Electrical relays help?

[berni109]

Let me try and explain what I want to do.
I want to power my reversing camera and reversing sensors from the hab battery via a switch on the dash powered from the cab battery which is also used to trigger the head unit to look at the camera video feed.
I read that both sides of the relay are isolated so the power from the dash switch to energise the coil in the relay is totally seperate from the power switching of the load (camera & r/s)???
How much power (how many amps) does it take to energise the coil in the relay ???
Since the hab and cab batteries have a common "earth" -12v it makes no difference where the negative connection is made ???

Would it be better to use a solid state relay (less trigger amps) ???
Suggestions for a SS Relay that is DC switching and DC load ???

Thanks in anticipation.
Bern

 

[Itsmcb]

Clever idea but since you would need to switch off at the dash to turn everything off after reversing, why not just power the camera and sensors directly from the dash switch, there's very little load

 

[xsparks]

Miliamps.

 

[pappajohn]

How much power (how many amps) does it take to energise the coil in the relay ???

It's in milliamps.....next to nothing.

 

[Lenny HB]

How much current the relay takes will depend on the relay coil, as you are only switching power to the camera you will only need a small relay with 1 or 2 amp contacts. Use one from the electronics industry rather than an automotive one. You will be able to get one that only takes 30 ma or less.

I assume you want to run them off the hab battery so you can have them on when the ignition is off so why involve the cab battery?

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[berni109]

The radio / head unit is independently powered from the cab battery and that switch is adjacent to the switch for the camera trigger.
Radio on at any time irrespective of ignition state and if I want to have a look who's out the back I can at the flick of the switch.

Thanks for the replys no need to worry then about cable sizing for milliamps and a 2 amp fuse should cover it.

 

[Lenny HB]

It doesn't matter where the camera trigger to unit comes from it just need to see 12v relative to ground.
I have 2 rear cameras, a service camera & a front camera all ignition powered with the signal from the same source but the head unit is powered from the hab battery.

 

[berni109]

Lenny HB I understand what you are saying about trigger but most of the wiring already in place and I dont want to be changing it now when I can run a single thin wire off to the relay down the back.

You also said "Use one from the electronics industry". Just been checking Amazon and Ebay every one i have found seems to need a stabilised 12v and a momentary trigger rather than permanent switch 12v. Any pointers?

I know you are on hols just at the moment but if you have time after your siesta or when you get bored eating out or sight seeing let me know.
Thanks.

 

[Lenny HB]

This is a standard relay, coil power is 200mw which is about 17ma, if you want a really low power one search for high sensitivity relays.
Amazon product ASIN B09JGPX76Y
This is a higher power one and will draw more current but easier to wire.
Amazon product ASIN B09RWL4ZFV
If you want to switch video signals best to use telecoms relays with gold contacts.

 

[autorouter]

I just looked at a couple of relays at random, and a typical 30A automotive relay has a coil resistance of about 90 ohms, so the current through it would be 12/90 = 0.133A = 133mA. A smaller 2A relay has a coil resistance of 400 ohms, so its coil current would be 12/400 = 0.03A = 30mA.

You just want a simple on/off (SPST) or changeover (SPDT) relay. The ones requiring a momentary trigger are latching relays, that have two stable states (on and off) and require a momentary pulse to flip between those states.

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[Lenny HB]

Brain not in gear today just edited my post coil on first relay spec was in mw so 17ma.

 

[Richard n Ann]

A mosfet transistor will use Zero current

I had a relay fail on a little thermostat module last year, swapped it for a mosfet

 

[berni109]

Thanks again, ordered the 2nd one Lenny HB posted. Think I have got my head around it now as far as relays go.
Richard n Ann had a brief read on the RS site and decided electronics at that level might lead to me burning the van down.

 

[Richard n Ann]

Thanks again, ordered the 2nd one Lenny HB posted. Think I have got my head around it now as far as relays go.
Richard n Ann had a brief read on the RS site and decided electronics at that level might lead to me burning the van down.

I'm not very good either, I just keep messing about with stuff until it works