Can I Use 'Radio Illuminate' As A 12v Source?

**we87** · 07-16-2008, 02:11 AM

I have a 2002 Trailblazer LS and I am working on putting a powered USB hub in my dash. I got everything USB wired and good to go, but now Im stuck on the power.

I got a 7805 5volt regulator to take the 12volts from the car and regulate it to 5volts for the hub, but I need a 12v source. In the radio harness I have a wire for RADIO ILLUMINATE|brown/white (dimmer)|+ |radio. I just checked with a volt meter and this is only 12v when the car is on.

My question is: Can I use this as a 12v source for my USB hub? Or would this be best to be left alone and find another source.

Sorry for the dumb question, thanks in advance for your help.

**skuller** · 07-16-2008, 02:19 AM

It might work, but not recommended.
You don't know how many amps the circuit can handle.
Get a relay, use that radio illumination input as the coil and run a constant 12v wire from battery for the switching.

I wouldn't use LM78xx voltage regulator.
The excess voltage is converted to heat, very inefficient.

**we87** · 07-16-2008, 02:25 AM

Originally Posted by skuller

I wouldn't use LM78xx voltage regulator.
The excess voltage is converted to heat, very inefficient.

Thanks for the reply.

What would you recommend in place of the 7805?

I would like to avoid using a relay if possible. I would perfer a straight switched 12v source. But if I need a relay, then I need a relay....

**skuller** · 07-16-2008, 02:34 AM

lm2575 is a good option
83% efficiency, compared to 40% for 7805

There's an exception though, if your device draw very small power from the voltage regulator then 7805 is fine.
less than half watt or so.

**we87** · 07-16-2008, 03:49 AM

Well at the moment is only powering the USB going to the Lilliput Screen and a DeLorme Earthmate GPS. Eventually I'd like to put a USB DVD Drive in the front, and that would be it. Im not even sure I need the extra voltage, but I figured it is always better safe than sorry...