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Usb trigger for 12v TV backlight

carpenter940

Mar 27, 2013
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I have a strip of 12 volt LED lights that I'd like to use for a TV backlight. I'd like the LEDs to come on/off when the TV is powered on/off. I'm using the USB on my TV as my trigger.
Will someone take a look at it to see if it's OK to use on TV w/o destroying TV?

The parts I will use are:
USB cable connected to 5v USB out on TV
R153S Diode (the R153S was the only indicator of it's value)
1/4W 0.25W 5% 220 R OHM Carbon Film Resistor
IRF520 Power MOSFET N-Channel TO-220 GM
Led Strip 3528 SMD White lamp DC 12V
12v source for LEDs

Below you will find a link to the schematic.
http://tinyurl.com/y84rxw35

Thanks for any help.
NOTE: I'm not very good at electrical circuits.
 

Externet

Aug 24, 2009
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I do not see anything wrong with it.
You could choose a relay if makes you sleep better.
 

BobK

Jan 5, 2010
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IRF520 requires 10V to turn on fully. It might work at 5V, but typically cannot pass more than 2A at that gate voltage, and will be dropping between 1 and 2V at that current. You need a logic level MOSFET like the IRLZ44.

Bob
 

carpenter940

Mar 27, 2013
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Thank for the replies. Now that I went back and looked at the data sheet for the IRF520 I see it DOES require 10V, but you wrote it may work at 5v too. I decided to take a chance since I already had a IRF520 and constructed the project and it seems to be working fine plugged into a 5v phone charger. I'll leave it plugged in for awhile to make sure it doesn't melt before plugging it into my TV.
 

Harald Kapp

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Nov 17, 2011
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but you wrote it may work at 5v too. I decided to take a chance
If it works, it may not be reliable. It may be seemingly on, but have a comparatively high drain-source resistance which will heat it up, shortening its lifetime considerably. I really recommend you follow Bob's advice.
 

BobK

Jan 5, 2010
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I would expect the MOSFET to get quite warm if not hot with 5V on the gate. How much current is the strip using?

Bob
 

carpenter940

Mar 27, 2013
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The strip had 75 LEDs on it so about 1.5 amps. Actually I plugged it into a phone charger as a test for over 2 hours and the MOSFET was still room temperature, didn't feel warm at all, kind of a surprise.
 
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