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I am a newbie trying to build a simple circuit for reversing polarity (using the components I have). The intention is to control a DC three way motorized ball valve. The valve has two positions and to switch I have to change the polarity of voltage applied to the input wires. I would like to make the switch happen via output from a couple of RPI GPIO pins.

The schematic is attached. enter image description here

Most common H-Bridge's that I saw on the net use some sort of BJT combination. Unfortunately I do not have the luxury of ordering more components that what you see in the circuit. Will the schematic work, or should I really go hunt for some new components?

Thanks for your comments..

Vijay

EDIT: Modified the schematic based on inputs is below, modified schematic

Vijay Seshan
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    Take a look at this [Similar Question](http://electronics.stackexchange.com/questions/209212/will-this-simple-four-n-channel-mosfet-h-bridge-circuit-work?rq=1) regarding high side switching transistors – Marla Aug 03 '16 at 20:18
  • I see 2 problems so far. The 74HC138 shows floating control pins. You can never have floating inputs, because they will pick up local noise, making the IC behave erratically. The 4.7K pull down resistor are too high of a value to switch OFF the mosfets quickly enough. **PLEASE** read the article posted by @Marla. –  Aug 03 '16 at 21:11
  • @Marla thanks for the link, looks like I have to read up more on the high side switching. – Vijay Seshan Aug 04 '16 at 12:33

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As drawn, the H-bridge will not work.

You have N-channel MOSFETs for all components of the H-bridge. This is OK if you drive them appropriately. Imagine turning on Q1. Initially, Vgs will be 12V, but as the voltage on the output of Q1 rises, then Vgs will become smaller and smaller... until Q1 is off.

Additionally, optocouplers aren't very fast, so your switching losses on your MOSFETs are going to be much higher than they needs to be.

There are chips designed for this exact application. Have a look at the FAN7842MX for a good example. There are other designs, but this chip works great. You will need one chip for each half-bridge. There are full H-bridge drivers out there, I just haven't used them.

Enjoy, motors are fun!

slightlynybbled
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  • @slighlynybbled, I only have a couple of ICL7667's. Is it possible to use this to drive the mosfet's? If so, would you be kind enough to point me to some reference circuits? – Vijay Seshan Aug 04 '16 at 12:37
  • No. ICL7667 is good for low side but not for the highside, for the same reason given by `slightlynybbled` – matzeri Aug 05 '16 at 20:05
  • Agree with matzeri. If you want to make your own drivers, put P channel MOSFETS on the top and pull the Vgs 'down' to turn them on. You will need a circuit for that too, but it is reasonably done with discreet transistors. Optocouplers *can* work, but are going to be slow and increase your losses. Generally - with some exceptions that require isolation - optocouplers are not used for these types of circuits. – slightlynybbled Aug 05 '16 at 23:07
  • Ok, I *may* be able to source these, 1. P-Channel Mosfet (FQP27P06) - replace the high-side n-mosfet with this and drive all mosfet's using ICL7667's - Will it work with IRL2910 as the low-side n-moset? 2. Quad half H-bridge (SN754410NE) - replace all the mosfet's with this H-bridge (protected by fly-back diodes) - Will it be able to supply 0.5mA@12V required by the ball-valve to change position? (ball valve takes abt 5 seconds to switch position and then powers off) The requirement is to switch the ball-valve position once in a couple of days. Thx for your inputs.. – Vijay Seshan Aug 07 '16 at 18:55
  • slightlynybbled, @matzeri -- pls ignore my previous comment. I have uploaded a modified schematic based on parts that I would be able to source at my location. Would appreciate your comments. – Vijay Seshan Aug 08 '16 at 13:01
  • No obvious reason that it wouldn't work. Observations: (1) The circuit will *always* drive current in one direction or another, you can't turn it 'off'. (2) The optocouplers will slow your circuit down, but that may be ok if your frequency is very low (1Hz) vs. higher frequency (1kHz). If you plan to use PWM at all, you should find a way to eliminate the optocouplers to avoid high and frequent shoot-through currents. (3) Your circuit is complicated! Most low-frequency drivers are a few [transistors](http://www.talkingelectronics.com/projects/H-Bridge/images/H-Bridge-MOSFETs.gif). – slightlynybbled Aug 09 '16 at 15:41