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Topic: Help Choosing Correct Transistor (Read 4955 times) previous topic - next topic


Thats what I was thinking... Though I was worried because my car for whatever reason stays up around 13.5V.

I was worried that going over the specified 12V might be a problem. What do ya think?



Do a google search for "transistor relay schematic" to see how to operate a 12v relay using a 5v control. Pretty simple. There are other recent threads here on using optical isolators to do the same.
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Why I like my 2005 Rio Yellow Honda S2000  https://www.youtube.com/watch?v=pWjMvrkUqX0

Ran Talbott

And as for having to cut the wires, you will have to unless you can find a "plug" connector where the light plugs into the power source.

Nah:  there are "tap" crimp-on connectors intended to connect an additional wire in parallel with an existing circuit.  They do insulation displacement,  just like computer ribbon cable connectors.  Since Patrick is adding a function,  and not intending to interrupt the existing connection,  there's no need to cut.

Ran Talbott

my car for whatever reason stays up around 13.5V

It's supposed to:  the nominal "12V" battery is actually supposed to charge up to about 12.6V (some details on wikipedia).  Car alternators run about 13.5-14V because you need a voltage higher than the battery's to charge it.  Older alternators,  without the benefit of solid-state charging controls,  will peak even higher as the RPMs go up.

Relays are designed to tolerate a range of voltages around their nominal rating.  Unless you're planning to have it on for hours at a time,  I wouldn't worry about even a relay not designed for "automotive" use suffering from the higher voltage.

Since you're in TX,  I'd suggest shopping at either BG Micro or Goldmine for surplus "automotive" relays:  you'll probably save enough from retail just on the relays to pay for the shipping.  Plus you'll find other goodies you'll want for future projects.


Thanks again for your help guys.

Quick question as to an alternative to a relay for this... Would it be possible to use a darlington pair such as the TIP-120 instead of the relay?

Here's a datasheet for one that I found.

If this works it would allow me to switch the lamps at a much faster rate than the relays with a less complicated circuit...

Got the idea from this page:

What do you guys think?



A big enough transistor will work, and you don't want a Darlington.  The problem is that there is so much noise in most automotive environments, that you need to be fairly careful about using a bare transistor.  I do think a relay is a better choice.  It's more forgiving.

If you use a relay DON'T FORGET THE DIODE on the coil.

You can use an MPS2222 to drive the relay, with a 1k resistor in series from the Arduino pin to the base of the transistor.  

Why are you trying to measure the current?  Just look up the wattage of the bulb and divide by 12 to get the current it needs.  Power in watts = Voltage in volts times current in amps.  That means amps = volts divided by watts.  Be conservative.


Absolutely agree with the comments above regarding using a automotive relay along with a socket.  If you relay want to go the whole hog - you're fuse box probably has spare fuse and relay positions intended for equipment not installed in your vehicle.  I believe this is know as a "stock" look  ;) lol


Thank you all for your input. I have pretty much settled on a transistor / relay set up as per all of your suggestions... However, for the purposes of simplicity, cost, and speed of switching:

Can I ask if there is any real danger in using a MOSFET like this one?


Just to review it would be taking the 5V signal from a duino pin at the gate and controlling the cars 12V 2.2A(ish) current through the lamp.


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