• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help with simple voltage/current sensing relay.

Status
Not open for further replies.

Sketch_hs

New Member
This is for an automotive application.

I've done an engine swap in a car and have finished everything but the Air Conditioning.

This particular vehicle uses a computer to control the AC. It's working and outputs a 4.6v signal on the AC control wire when you press the AC button on the climate control. The current on the signal wire is only about 4-5 mA.

The original engine computer looks for that signal and then raises the engine idle and grounds the relay for the AC compressors magnetic clutch. Simple.

However, the new engine computer is looking for a 12v signal!

I've tested this by supplying the ECU with a 12v on the signal wire (everything works)


So what I need is a relay that will sense the small (4.6v, 5mA) voltage/current and switch on a 12v signal to the engine computer.

I assume this would be some type of solid state relay, maybe inductive?

If someone could point me to a product or alternative solution I would be very grateful.

-Tyler
 

tunedwolf

Well-Known Member
A couple of transistors and a couple of resistors is all you should need to level shift your 5v to 12v, no need for relays. I would doubt that the management computer can deliver the power required to pull-in and hold anything but the smallest of relays, nor will the output likely be protected against the back emf generated by them. If you are determined to use a relay, perhaps look for a small telecoms reed type relay and make sure that you put a fast protection diode across the coil to control the back emf.

rgds
 

Sketch_hs

New Member
I just need a way to supply the wire to ECU with 12v at this wire when there is voltage from the AC computer.

:D
 

unclejed613

Well-Known Member
Most Helpful Member
actually, using a MOSFET to drive the relay will use the fewest components. you can also use a bipolar transistor to drive the relay. the diode can be either a 1N4148, or a 1N4004. L1 is the relay coil. if you use the 2N2222 , make sure the relay board isn't where it's subjected to high temperatures. the MOSFET can be used without a heatsink. but should be at least bolted to the board so it doesn't wobble with vibration and eventually break the leads
 

Attachments

Last edited:

Sketch_hs

New Member
I'm in over my head... Is that a part I would have to make or could purchase?

I'm just getting into electronics....

I need to solve this problem in a simple way... ideally by purchasing a relay/controller/whatever and just soldering some wires.

Thanks in advance
 

Willbe

New Member
Find a relay with a 5v, 1kΩ coil whose contacts can handle the 12v current necessary.
Try Hosfelt, Allelectronics, Digikey, Mouser, Jameco.
 
Last edited:

Sketch_hs

New Member
i found a 5v relay with 250Ω coil but it wont open, i was assuming that it's because of the tiny amperage (just measured at exactly 5.6 mA)
 

Willbe

New Member
i found a 5v relay with 250Ω coil but it wont open, i was assuming that it's because of the tiny amperage (just measured at exactly 5.6 mA)
5v/250Ω= 25 mA = too much current req'd.
5.6 mA x 250Ω= 1.4v= you said 4.6v was available.
???
 

Willbe

New Member
So a 5v 1000Ω coil relay will work with the 4.6v and 5.6 mA available?

<--- Electronics noob
It should.
4.6v/5.6mA = 820Ω, so 1000Ω is OK.
A 5v relay should pull in at about 2/3rds of 5v = 3.3v.
Check the websites for Hosfelt, etc.
 

unclejed613

Well-Known Member
Most Helpful Member
an SSR (solid state relay) might work. most of them are made to switch 120Vac, so you'll need to find one for low voltage DC, some of the 120V models might work, some might not, as they use zero crossing detectors to switch the mosfets on and off.
 

Sketch_hs

New Member
I can't find a 5v 1000Ω relay. If any one can, let me know!

The specs and ratings for the solid state relays confuse me honestly... can anyone help me?
 

Willbe

New Member
With an electromechanical relay you may need to debounce the contacts in this application.
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top