DC to DC circuit for automotive relay - 5v to 12v

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Discussion Overview

The discussion revolves around the design and implementation of a DC to DC circuit for activating a 12V automotive relay using a 5V power source from an oil pressure switch. Participants explore various methods to achieve this, including the use of transistors and relays, while addressing challenges related to component specifications and circuit design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using a 5V relay to control the 12V relay, questioning the coil current required for the 12V relay.
  • Another participant proposes measuring the relay's coil current by applying 12V directly and observing the current draw.
  • A participant shares results from a bench test indicating the relay switches at 7.65V and 1.65A, and inquires about using a transistor circuit to amplify the 5V signal.
  • Suggestions are made regarding the use of a transistor to drive the relay, with discussions on necessary components like resistors and diodes for protection.
  • One participant raises the idea of using a MOSFET instead of a relay for switching, while another participant expresses confusion about how to implement this.
  • There is a discussion about finding a suitable latching relay for a motorcycle application, with references to existing wiring and relay types.

Areas of Agreement / Disagreement

Participants express various approaches to the problem, with no consensus on the best method to implement the circuit. There are differing opinions on the use of relays versus transistors or MOSFETs, and the specifics of component selection remain unresolved.

Contextual Notes

Participants mention the need for datasheets and specifications for the relay, indicating limitations in available information. The discussion includes assumptions about component behavior and circuit design that may not be fully validated.

Who May Find This Useful

Individuals interested in automotive electronics, circuit design, and those seeking to understand relay and transistor applications in low-voltage systems may find this discussion beneficial.

LUI-G
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Greetings E.E. Gurus!

I'm a noobie in need of your guidance!

Here's the situation:

I have a 5V DC, 0.25 amp (1.25W ?) power source from an oil pressure switch wire on an old truck that is activated only when the engine is running.

My goal is to activate a 12V DC, 2.5 amp, 30/40W relay, which will be activated continuously while the engine is running.

I have yet to take a course in electronics, and will do so in the future... but right now I have this dilemma...

I've done a bunch of reading online but have not had any luck finding the info I need to build this simple circuit...

Thank you all for your replies!
 
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LUI-G said:
Greetings E.E. Gurus!

I'm a noobie in need of your guidance!

Here's the situation:

I have a 5V DC, 0.25 amp (1.25W ?) power source from an oil pressure switch wire on an old truck that is activated only when the engine is running.

My goal is to activate a 12V DC, 2.5 amp, 30/40W relay, which will be activated continuously while the engine is running.

I have yet to take a course in electronics, and will do so in the future... but right now I have this dilemma...

I've done a bunch of reading online but have not had any luck finding the info I need to build this simple circuit...

Thank you all for your replies!

Welcome to the PF.

It sounds like you can use your 5V signal to power a 5V relay, and the output of the 5V relay can drive the 12V relay. What is the required coil current for the 12V relay? The 2.5A number is the allowed output current, I would think, and not the coil current.
 
Hmmmm... I'm not sure... How can I find out? I have the relay...
 
LUI-G said:
Hmmmm... I'm not sure... How can I find out? I have the relay...

Does it have a manufacturer name and part number? You can use that to find the datasheet on the Internet usually...
 
Here's a pic. thanks for your help!
 

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I'll do some research on that and post it ASAP... thanks!
 
Hmm. My googling is not showing APO as a relay brand so far...

Are there any other markings? Where did you get the relay?
 
I found a similar one on ebay. I messaged the seller to see if he has a data sheet...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370478844324
 
so according to the seller, there's no data sheet for the relay...

How can I test the relay to see what the minimum current to activate the coil would be? Would a variable resistor or something, help?
 
  • #10
LUI-G said:
so according to the seller, there's no data sheet for the relay...

How can I test the relay to see what the minimum current to activate the coil would be? Would a variable resistor or something, help?

You can just put 12V across the coil and measure the current.
 
  • #11
Berkeman, I appreciate you replying and helping me out!

So I ran a bench test with a variable AC/DC power supply and found that the relay did not switch at 5.75V, 1.7A, but DID switch ON at 7.65V, 1.65A

Couldn't a simple transistor circuit be made to amplify 5V, 0.25A ?

I found a transistor circuit example here, but I can't follow it too well... It doesn't allow me to adjust the base current...

http://www.bcae1.com/transres.htmIf so, what exact components would I need?

I read somewhere I should start with a diode to prevent any current leaking back to my power source and then use a resistor in series with the base, and add resistors to the emitter and collector, but I don't know what sizes, etc.

Also, a diode should be attached across the relay coil to prevent any voltage spikes when it's turned off, but I don't know what size...
 
  • #12
Yes, you should be able to use the 5V source to drive a transistor. Connect one side of the relay coil to +12 and the other coil terminal to the collector of the transistor. Put the emitter of the transistor to ground.

The sensor's 5V ouput would go to the base of the transistor through an appropriate resistor. Don't forget a flyback diode across the relay coil to protect the transistor.
 
Last edited:
  • #13
mdjensen22 said:
Yes, you should be able to use the 5V source to drive a transistor. Connect one side of the relay coil to +12 and the other coil terminal to the collector of the transistor. Put the emitter of the transistor to ground.

The sensor's 5V ouput would go to the base of the transistor through an appropriate resistor. Don't forget a flyback diode across the relay coil to protect the transistor.

Awesome! Thanks mdjensen22! Can you tell me which transistor and diode to use?
 
  • #14
The transistor is dependent upon the current of the relay coil for energization. You need to measure this or find a datasheet for the relay. I assume something like a 2N3415 should work.

For the diode, I typically use a 1N4004, or a 1N4148 depending on the design.

Your base resistor will probably be something like a 1K or less (470 maybe?). This is calculated based on the collector current (relay coil current) and the current gain of the transistor that is selected (Hfe).
 
  • #15
berkman; similar question. I have a set of auxillary lights on my motorcycle, I'm attempting to turn them on using the flash to pass switch. I have a relay and existing wiring in the head lamp housing, but the relay is momentary. Is there a switch I could put inline, or another type relay, that would stay on, then turn off with the next flash to pass input? Just joined the site while searching for a switch. Thank you in advance.
 
  • #16
Why not dump the 12V relay completely and just replace it with a suitable MOSFET? You can then switch the MOSEFT with the 5V supply connected directly onto the FET's gate.
 
  • #17
cmb, thank you for the reply. However, I don't have a 5v, it's 12v. I'm trying to use the flash to pass, which sends a 12v signal, to turn on the auxillary lights on my motorcycle. A latching relay would work if I can find one. I googled a moseft but not tech savy enough to understand how this would work.
 
  • #18
Are you trying to wire into your flasher relays?
Those typically are not relays in the conventional sense (coil and contacts) - flasher relays are typically just bimetallic contacts that heat when the current passes through them and then they open until they cool at which point the cycle repeats itself.
http://auto.howstuffworks.com/turn-signal2.htm
 
  • #19
mdjenson22; no, fortunately a little more savy than that, but not by much;o) My bike also has an empty 12v, ground, and plug for a standard three prong headlamp for the U.K. It also has a flash to pass function. I can use the empty 12v and ground to drive the lights through a relay, and the flash to pass function from the high beam side of the empty UK socket, obviously a standard relay would only power the lights when powered, but a latching relay will stay in the last keyed position, that's what I'm trying to find.
I found one on line, but after reading the review it stated that it was infact only a standard relay.
 
  • #20
search Digikey or Farnell - both sell latching relays.
 

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