I created a mosfet h bridge based on the design from bristolwatch.

http://www.bristolwatch.com/ele/moshbridge/mosh7.png

I'm using it to power a solenoid (that needs +24v or -24v to open/close a valve). If I connect the solenoid directly up to my 2200uf capacitor, I can get it to turn on/off. But when I connect it through the h bridge, it doesn't appear to be getting enough power (a multimeter on the capacitor shows it only draining halfway). I've tried using a 10k, 30k, and 100k between the source-gate of the p channel mosfet, but none appear to be working. Q5/6 are PN2222ATF powered by a pi. (slight modification is a 100k between the base-emitter, but I'm pretty sure I've tried it with and without this)

My hunch is that it has to do with the gates not getting enough power to open up my mosfet enough for 5a. The gate threshold voltages are 1-2v (N) and 2-4v (P), but I'm not really clear on whether that is a difference of 1-4v from my 24v or 1-4v total. And I can't figure out how to calculate the ideal resistor for gate-source and if i need a separate resistor for my n channel gate since it has a lower threshold.

Thanks for any help!

Datasheets

P Channel MOSFET

N Channel MOSFET

  • Unless you are trying to actively drive the solenoid in both directions (is the plunger a permanent magnet?) you do not need an H-bridge, but rather only a single N-FET. Also, what are you doing with the capacitor? – Chris Stratton May 22 '16 at 1:26
  • @chris I need +24v to open the valve & -24v to close it. The capacitor is used to store the 24v 5a needed to do this (since my source can't supply 5a at once) – mosfetnoob May 22 '16 at 11:37

The gate threshold max is 10V on my N channel MOSFET (and 25v on my P channel). My previous design assumed both MOSFETs had a max Vgs of at least 24v. I was sending 24v to the N channel which was too much causing it to limit my drain to source current. I decided to use a 10k from 24V source to my N channel gate & transistor. Then 10k to the P channel gate. That divided the voltage enough for my N channel to work correctly (not sure how I didn't damage it previously) and allow the full 5a flow between drain & source. I guess that's what I get for only testing the voltage previously & assuming the full load current would work.

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