using a transistor in place of a relay?

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95ta04goat

Member - Posts: 4

Joined: March 08, 2009

Location: New York, United States

Posted: March 08, 2009 at 9:00 PM / IP Logged

Hi. I'm new to this site. I've done basic wiring before but have run into a problem that at first glance seems a little more complex them I'm used to handling.

I would like to turn on an extra set of reverse lights only when in reverse (obviously) and only when the fog lights are on. I'd prefer to make this as simple and compact as possible. I've figured out how to wire it up using two relays but is it possible to do it with using a transistor and a relay? I've never wired in a transistor before so I'm a little iffy on how to or if it's the right application.

I'd prefer to draw something up but don't have anywhere to host it so I'll attempt to explain what I'm thinking will work.

Have a (+)trigger from the fog lights go to base of the transistor, 12v to the collector and the emitter go to the constant 12v(30 of a typical bosch relay) of the relay. Then a (+)trigger from the reverse lights to the switched power(85) of the relay.

Any thoughts? Will this work in theory?

loneranger

Copper - Posts: 572

Joined: December 28, 2008

Posted: March 08, 2009 at 9:20 PM / IP Logged

Your idea all depends on the type and current capacity, of the transistor. You will be better suited to to use the transistor to drive the ground side of the relay coil. Your circuit driver will be more complex than just a transistor. However, on the whole, yes it's capable. What exactly is "the switched power(85) of the relay"?

Ideal - cmon dude, add to topics in a useful manner, not stuff that is obvious.

Story - Phzzzt! Hey, what happened?! ... Isn't it obvious?

Moral - Never dismiss the obvious.

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: March 09, 2009 at 3:48 AM / IP Logged

Relays 85 ground side of coil

86 Pos side of coil

87 NO switch

87a NC switch

30 Common switch

That's the DIN ISO convention relating to ALL the pins on switches etc. Please use this format and we will know what you are getting at.

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: March 09, 2009 at 7:56 AM / IP Logged

What are you using for a fog light signal?

What are you using for the reverse light signal?

How much current do your "new" lights need?

Depending on how you answer these questions you might be able to get away with just one relay and no transistors.

If you are set on using the transistor, as others have mentioned, you will connect your 12vdc to the transistors base (through a 1000 ohm resistor), connect the emitter to ground, and then connect the collector to the (-) side of the relay. You will need a diode across the coil otherwise you will burn up the transistor over time.

Kevin Pierson

95ta04goat

Member - Posts: 4

Joined: March 08, 2009

Location: New York, United States

Posted: March 09, 2009 at 7:35 PM / IP Logged

I appreciate the quick and helpful responses guys. Looks like i got the numbers on the relay confused. Sometimes knowing just a little about something is a dangerous thing. using a transistor in place of a relay? -- posted image.

using a transistor in place of a relay? -- posted image.

Both the reverse and fog light signals are 12vdc and the new lights only need 10 amps. Is it possible to only use one relay with this application?

If i were to use the transistors would the diode go between the collector and the (-) side of the relay?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: March 09, 2009 at 9:06 PM / IP Logged

With 10A you should use a relay with the 30 pin connected directly to the battery - it wouldn't be safe to try and tap 10A from an existing circuit.

You can use the transistor connected to the fog lights to provide the (-) side of the coil and then use the +12vdc signal from the reverse light connected to the (+) side of the coil.

The diode would go between Pin 85 and Pin 86 (the coil) of the relay.

Kevin Pierson

95ta04goat

Member - Posts: 4

Joined: March 08, 2009

Location: New York, United States

Posted: March 09, 2009 at 10:03 PM / IP Logged

I just read the diode sticky. Now I understand why it's needed and what to do with it. So if I'm understanding correctly..

Transistor -

base - 12vdc signal from foglights with 1000 ohm resistor

collector - to ground

emitter - to (-) pin 85 of the relay

Relay -

diode from 85-86

85 - transistor emitter

86 - 12vdc signal from reverse lights

30 - fused 12vdc from battery

87 - to "new" lights (+)

Actually seems pretty easy to wire up now. I'll probably put it together this weekend.

i am an idiot

Platinum - Posts: 13,667

Joined: September 21, 2006

Location: Louisiana, United States

Posted: March 09, 2009 at 10:19 PM / IP Logged

Emitter to ground, collector to 85 of the relay. While I was drawing you posted. but here is a picture of what Kevin was suggesting. The second relay at bottom right is just to show you how the diode goes on the other relay. If the setup does not trip the relay every time you need it to come on, you may have to change the resistor to a 680 ohm or a 470 ohm resistor.

95ta04goat

Member - Posts: 4

Joined: March 08, 2009

Location: New York, United States

Posted: March 10, 2009 at 10:00 PM / IP Logged

Awesome, thanks for the picture. Time to make a trip to radioshack.

reactor

Member - Posts: 1

Joined: November 30, 2007

Location: California, United States

Posted: May 27, 2009 at 1:16 AM / IP Logged

Hi, i'm new at posting here but have been reading this forum for some time. I have a very related question may be someone can answer.

I need to provide positive power to the LED light (0.05Amp draw-very low current), but the control wire i have available is a ground signal. So i want use ground to switch the transistor On and Off. Can i use same wiring diagram posted above for wiring the transistor exept the ground will not be permanet but switched? Is it OK for the transistor to have constant power supplied to and will it work properly by having ground turned On and Off?

I'm planning to use same pn# as posted above since it will be running much lesser current than needed for relay coil.

Thanks.

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