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ThSpeakerDude88 20th January 2007 01:32 AM

relay coil on ESP's project- bd139 equiv?
in Rod's smps supply for a car amp, the head remote turn on relay is driven by a bd139 transistor. I have an amp that I built, and would like to use the remote turn on from my head unit.

I was just planning on using the same thing he used in his power supply, the bd139 w/ 1k resistor driving the 12v relay coil.

what is the equivalent? will just any transistor work, and does it even have to be driven by a transistor or is the head capable of running the coil?

ThSpeakerDude88 22nd January 2007 05:38 AM

eh, anyone? I have searched to no avail on google...:(

ssanmor 22nd January 2007 05:47 AM

It simply depends on the relay. Usually you will need a small-mid power transistor.
Any of these should work for you:
BD137, BD139, BD241, etc, etc, etc. Just measure the resistance of the coil of your relay and tell us what similar transistors you have for checking.

BTW: there is another way to turn the amp. on/off: there is usually a pin in the controller IC that shuts it down (SG3525, etc). You can use that pin instead of a relay cutting the supply of the IC. Just be sure of its polarity (the head unit provides 12V when turned on, and the shutdown pin of some chips is inverted (they require it to be at 0V for operation), so you would need a small transistor (BD547, 2n2222, etc) for inversion of polarity.

ThSpeakerDude88 22nd January 2007 05:35 PM

I'm not using the sg3525, I don't have a smps amp yet. I just have a small TDA7375 amplifier good for 15w/channel rms.

I need it to turn on by my head unit's remote out. I have a bosch style automotive relay from radio shack. Coil rating of 12VDC, 160mA, 400ohms are whats listed on the 'shacks site.

I've got... a couple of 2sc1846's, 2sc3619,d2012 ( 2sd?),B1185 ( again , 2sb?), tip50, 2sc3866, and a couple of 2n3055s. I do have a box of mosfets too.

ssanmor 22nd January 2007 06:32 PM

If your relay is 400ohm then it doesn't draw 160mA, but 30mA :)

Anyway, TIP50 is more than enough.

ThSpeakerDude88 22nd January 2007 09:33 PM

if its 30 ma..... woudn't a 3904 work?

AKN 22nd January 2007 09:48 PM


Originally posted by ThSpeakerDude88
if its 30 ma..... woudn't a 3904 work?
Check the current with coil connected to 12V or measure resistance since specs seems to be contradicting, just in case to be shure.

ThSpeakerDude88 23rd January 2007 01:12 AM

coil R is 70 ohms. , current is 185ma.

ThSpeakerDude88 23rd January 2007 01:24 AM

70 ohms, 180 ma max, my "12v" radioshack dc power supply is putting out 18v right now... so I switched it to 9v and got 13.4v, and a lowered reading of 140ma.

I also went through my parts box and found a 2sd974, came from a video camera, has a collector current of 1 amp.

ThSpeakerDude88 23rd January 2007 03:24 PM

2sd974 heats up and stays on, I think its shorted. I used a 2sc1846 and it seems to work just fine. Thanks! :)

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