solid state switching

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URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=123578
Printed Date: July 07, 2025 at 2:16 PM

Topic: solid state switching

Posted By: 24inchranger
Subject: solid state switching
Date Posted: September 19, 2010 at 4:13 AM

Hi everyone need a bit of a hand with this one.

i am trying to eliminate the clicking of a mechanical relay for oem purposes and was thinking of using a fet as solid state relays are to expensive.

Could someone point me to or draw something that could eliminate the relay with the fet while using only the four terminals on a conventional relay

Relay is 12v 15a Max

Thanks

Andrew

Replies:

Posted By: oldspark
Date Posted: September 19, 2010 at 6:39 AM

You can buy MOSFETS that handle over 60A for $2-$3.

Posted By: 24inchranger
Date Posted: September 19, 2010 at 6:47 AM

but how will i incorporate them to replace the relay

Posted By: oldspark
Date Posted: September 19, 2010 at 8:23 AM

In simplest form with an N-channel MOSFET, connect the Drain & Source between the load and ground.
Then (say) +12V to the Gate to turn it on.

Add a resistor (1M-Ohm etc) from the Gate to Ground to ensure it turns or stays off if your control voltage/switch is floating when off.

Often a series resistor is placed inline with the Gate - say from 100 Ohms to 100k Ohms or 1 M Ohms....

And if the FET does NOT have a reverse biased "freewheeling" diode across its D & S, then add one yourself (IN4004 etc).

It sounds complex, but isn't.
Often just the 3 straight connections (no resistor/s) work fine.
Unlike a transistor where Base (Gate) current somewhat critical....

For hot switching (+12V), P-channel FETS can be used, but its Gate needs to be pulled low (down from +12V - eg, to +6V or GND etc) and that requires an extra FET or transistor etc if the control signal is "high" for ON.

Posted By: 24inchranger
Date Posted: September 19, 2010 at 5:37 PM

I'll give it ago on the weekend can you throw as a quick schematic just for double checking

Posted By: oldspark
Date Posted: September 19, 2010 at 9:26 PM

Good 'ol Wikipedia!
posted_image

... from wikipedia-MOSFET, and its...
Fred the Oyster's Mosfet_n-ch_circuit....
"Example circuit with an N-Channel MOSFET. When the switch is activated. The LED is activated through the current limiting resistor. The resistor below the switch ensures the capacitance of the MOSFET gate is depleted when the switch is not activated."

FYI - I've been meaning to order various MOSFETs from Oatley Electronics (Australia - see typical MOSFETS for $2.50. These are typically many tens of Amps with low ON resistance.