# I need some help with a circuit in my car

Discussion in 'General Electronics Discussion' started by anbrsi, Apr 26, 2013.

1. ### anbrsi

2
0
Apr 26, 2013
Hello, everybody.

This is my first post here. =) I need some help...

I created a circuit for a specific bulb in my car to work in pre-determinated situations. However, for this circuit to work, I would need a component that would bar any current under 13V, but allow the current to flow when it's above 13V.

Is there any component with such characteristics?

The idea is to have a relay fed only when the voltage in my car is over 13V (in other words, when the engine is on and the alternator is charging the system).

Could you help me?

Thanks.

André

2. ### Solidus

349
4
Jun 19, 2011
You could use a 13V Zener diode feeding a relay.

Essentially, 13V is the breakdown voltage, so you connect the diode in reverse. When voltage is below 13V, it will not pass current, but at 13V or higher the diode will conduct in reverse and feed the relay.

Now, car voltages in the electrical system vary by load so it would be wise to have a regulator network or IC on your main circuit to make sure the lamp sees constant voltage.

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3. ### anbrsi

2
0
Apr 26, 2013
Thank you very much, Solidus.

And, what about the wattage of this Zener Diode? It's meant to let feed a 0,5w bulb.
A 1W 1N 4743 would be enough?

Regards,

André

4. ### Solidus

349
4
Jun 19, 2011
Well, the Zener diode would be feeding the relay coil, as it is simply allowing the voltage to cause the coil to turn on/off, so I would venture to say 1W would suffice.

If your relay comes with a datasheet you could look up the current draw of the actuator coil, and construct a resistor network to drop the voltage down from 13 to 12V and to only supply that rated current, but that's only to stay 110% safe.

I'd say most relays capable of supplying a good deal of power are capable of tolerating overvoltage fairly well. At any rate 13V won't blow the coil.

Are you going to be running the 0.5W lamp directly off the 12V positive rail? That is, is it a 12V, 40mA or so bulb? If that is the circuit in its entirety then simply connect that lamp between the "Circuit Trigger" line (coming out of the relay) and ground.

Also, I neglected to think of this earlier, but I'm assuming your relay is NO, that is, normally open (disconnected)?

5. ### Rleo6965

585
9
Jan 22, 2012
You can try this circuit with a transistor to drive a relay.

I use 12V zener diode. When dc supply was 12V. NPN transistor will not conduct and relay will not be activated. That is. npn base was at 0V.

When Battery voltage increase to 13V. a 1V will be applied at the junction of resistor 330 ohm. This will forward bias npn transistor and relay will be activated.

Ooops. I forgot to place wire connection on relay switch contact. Put it in series with your load or bulb. Select relay contact current ( A ) that can safely drive your load.

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