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I'm trying to drive a solenoid to create a magnetic field. I've got a constant current controller circuit that supplies the current I need, but I'd like to control this using an H-Bridge so the the polarity can be changed and/or the coil can be turned off quickly. (<1ms)

I'd been planning to do this with an IC H-bridge, but am running into problems. Typical currents for my application are ~10-300mA which can require voltages of 5mV or less. The current controller is capable of supplying this and regulating itself, however the H-briges I've seen can't operate at such low voltages.

For instance, the SN754410's datasheet says:

The SN754410 is a quadruple high-current half-H driver designed to provide bidirectional drive currents up to 1 A at voltages from 4.5 V to 36 V. The device is designed to drive inductive loads such as relays, solenoids, dc and bipolar stepping motors, as well as other high-current/high-voltage loads in positive-supply applications.

The SN754410 has seperate supply pins for the logic circuits and for the outputs but the lowest recommended voltage I can find for the supply of an H-bridge is around 4.5V.

How can I quickly turn on/off a solenoid that only requires mVs and mAs?

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  • \$\begingroup\$ What are you using to produce your constant current? There may be a way to configure that for use with a standard H-bridge if you give us some details. \$\endgroup\$ – brhans Apr 2 '15 at 15:16
  • \$\begingroup\$ Do you need the polarity changed quickly? (You could use a double pole switch or relay.) For fast turn off can you just "ask" the current source for zero current? (Toggle the reference voltage assuming it's a VCCS.) \$\endgroup\$ – George Herold Apr 2 '15 at 15:40
  • \$\begingroup\$ @brhans The current is coming from an FL593FL controller which is designed to power laser diodes. \$\endgroup\$ – CharlieB Apr 8 '15 at 12:58
  • \$\begingroup\$ @GeorgeHerold The final device should have the ability to switch polarity quickly, yes. I haven't actually checked yet how quickly the current source can power down: I'll have a look. \$\endgroup\$ – CharlieB Apr 8 '15 at 13:00
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So let's talk this out. An integrated H-bridge circuit is going to do one of two things - it is going to apply it's own supply voltage to the load (for instance, the DRV8836), or it will give you four electronic switches with integrated drivers to connect however you like.

In either case, your current source becomes a problem. The totally integrated module isn't going to work without a constant supply voltage, period. The other module still needs a constant control voltage, but the source could be attached to the switches as you please. The problem is, without really careful design work, the constant current source is going to attempt to keep pushing through the switches when they transition from on to off, potentially destroying the module - so, a change in direction of current flow now requires turning off the source, changing the H-bridge configuration, and then restarting the source. Perfectly workable, but a cursory search hasn't yielded a good module for this yet.

If it were my design, I would compare the cost of any module I found that was suitable against the cost of simply building an additional current source with the opposing direction. A finished, contained project was going to have to have a controller of some type anyway, so I would then abstract away timing and control in firmware. At these currents and voltages, I could imagine accomplishing this with a 5 volt power supply, an arbitrary microcontroller, a couple of diodes, and possibly as few as one quad opamp chip, depending on how clever this really needs to be.

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  • \$\begingroup\$ Thanks for your in depth reply and sorry I've taken a while to get back to you. You said that the H-bridge type with seperate supply / control voltages just provides a set of switches to be configured as I wish. That was exactly my intuition about how I'd expect H-bridges to work, and was why I was suprised when the spec sheets stipulated a minimum for both the control and the supply voltages. Does this mean that, disregarding the second problem, simply using a dual supply H-bridge should work? \$\endgroup\$ – CharlieB Apr 8 '15 at 12:57
  • \$\begingroup\$ The second problem about the current source attempting to force current through an off H-bridge I'm not sure is an issue for me: I'm using an FL593FL board designed to drive laser diodes and our setup has it powered by only 5V so I don't think it has the oompf to damage the H-bridge. Your idea for a design: the op amps here are to build a constant supply? The ultimate requirements are programmable currents in either direction, switching from setup A to setup B as quickly as possible (possibly switching polarity depending on user's choice) and driving an inductive load. \$\endgroup\$ – CharlieB Apr 8 '15 at 12:57
  • \$\begingroup\$ @CharlieB, Opamps are useful for implementing small voltage controlled current sources, and can handle some small inductive loads just fine with proper design work. You said you were using a current source - placing a resistance in the path of a current source is a great way to break stuff. If your current source won't have this particular problem, yes a standard H bridge module will work great. Check the prices though and be sure that you don't want to build your own, as MOSFET driver ICs are very reasonably priced. \$\endgroup\$ – Sean Boddy Apr 8 '15 at 13:45
  • \$\begingroup\$ @CharlieB, also, the right opamp with a bipolar supply could also give you programmable current flow in both directions, with the right input control. Make sure you are reviewing your options. \$\endgroup\$ – Sean Boddy Apr 8 '15 at 19:17
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I have zero knowledge or experience, but for a problem where I was trying to use a low voltage motor (1.5-3 volts) someone pointed me to DRV8835 Dual Motor Driver Carrier which has a minimum voltage of zero. It worked great.

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