Thread: Whats a semiconductor substitute for a Relay?

You pretty much answered your own question, a Mosfet can easily be used in place of a DC relay, you'll need to use a diode across the coil to prevent the high voltage from destrorying the mosfet, if you're not currently using one on your solenoid that may be a big part of what's killing your relay in the first place.

Originally Posted by Sceadwian

You pretty much answered your own question, a Mosfet can easily be used in place of a DC relay, you'll need to use a diode across the coil to prevent the high voltage from destrorying the mosfet, if you're not currently using one on your solenoid that may be a big part of what's killing your relay in the first place.

Thanks for your reply. Yeah, I got triggered that the solenoid would spark heavily when switching it off, rather than when switching it on(back-emf !)

So, I guess I should modify my circuit this way.


But Would you please tell me the product No. of a common Mosfet that can be use here?? Also, of what votage rating should I use my diode?

(Edit: Oops, I don't think that the diode I inserted will prevent sparking in the relay, or damaging the mosfet)

Attached is a simple image of how a protection diode is supposed to be used. Simply replacing the transistor with a Mosfet rated for 5+ amps is all you need to do. Any general purpose power switching mosfet will do. What voltage are you using as you'll need 10volts or better to fully turn on most power mosfets. Also what are you using a capacitor in parallel with the relay for? EMF protection?

Once the circuit is modified so a MOSFET will survive, keep the relay. It will last forever if properly protected.

mneary he's modifying the circuit to use a mosfet so he doesn't have to use a relay =\ The diagram I posted was just a quick one I grabbed off the net to show the placement of a protection diode on an inductive load, the relay in that schematic would actually be the solenoid he's using. Though with a protection diode properly in place over the existing relay the problem may go away.

If you're going to keep the relay you would put them across both the relay trigger coil, and the solenoid. The one on the relay protects the transistor or whatever you're switching it on with and the one on the solenoid will eliminate the back EMF when the solenoid is switched off. You probably shouldn't be using a capacitor in that manner. Can you provide a complete schematic of how you have things hooked up currrently? To get the kind of behavior you're describing so far you should be using a 555 to trigger the relay or mosfet that feeds power to the coil.

The IRFz44n is rated to almost 50 amps, that's overkill to say the least.

thecritic, I think you need to go back to electronics 101.
The diode will do absolutely nothing when power is applied to the solenoid because it will be reverse biased and the solenoid will function normally. When you cut the power to the solenoid it resists the current change by creating an inverse voltage potential, this forward biases the diode causing it to conduct, basically short circuiting the solenoids terminals. This causes the current the magnetic field produces as it collapses to run around in an endless circle in the coil through the diode till it all dissipates as power in the resistance of the coil, walla, no arcing. It's needed on both the relay primary AND the solenoid, because they're both inductive loads.

Forget about the fet for right now, the diodes alone may solve your problem.

Here is a thumnail of where I have reached so far.


Am I correct ?