Hello,

I need to build a 4 port solenoid driver. The solenoids operates at 24VDC, 54Watts, which is if not wrong around 2.2amps. Could you kindly suggest whether to use a MOSFET or bipolar transistor and any reference part for that? The driver board needs to be isolated, so 4N35 optocouplers are to be used as well. Thanks in advance for any suggestion!!

 

Personally I would use transistor switching relays. Provides the isolation you need as well.

 

True, you can find solid state relays in the lower voltage/current range at fairly reasonable prices nowadays.

 

I didn't actually mean solid state relays. I meant transistors turning relays on and off. I forgot an "s" on transistors. Makes much more sense when all the letters are there.

Being of the old school I much prefer real relays that you can actually hear the click from.

 

Hello thanks for suggestions!! But I have to switch around 3amps current with a frequency of one every 10 seconds in the worst case. So it seems to me that a normal relay mechanical capacity may be exhausted quick. What is your opinion?
peyro

 

I would stick with a MOSFET, perhaps an IRF540 or similar.....

 

I agree with Bmorse. It would be easier for the 4N35 to drive a MOSFET gate than a transistor base since speed doesn't seem to be an issue anyway. Use a diode across the solenoid to suppress the back emf.

Regards,
Ifixit

 

I suppose now MOSFETs are so cheap you might as well use them. But plain old bipolar transistors would certainly do the job.

Actually I claim that driving a transistor base uses fewer components than a MOSFET gate--as long as it's a Darlington transistor. Just have the collector of the isolator go to the +24V rail, and the emitter to the transistor base. With a MOSFET, you'd need an additional pulldown resistor to turn it off, but with a Darlington, that's likely to be built in.

See TIP102, 39 cents at Jameco:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_32977_-1

 

You might consider something like a DRV102; Texas Instruments makes them.
Product page and datasheet: http://focus.ti.com/docs/prod/folders/print/drv102.html

The idea behind these drivers is that they provide a high initial current through the coil to ensure that it engages, and then a lower hold current. This will help keep your electric bill down in the future.