a base current of 19
mA would allow them to be completely on. (they can only do about 8Amps).
> I'm using a 6V supply, with a 6V motor. (However, it's not one of the
> motors I'm actually going to be driving, I haven't gotten that far yet!)
> Do these readings sound right?
To test your bridge, you can do the following:
- Attach it to a power resistor so that it drives its near max
load. (This should be the maximum current you expect to draw
from the bridge) (5A is max for the TIP 120/125)
- Measure the voltage across the emitter/collector of both
the high side and low side transistors.
- You can reduce the base current (increase the base resistor)
until the emmiter-collector voltage *just* starts to rise.
Then raise it back up a notch (reduce the base resistor).
This is the "optimum" bias current for the set voltage. Now
reduce your voltage to the minimum source voltage. Do the
same thing.
What you will find is you need smaller resistors to get the
same bias current at low voltage. This is required at low
voltage but a bit wasteful as higher voltages (at higher
voltages you get more current). The voltage where the
base current is just enough to keep the transistors
fully on is your "Absolute Minimum" voltage spec, now
leaving the circuit alone, the voltage where the base
current would exceed the transistors maximum input base
current in your design's "Absolute Maximum" input voltage.
> How can I test to find out if all the transistors are properly saturated in
> this circuit?
See above.
> One other thing - I noticed that in the original circuit, the base
> of the 2n2907's would be _shorted_ to ground when they were turned on. This
> didn't sound like a great idea, so I inserted a 1K resistor between the base
> and ground.
The current through the base of the 2907 will be limited by the current into
the emitter. You're correct though, a 1k to ground is a good idea.
Once you've characterized your bridge for your design voltage and design current
you've got your very own custom H-bridge that does exactly what you need.
If you decide you want to market your design you should also run the
characterization the coldest and hottest temperatures you want the bridge
to operate under.
--Chuck
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Message is in Reply To:
 | | it works!
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| Hey, it's time to celebrate! :-) I managed to get the h-bridge circuit working, after thinking about it for a while, tinkering a bit, and reading a couple chapters on transistors... :-) I can run a motor forward and backward, with no problem. The (...) (29 years ago, 21-Apr-96, to lugnet.robotics.handyboard)
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