# Replacing relay with transistor



## basshead (Sep 12, 2008)

Hi, i want to replace my mechanical relay with something less noisy. So after digging a little on the net I found some Solid State Relay, but I also found a very simple circuit that should cost less and more DIY! This is to control my 3 amps remote and my 2 fans.

I need a transistor like the TIP120 (TO-220,NPN,10A), 1k resistor and a 100k resistor.



The remote 12v from the HU will go to the 1k resistor then the other end of the resistor to the base of the TIP120, constant 12V (fused) at the collector and the devices (fans or amps remote) at the emitter. Also a 100k will be connected between ground and the emitter.

Any problem with this circuit?


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## basshead (Sep 12, 2008)

so is it safe? Anyways, ill go buy the components this weekend and try it out.


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## thehatedguy (May 4, 2007)

You should be good to go.


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## sonikaccord (Jun 15, 2008)

I forgot all about transistors as switches. I found that schematic somewhere too. I believe you can scale it too for more power...


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## 24th-Alchemist (Jun 16, 2011)

I think your circuit should work.

But, be careful with the current draw of your fans. Talk is cheap but my talk says there's no way you're gonna get 10A from a TIP120 without a massive heat sink -- and even then 10A is questionable. (The data sheet I looked at says max current is 8A pulse, not constant; see here.)

Another thing is that there are more efficient ways of making such a switch. Figure 2 in the link above shows the voltage drop from the collect to emitter for various currents (the Vbe(sat) line). A slightly different circuit that runs a transistor in what is called "saturation" results in less voltage drop (the Vce(sat) line), and hence less heat and more current capacity.

If your fans are low current draw (<250 mA?) you should be fine though (the amp remote turn ons should be very low current). Also, I'd put a 100 k resistor between the transistor base and ground to be sure the transistor is fully off when it's supposed to be.


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