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Quick help needed with simple circuit

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Odysseas

Member
Hello everybody,

I need some help with a very simple circuit, I think it should work but I don't feel that self-confident to really say it does work :)

The circuit has the task of controlling a relay when 12V are applied (Remote-output from a car radio, that apparently can't handle all the load applied to the wire) through a transistor, so that the radio only has to output about 1mA. Please tell me if this circuit works and if not, what's wrong with it :)

Thanks a lot!!
 

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audioguru

Well-Known Member
Most Helpful Member
How much current does the relay coil need?
The max allowed current for a 2N3904 is 200mA but it does poorly above 100mA.

With only 1mA of base current then it saturates well with a load current of only 20mA.

Does the input voltage drop low enough to turn off the 2N3904?
 

Odysseas

Member
The relay coil has a resistance of 180 Ohms, so it'll need about 66mA to turn on. Should I reduce the base resistor value a bit?

The input voltage should be either 12V or 0V.
 

RadioRon

Well-Known Member
Odysseas said:
Hello everybody,

I need some help with a very simple circuit, I think it should work but I don't feel that self-confident to really say it does work :)

The circuit has the task of controlling a relay when 12V are applied (Remote-output from a car radio, that apparently can't handle all the load applied to the wire) through a transistor, so that the radio only has to output about 1mA. Please tell me if this circuit works and if not, what's wrong with it :)

Thanks a lot!!

The circuit topology is OK, but whether this will work well or not depends on the resistance of the relay coil. The problem that might occur is that the 2N3904 has a DC current gain when passing upwards of 50 mA of only about 50 or so. With a drive current into the base of maximum 1 mA, you can only get it to saturate if the relay coil is 270 ohms or higher. I would suggest some extra margin is needed on that because the transistor will heat up somewhat, losing gain in the process, so perhaps 500 ohms is a reasonable minimum.

You have other options. For example, an N channel MosFET could be chosen that requires effectively no current to drive it.

EDIT: looks like I took too long to type my answer, as the one above is fine too.
 

RadioRon

Well-Known Member
Odysseas said:
The relay coil has a resistance of 180 Ohms, so it'll need about 66mA to turn on. Should I reduce the base resistor value a bit?

The input voltage should be either 12V or 0V.

For 66 mA, I would choose to drive the transistor with about 3 mA. This will keep the transistor a bit cooler too by saturating it for minimum Vce.
 

Odysseas

Member
Okay, then I'll use a 4k resistor as the base resistor. I'm afraid I can't use a MOSFET since I don't have one at home, and I need to rush to finish that circuit :) Thanks a lot!!
 

Odysseas

Member
Circuit works now, thanks a lot everyone!
 

Hero999

Banned
How about a darlington pair like the MPSA43?

Are you sure that you don't have a small signal MOSFET like the 2N7000 lying around in your junkbox?
 

Odysseas

Member
Yes, my junkbox is rather small, so I don't think I need to start looking for one. I'm sure my electronics supplier or a store a few miles away does them but a few milliamps of current drain at the transistor isn't important anyway :)
 
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