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Simple transistor question

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forcedfx

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I'm looking for a particular type opf transistor for the following application.

I need to switch off a 12VDC circuit when another 12VDC circuit is turned on. When the transistor is conducting it needs to handle at least 3amps at 12VDC.

Can anyone offer any examples that I can use fot his?

Thanks
 

Can't you just do this with Diodes? Sort of like the battery backup application, where the battery circuit conducts when the voltage falls on the primary circuit.
 

jdhar said:
Can't you just do this with Diodes? Sort of like the battery backup application, where the battery circuit conducts when the voltage falls on the primary circuit.
Click to expand...

If that's possible I'm definately all ears since it sounds much more simple.

I was going to try using a relay that is normally open and then closes when you power it up, but haven't had much luck finding one locally.

How can I try using relays to do the job? Anyone know if Radioshack carries a relay that will do this job?
 

Why do you need to use a transistor?. Would not just a relay do. If the power supply that you want to switch off is 0.7v or less that the other supply, you can just put a diode in each + lead, cathode(stripe) towards the load. The higher voltage supply will reverse bias the other one.
 

GrandAlf said:
Why do you need to use a transistor?. Would not just a relay do. If the power supply that you want to switch off is 0.7v or less that the other supply, you can just put a diode in each + lead, cathode(stripe) towards the load. The higher voltage supply will reverse bias the other one.
Click to expand...

I'm not dead set on using a transistor at all. I know very little about electronics and the theory behind it. I'm slowly learning more and more.

Both voltages are identical to one another since they both feed of the same 12VDC supply, so I guess the diode trick is out for me.

I can't find a normally closed relay that can carry 3amps from Radio Shack... and if I can pick up a transistor that will do the trick I'd rather do that than wait on an order from Mouser.
 

You still better off with a relay. There are places where you can find 12V relays, 3A.

Using a transistor, you may also need a heatsink.
 

I dont think you will find a relay with just a n/c contact. You just need one with a changeover contact, then just use the normally closed pair.
 

There are some relays that are dual-functions. Thay have a normally open, a normally closed and a common contact.

Look for SPDT relays.
 

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