Help replacing relay with MOSFET or transistor

[gmorfeus]

Hi ppl!

I would like to change relay in this circuit with mosfet or transistor but don't know how?! Can anybody help me out? The current running on OUT_1 and OUT_2 is 200mA and voltage is 14V. Thanks!

 

[Hero999]

Please post the rest of the circuit showing what's connected to the relay.

From what you've already posted it looks like you can't replace the relay with a transistor because the circuit being switched by it is isolated.

 

[gmorfeus]

...

I don't know exactly while this is an project I am making by myself and it's for switching high and low headlight beam on my car by an touch switch. Now I managed to make the JK to change states with touch (and the original switch) but it seems that it doesn't give enough current (while voltage is OK). I think it is going to some sort of relay :-/ The information on current is based on measurement (cut the wire and measured), so the circuits don't need to be isolated.
I was trying to replace the relay with two nMOS BS170 (theoretically) but as I don't have 4 wires (two for low beam, and two for high beam), it's impossible (correct me if I am wrong) to make it that way. Also, OUT_1 must be high when low beam is active, and OUT_2 when high beam is active (the outputs are complementary) but that can be done directly with Q and /Q of JK.

So basically, what I need is to be able to switch 14V with current of 200mA with every change of JK state. Something like: OUT_1 goes to 0V and OUT_2 goes to 14V and vice versa driven by JK outputs...

 

[audioguru]

You need two Mosfets.
One has its gate connected to the Q output of the CD4027 flip-flop and the other Mosfet has its gate connected to the Q-Not output of the CD4027.
Then one Mosfet turns on the low beams and the other Mosfet turns on the high beams.

The lights probably have one wire grounded so you need P-channel Mosfets.

You might want to have a power-on-reset resistor and capacitor so that the low beams are always turned on first.

 

[gmorfeus]

Audioguru, thanks!

I secured the Q is initially on by putting RC at the SET pin of JK.

So the nMOS is impossible to use in this application? Ok, how to choose pMOS? (web page with list and specifications?!) Can anybody make me an sample circuit on how to connect Source, Drain and Gate? Gate shouldn't be directly connected to Q? I newer worked with MOSFET so I don't know much about them... :-/ Help, help, help!!!! If someone can provide me an diagram, I would be very happy...

 

[audioguru]

A P-channel Mosfet has its source connected to the positive supply voltage and its gate needs a low value resistor near it and in series with its gate to avoid very high frequency oscillations. Use 47 ohms. When the gate voltage goes to 0V then the Mosfet turns on.
The load is connected to its drain which goes High (+12V) when the Mosfet is turned on.

 

[gmorfeus]

So the the final schematic with mosfet should look like this in attachment?

 

[Hero999]

Remove the two 47R resistor going to 0V, they only burn up energy; so much so that your CMOS IC will overheat and won't switch properly.

 

[gmorfeus]

Ok, thanks!

Only one more question: can I use nMOS and put drains to +12V and sources to outputs? Or I MUST use pMOS?

 

[audioguru]

If you use N-channel Mosfets as source followers then the gate voltage must be 22V for the source voltage to be 12V. The CD4027 has an output voltage of only 12V so the source voltage of the N-channel Mosfet will be only 2V when it is turned on. Then most of the power will be spent heating the Mosfet instead of driving the load.

 

[gmorfeus]

Ok, thanks! All this information was very helpful to me.