polarity sensing relay

Other Hardware General Electronics
1

I understand that DPDT relay can reverse polarity... but that's not what I'm looking for.

I'd like the relay to respond/open only if it see's the correct polarity.

The use case is to accomplish this (if you're interested):

Currently:
Car window switch (on old antique car) sends power to window motor. The power polarity may be either direction depending on raising or lowering the window.

The Problem:
Due to switches, safety checks, fuses, etc, the power once it gets to the window motor is reduced to the point that the motor moves slowly and doesn't have full voltage/amps.

The Potential solution:
Send fresh wires directly from the battery power to near the window and terminate at a relay (then extend through the relay to the window motor). Use the reduced power (with its corresponding polarity) to signal the relay. There should be two relays for each polarity (one to each motor power side, to turn on the appropriate power direction).

So the power red wire is received on two different relays, one relay opens when it sees a positive signal, the the other relay only opens when it sees negative. Depending on what polarity the red wire comes with, only one of the two relays that Red is hooked up to will open (allowing the corresponding battery power polarity through the relay to the window motor) . There would be a corresponding setup on the Black wire side.

The relay would be operated with 12v signal (the original power source), and it would be sending through the new direct from battery power 12v of power.

Other Potential Solution:
I believe I could use a polarity sensor with a pi/uno then process that input to create an output to signal the corresponding relay to open/close based on that input, however that would require the additional pi/uno board and processing. I'd imagine that there is a relay out there that only works with the correct polarity (I see the large/expensive ones for industrial control environments, but I've not seen something for the Arduino world)

(I've drawn this up at home, when I get there I'll add the pictures to help explain what I'm thinking)

Thanks for taking time to read.

I appreciate any help or ideas!!

2

But most of the times both wires are just grounded when the motor is off. Aka, you simply have or wireA high or wireB high. Now just connect them to two relais.

The COM of the two relais to the two sides of the motor.
NC to GND
NO to battery positive

Be sure not to forget a fuse!

3

Ok, thanks!... took me a second to wrap my head around it... but I get what you're saying. the side that is NO when goes to closed with the Positive feed, it will power that correct side giving the motor spin the correct direction...

While that did help me remove two relays (Thank you very kindly!)

I'm stuck at the initial question: Are there Relay's that sense polarity?

4

Are there Relay's that sense polarity?

Not sure what you want.

You can put a diode in series with the relay's coil.

If the correct polarity is attached, the diode will allow the coil current to flow.

If the incorrect polarity is attached, the diode will disallow the coil current from flowing.

5

Fix the original wiring. A connection that is slowly falling apart might finish the process the day after you install your relays.

It might finish in a way that shorts to chassis and starts a fire.

6

MorganS:
Fix the original wiring. ...
It might finish in a way that shorts to chassis and starts a fire.

I appreciate the concern... but this is a known issue... and the approach doesn't have a threat of a fire, (which is why there are fuses to protect the circuit, both the signal and the new power source)

There are other designs to over come this issue but they require additional wiring (to accommodate the different relays to allow it to work with the different parts....

also rewiring the entire car with new switches, fuses, etc, would not be economically viable, and would take away from the OEM look of the car..

7

larryd:
Not sure what you want.

You can put a diode in series with the relay's coil.

If the correct polarity is attached, the diode will allow the coil current to flow.

If the incorrect polarity is attached, the diode will disallow the coil current from flowing.

Awesome!!!!
I think this is the answer. (I really appreciate your help!)
I'm going to do some looking for a diode that can handle the signal current of 12v. (let me know if you recommend one)

8

1N4001 thru 1N4007

9

But you don't need to if you have a GND :wink: One side of the relay to GND, other to one of the control lines / original motor lines.

10

Cyberlink47:
I appreciate the concern... but this is a known issue... and the approach doesn't have a threat of a fire, (which is why there are fuses to protect the circuit, both the signal and the new power source)

There are other designs to over come this issue but they require additional wiring (to accommodate the different relays to allow it to work with the different parts....

also rewiring the entire car with new switches, fuses, etc, would not be economically viable, and would take away from the OEM look of the car..

You can get antique-look wire to recreate any period. If looks is a real problem you can probably fit newer compact electronics inside the original cases.

What car is this?

Poor connections are poor connections. Just wire-brushing the fuses snd fuseholders might fix it.

11

I’d fix the original wiring too- it must have worked at some time - get your multimeter out and find where the voltage drops are and fix them -
Also check the mechanism is free and the motor in good condition ( brushes , bearings).
Corroded earth points is a common problem .

Circumventing the current issue is a bit of a bodge IMO.