Simple Fan Power Circuit Needed (relay?)

[mjrgroup]

Greetings, this is my first post here so I'm hoping to both get some help and create a nice thread for others to see how this is done.

Problem:

I have a car audio amplifier which has a 12V 0.20A fan connected to the amplifier via a dedicated power channel specifically for the fan. On the PCB, the fan power connector is labelled "10/0.5W" and the fan itself draws roughly 0.20A. If you divide 10 by 0.5W, you get 0.20A so the current fan seems perfectly mated with the provisions of power on the board.

I want to put a significantly more powerful fan into the unit and connect it to the same header, without blowing it. The fan I want to use draws 0.32A @ 12V. I don't want to connect it to the board because it might fry something if it isn't rated for a higher-draw fan.

I want to build a device to detect when the 12V power is coming out of the fan power connector on the PCB and then feed power to the fan from the main car battery. This device would thus be connected to 3 things (2 power inputs, 1 for the low 0.20A 12V feed from the amp's board, and one 12V main feed from the car battery; the device would have 1 output for the new higher-draw fan).

How can I build this and is a relay part of the equation?

Thanks in advance.

Here's a concept drawing:

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BTW, the car audio amplifier is an Audiobahn A8000T and here are some pics of the internals to visually see what it looks like.

http://www.db-r.com/Extras/index4.html

 

[Resqueline]

You could use a relay, but a just as good (& smaller & cheaper) way is to use an NPN transistor. Any medium power type will do, but choose a TO-220 size to be sure.
(That's the size next to the fan connector.) Just connect the old fan output to the base, the new fan to emitter, and +12V to collector.
You'll loose 0.6V on the output relative to the input but it'll draw nex to nothing from the old fan connection so the voltage there may rise a little, compensating for the loss.

 

[mjrgroup]

You could use a relay, but a just as good (& smaller & cheaper) way is to use an NPN transistor. Any medium power type will do, but choose a TO-220 size to be sure.
(That's the size next to the fan connector.) Just connect the old fan output to the base, the new fan to emitter, and +12V to collector.
You'll loose 0.6V on the output relative to the input but it'll draw nex to nothing from the old fan connection so the voltage there may rise a little, compensating for the loss.

Thanks for the response. And I would just leave the negative fan lead connected to the output on the connector or should I ground it to the car's ground (chassis)?

Is this the transistor sufficient? http://www.newark.com/jsp/search/productdetail.jsp?SKU=38C8658&CMP=AFC-GB100000001

 

[Resqueline]

Yes, that'll be just fine. Make sure the metal tab doesn't touch ground btw. since it'll be at +12V. I see Newark claims it's a Darlington btw., but it's not, it's the ordinary kind.
I really can't tell for sure about the negative. If you Ohm (or trace) it to be connected to amplifier ground then you can connect it there, or to car ground if you're unsure.