Choosing a Mosfet for a Constant Current H-bridge

Ian0

Joined Aug 7, 2020
9,802
OR -

An H-bridge is a trick to effectively double the apparent differential voltage across a load. But the goal is a controlled bipolar current with a relatively high (for simple opamp circuits) voltage compliance. Given the very low current and power levels, what about a single high voltage opamp and +/-40 V supplies? Apex and Burr-Brown still make them (National's LM143/343 would be perfect if you can find them), or you can put a couple of 60 V transistors around a normal high-precision opamp.

There are jillions of opamps with very low offset and bias errors, so that part is easy. Opamp "output booster" and "current booster" circuits have been around since the 60's. Fun fact, one of the versions of the Crown DC-300 is basically just that, an opamp with a dozen power transistors wrapped around it.

https://how-to.fandom.com/wiki/How_to_boost_the_output_voltage_swing_of_an_operational_amplifier

The smallest 24 V transformer you can buy will give you +/-33 V rails, slightly less with regulation; but given that the circuit is a current regulator, stiff rails are not mandatory.

I'll redo the #6 circuit. Basically, delete all 5 transistors, and put the load between the opamp output and the top of R3.

ak
You can't go too high in voltage before the electrical safety regulations stop you!
 

AnalogKid

Joined Aug 1, 2013
11,036
You can't go too high in voltage before the electrical safety regulations stop you!
Maybe. For the most part, those rules assume a large, sometimes unlimited, current capability. When the max total energy is only 24 mW, I don't think a human subjects or patient contact compliance review will be an issue.

Note that a Taser is approved for virtually-untrained use in all 50 states, and that thing actually penetrates the skin.

ak
 
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