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Sanity check - FET H-Bridge circuit
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danners430:

--- Quote from: Hero999 on September 12, 2018, 03:34:26 pm ---The problem with a series resistor is the voltage drop. You have a 250R, so the maximum current your circuit can supply into a short circuit at 5V is 20mA. When the voltage across the load is 2.5V it can deliver just 10mA.

--- End quote ---

The series resistor would be necessary anyway for a bi-colour LED - I always go on the safe side, and say 20mA, no voltage drop. I never need LEDs that are uber bright anyway :-)

I'm not sure where you got 2.5V from....

Finally, when I use it for an actual motor, I'll eliminate the resistors, and simply drive all four outputs directly from the MCU, or use the circuit posted by Alex earlier... if I can work out how it works :-)
Alex Nikitin:

--- Quote from: danners430 on September 12, 2018, 03:38:05 pm ---use the circuit posted by Alex earlier... if I can work out how it works :-)

--- End quote ---

Is it that difficult? If both inputs are at 0V, M2 and M4 are conducting and on both sides of the load voltage is close to 5V. If, say, the input 1 is at 3-5V, M1 is conducting, at the same time M2 is switched off, as the voltage on the gate is close to 0. The load current goes from +5V supply through M4, R load, D1 and M1 to ground. If the input 2 is at 3-5V, the current in the load goes in the opposite direction through M2, R load, D2 and M3. If both inputs are at 3-5V, M2 and M4 are off and no current goes through them or the load, so the circuit is safe if you drive both inputs high.

Cheers

Alex
T3sl4co1l:
No one has so far noticed(?) that the P-channel transistors are upside-down. :o

H-bridge is perfectly fine with all N-channel, you just need a bootstrap driver.  These are particularly abundant for low supply voltages like this.

You can even get drivers with dead-time protection built in, so that you can drive each side of the H-bridge as a logic inverter and not worry about shoot-through. :)

Tim
boB:
If Vcc is only 5 volts and you are just driving a Bi-color LED, why not just use a TTL logic gate of some sort ?
Then just one limiting resistor and now worry about shoot through.

Could make use of a hex inverter (eg. 74HC14)  and would also include the extra inverters to drive
the opposite side inputs.  OR, almost any 4 gate TTL chip for control of the LED.

boB
max_torque:
in this day and age, i have NO IDEA why you'd want to roll your own H bridge, other than for personal learning reasons?

You can buy lots of smart H bridge IC in lots of different voltage and current flavours, and they have lots of safety, diagnostics and feedback built into them, and are fully proven and certified. 

ie  : http://www.infineon.com/cms/en/product/power/motor-control-ics/intelligent-motor-control-ics/


The only reason not to use one i can think of is because you want to save every last $ from your BOM, and are happy to spend many $$ during development to roll your own solution. (and even then, you'll still find it hard to get to any significantly lower cost with a much larger BOM parts count)
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