using a transistor in place of a relay?
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Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=112216
Printed Date: April 28, 2024 at 3:00 PM
Topic: using a transistor in place of a relay?
Posted By: 95ta04goat
Subject: using a transistor in place of a relay?
Date Posted: March 08, 2009 at 9:00 PM
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?
Replies:
Posted By: loneranger
Date Posted: March 08, 2009 at 9:20 PM
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"?
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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.
Posted By: howie ll
Date Posted: March 09, 2009 at 3:48 AM
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.
Posted By: KPierson
Date Posted: March 09, 2009 at 7:56 AM
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
Posted By: 95ta04goat
Date Posted: March 09, 2009 at 7:35 PM
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. 
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?
Posted By: KPierson
Date Posted: March 09, 2009 at 9:06 PM
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
Posted By: 95ta04goat
Date Posted: March 09, 2009 at 10:03 PM
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.
Posted By: i am an idiot
Date Posted: March 09, 2009 at 10:19 PM
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.
Posted By: 95ta04goat
Date Posted: March 10, 2009 at 10:00 PM
Awesome, thanks for the picture. Time to make a trip to radioshack.
Posted By: reactor
Date Posted: May 27, 2009 at 1:16 AM
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.
Posted By: KPierson
Date Posted: May 27, 2009 at 7:06 AM
reactor wrote:
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.
You really should start a new thread for this - but the simple answer is why can't you just reverse the connections on the LED? That would be the easiest way I would think. If, for some reason, that isn't possible then a transistor would work. ------------- Kevin Pierson
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