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[on:]

So, what i've got here is one of the sources for an h-bridge i'm working on, isolated from the rest of the circuit for testing. I can't seem to figure out for the life of me why it can only source about 1.7-1.9 A. The motor it's driving, when attached directly to the exact same power supply driving the circuit (only adjusted to match Vout), draws about 2.3-2.5 Amps stalled, but in when attached to  Vout, draws only 1.7-1.9 Amps stalled.

What I don't understand too, is why the voltage drop between +V and Vout is only around .5 V, when Vce of Q2 and Q3 are far greater than that.

There's a flyback diode installed between Vout and +V that I neglected to include in the schematic, but I don't think that has anything to do with it.

So yeah, help would be much appreciated.

EDIT: Whoops, forgot to include the schematic.

http://i218.photobucket.com/albums/cc201/sinkoman_iii/2011-08-19_22-46-48_925.jpg

[Reply #1 on:]

Whoops, forgot to include the schematic.

Is that actually a H bridge?

[Reply #2 on:]

Whoops, forgot to include the schematic.

Is that actually a H bridge?

So, what i've got here is one of the sources for an h-bridge i'm working on, isolated from the rest of the circuit for testing

[Reply #3 on:]

There's a flyback diode installed between Vout and +V that I neglected to include in the schematic, but I don't think that has anything to do with it.

Well if it were the wrong way round it would explain:-

is why the voltage drop between +V and Vout is only around .5 V, when Vce of Q2 and Q3 are far greater than that.

[Reply #4 on:]

Cathode is connected to +v, anode is connected to Vout

[Reply #5 on:]

OK have you measured Vout under the load of the motor or unloaded?

[Reply #6 on:]

Hadn't bothered to measure load voltage till now. Drops about .25 V with the motor+gearbox+wheel connected.

EDIT: When stalled, the drop is 1.7 - 1.75 V

[Reply #7 on:]

I think that is your problem then.

[Reply #8 on:]

I already tried adjusting the power supply to offset Vout with the motor connected (but unloaded), and the current draw when stalled is still topping out at 1.8-1.9 A :/

[Reply #9 on:]

check the transistor datasheets (for 3A or the TIP42's guaranteed gain is only 15) you'll probably find you don't have enough gain to fully saturate under your low drive conditions - try replacing the 1k on Q1's collector with 330 ohms? 

If Q1 can drive enough current then you could lose Q2 and have a much smaller Vce sat from your output stage - Darlingtons drop a lot of voltage compared to 3.5V...

[Reply #10 on:]

3.3v is very low to be driving an H bridge made from bipolar transistors. Power transistors typically have Vce(sat) figures of 1v or more at a few amps. If you really need to use such a low voltage supply and motor, I suggest using mosfets instead. You can get P-channel mosfets that are good with gate drive voltages down to 1.8v. The only snag is that all the ones I can find are designed for surface mounting (mostly SO-8 packages), so they are a little tricky to solder.

Mosfets generally incorporate reverse diodes between drain and source, so in a full H bridge configuration you don't need any extra flyback diodes.

[Reply #11 on:]

3.3v is very low to be driving an H bridge made from bipolar transistors. Power transistors typically have Vce(sat) figures of 1v or more at a few amps. If you really need to use such a low voltage supply and motor, I suggest using mosfets instead. You can get P-channel mosfets that are good with gate drive voltages down to 1.8v. The only snag is that all the ones I can find are designed for surface mounting (mostly SO-8 packages), so they are a little tricky to solder.

Mosfets generally incorporate reverse diodes between drain and source, so in a full H bridge configuration you don't need any extra flyback diodes.

While that's true in general there are newer high performance bipolar supertransistors out there with very low Vce(sat) at several amps such as the phenomenal ZTX851, 0.2V at 5A collector current - although its only in a small package so probably not rugged enough for motor control.

All _power_ MOSFETs incorporate the diodes because its necessary to prevent latch-up: http://en.wikipedia.org/wiki/Power_MOSFET#Body_diode

[Reply #12 on:]

I'm thinking maybe I do just need to go with MOSFETS, because i've already tried connecting Q1 to Q3 as a Sziklai pair, thinking the voltage drop was the problem, and I still had the exact same problem. I was hoping to avoid using MOSFETS, as p-channel ones are impossible to find locally, and shipping costs here are pretty ridiculous. Oh well :/