5v relay wiring help!

**pRoFiT** · 11-29-2007, 12:09 AM

Originally Posted by JimmyFitz

Thanks for pitching in and helping with suggestions guys. I agree that a circuit is a better solution but I don't know if the OP can deal with it. That transistor driver needs for there to be a protection diode clamper across the relay coil or it will probably only turn on and off one time. I suspect that a 3V electromechanical relay will not work any better than the 5V one did without some extra help.

CRAP, your correct. a diode across the coil will keep the transistor from blowing up and possibly your control circuit.

Code:

        Diode
  5+ ----|<------ GND
     |           |
     |/\/\/\//\/\|
      Relay Coil

Code block to help make it look better. forum removes empty spaces

**aalouie** · 11-29-2007, 03:45 AM

The option did come across my mind of using a transistor, i had never set one up before so i opted not to go that route. the 3.54 volts is from the output of the keyfob sensor without load. The manual just says its a 5v signal. Hopefully this relay i ordered will work if not ill try the transistor method. Thanks

**pRoFiT** · 11-29-2007, 11:43 PM

One thing to be carefull of is the current draw on that keyfob output. Just because it says 5 volts or 3.54, doesn't mean it will be able to control a relay. You may end up burning out the keyfob by pulling too much current.

The output is probably made more for a digital signal to a processor that can handle turning on or off a relay.

Current draw from the relay can be figured out by (voltage * coil resistance) then take that number and check the manual of the keyfob for ouput current on the (5V or 3.45V) output.

a small circuit board from radioshack, resistor, 7805, npn transistor and your relay. do a

keywords: transistor, relay, schematic, (maybe PIC or LPT Port examples)

Good luck.

**aalouie** · 12-03-2007, 02:34 AM

Ok i just picked up the stuff i needed. I got eveything soldered up i just want to double check to make sure i got this right before i power it up.

Im thinking i might need something else. The transistor i have is 800ma on the collector is this ok to switch a 50 or 60amp relay for the starter

**JimmyFitz** · 12-03-2007, 09:03 AM

Originally Posted by aalouie

Ok i just picked up the stuff i needed. I got eveything soldered up i just want to double check to make sure i got this right before i power it up.
Im thinking i might need something else. The transistor i have is 800ma on the collector is this ok to switch a 50 or 60amp relay for the starter

Your circuit is wrong. Use the circuit that Profit showed with a diode added. Yours has the relay coming off the emitter to ground. The relay should be connected between +12V and the collector with a diode in parallel. The diode should be connected with the anode to the collector and cathode to +12V. Be sure that your transistor is NPN.

**aalouie** · 12-03-2007, 08:37 PM

Ok cool finaly figured it out with a little help from a co worker that did electronics in the military, Its so simple now, 1 last question what type of diode should i use. will a 1N4003 Micro 1-Amp Diode work?

**JimmyFitz** · 12-03-2007, 09:33 PM

Originally Posted by aalouie

Ok cool finaly figured it out with a little help from a co worker that did electronics in the military, Its so simple now, 1 last question what type of diode should i use. will a 1N4003 Micro 1-Amp Diode work?

That diode will work fine.

**TheGemInIMan** · 12-04-2007, 08:55 PM

Originally Posted by JimmyFitz

Your circuit is wrong. Use the circuit that Profit showed with a diode added. Yours has the relay coming off the emitter to ground. The relay should be connected between +12V and the collector with a diode in parallel. The diode should be connected with the anode to the collector and cathode to +12V. Be sure that your transistor is NPN.

His circuit WILL work fine if it is a 12v relay and he adds the diode across the relay coil ... a 1Amp 1N4001 will be fine ... or the IN4003 ... just means is can handle more voltage that's all but that doesn't matter here ...

you can wire transistors any way round ... common collector or common emitter modes ... the voltage and current flows the same ...

I would personally use a 1k resistor on the base to limit the base current ... the transistor will saturate fine and pull the relay in ... it will also limit the current available from the FOB ... you could probably get away with a 3k3 resistor actually ... rule of thumb: 10v supply would need a 10k base resistor ... 5v = 4k7, etc, etc ... for most of the common switching transistors ...

The 800mA Ic current value is fine to switch the relay ... it's not the switching current rating of the relay itself, it's the coil resistance that determines the Ic (Collector current) value of the transistor ...

EDIT: The right hand setup will not work as discussed ...

cheers,
Gem

**Rob Withey** · 12-05-2007, 01:19 AM

Originally Posted by TheGemInIMan

His circuit WILL work fine if it is a 12v relay and he adds the diode across the relay coil ... a 1Amp 1N4001 will be fine ... or the IN4003 ... just means is can handle more voltage that's all but that doesn't matter here ...

It will NOT work. With an emitter follower wired with the emitter to the relay, the highest the emitter can go is about one diode drop below the base. With a max base voltage of 3.5V you can expect approx 3V max at the emitter. Go back and read your books about how bipolar transistors work...!

**aalouie** · 12-05-2007, 06:15 AM

Ok i think he is right, with the 3.5v even with out a resistor the transistor is only giving out 7.90v. Whats the minimum voltage needed to switch on the base. I tried it with a 5v and it worked. Maybe theres another transistor i could try?