Relays: can 12v be used for 24v

Submitted: Thursday, Jun 22, 2006 at 18:26
ThreadID: 35184 Views:9995 Replies:8 FollowUps:10
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One for the thinkers among us. Can I use a normal 12v 30A relay to wire up some spotties, or do i need a 24v one?

I understand the amps are halved for equiv watts when running 24v but, does this have any application to relays.

I already have a 3rd aux battery in the rear for 12V, but wish to use under bonnet power sources for my lights. (damned imports lol)

Thanks

Rev

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Reply By: Member - John L G - Thursday, Jun 22, 2006 at 18:29

Thursday, Jun 22, 2006 at 18:29
i gather the spotties are 24 V????????????????
AnswerID: 179923

Reply By: GaryInOz (Vic) - Thursday, Jun 22, 2006 at 18:36

Thursday, Jun 22, 2006 at 18:36
You will need to use the 12 V for the coil of the relay (24 V may lead to an early demise, just what you need at 100 km/h).

The switching part of the relay should be fine with 24 V.
AnswerID: 179925

Reply By: Peter 2 - Thursday, Jun 22, 2006 at 18:44

Thursday, Jun 22, 2006 at 18:44
Having tried it on my Humvee the smoke escapes very quickly and the relay will no longer work ;-))) but as has been said you can use a 24v relay to switch 12v and vice versa.
The actual relay coil requires the corredt voltage. The switching contacts can handle either.
Peter
1996 Oka Motorhome

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AnswerID: 179930

Follow Up By: Rokkitt - Friday, Jun 23, 2006 at 10:03

Friday, Jun 23, 2006 at 10:03
This is the basis for those little electrical/electronic black box relays and other things - they all work on smoke! - you let the smoke and they dont work no more.....
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FollowupID: 436220

Reply By: drivesafe - Thursday, Jun 22, 2006 at 18:48

Thursday, Jun 22, 2006 at 18:48
Hi revhead307, you either need to get a 24 volt relay and use it for you job or you could get another 12 volt relay and wire it in series with the 12 volt relay you already have. This way you could also wire one driving light to each relay’s switched output.

cheers
AnswerID: 179931

Reply By: Redeye - Thursday, Jun 22, 2006 at 19:06

Thursday, Jun 22, 2006 at 19:06
The only problem I can see happening is a lot of smoke.

If you use 12 volt products on 24 Volt systems you will end up with double the power consumption by the end product. Power consumption is Volts times amps. If volts is doubled the power is doubled. The smoke may be excessive.

Redeye
AnswerID: 179933

Follow Up By: GaryInOz (Vic) - Thursday, Jun 22, 2006 at 19:29

Thursday, Jun 22, 2006 at 19:29
Wrong!!!

As stated above, using the switching part of the relay is fine for 24 volts, you just can't use the coil on 24 volts. Best idea would be to use the center tap of the 24 volt battery (12 volt) through a switch to the coil of the relay and connect the 24 volts to the switched part. Better still, buy a decent truck relay meant for the job.

FWIW you actually quadruple the power by doubling the voltage into a resistance, as the current is also doubled, therefore you end up with power = I x V = (2I^2) x (2V^2) = 4 IV (Limited characterset makes it a bit fiddly to understand)
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FollowupID: 436119

Reply By: Redeye - Thursday, Jun 22, 2006 at 19:51

Thursday, Jun 22, 2006 at 19:51
Thanks,

Did my electronics in NSW not Victoria.

P = V x R.

Resistance is a constant. Voltage is doubled from 12 volts to 24 volts. So as a resultant the power consumed is also doubled. Where did the quadruple come from ?

Garry now in Queensland.

AnswerID: 179943

Follow Up By: Redeye - Thursday, Jun 22, 2006 at 20:00

Thursday, Jun 22, 2006 at 20:00
Oops.

P = I^R

Appologies

The last scotch is taking hold.

Redeye

Or Garry From NSW and QLD

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FollowupID: 436124

Follow Up By: GaryInOz (Vic) - Thursday, Jun 22, 2006 at 20:51

Thursday, Jun 22, 2006 at 20:51
see above entry...

P = I^2 R
P = I V
("^2" = squared)
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FollowupID: 436136

Follow Up By: agsmky - Friday, Jun 23, 2006 at 11:50

Friday, Jun 23, 2006 at 11:50
in this case, P=E²/R (I was always taught E for Emf instead of V :-)), therefore due to the fact that R will be constant, the doubling of voltage will result in the quadrupling of Power consumed......

Andrew
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FollowupID: 436238

Follow Up By: Mike Harding - Friday, Jun 23, 2006 at 12:02

Friday, Jun 23, 2006 at 12:02
My understanding is that V is the unit measure of emf - but even Wikipedia is a bit confused on the issue:
en.wikipedia.org/wiki/Electromotive_force

Edward Hughes - Electrical Technology 5th ed. says it is too and I haven't found him to be wrong in about 30 years so far :)

Mike Harding
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FollowupID: 436239

Follow Up By: agsmky - Friday, Jun 23, 2006 at 12:20

Friday, Jun 23, 2006 at 12:20
My understanding from the good (?) old TAFE days when sitting down to some boring lecturer just to complete our Electrical Apprenticeships was that:

Ohm's Law, E=IR

E is measured in Volts
I is measured in Amp(eres)
R is measured in Ohms

Therefore to me it seems like people are mixing symbols with measurements (BTW i believe E was a Greek symbol :-))

Tell me if i passed the subject adequately ;-)

Andrew
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FollowupID: 436241

Follow Up By: Mike Harding - Friday, Jun 23, 2006 at 12:56

Friday, Jun 23, 2006 at 12:56
Agsmky! I could go off you :)

MG Scroggie - Foundations of Wireless and Electronics, 9th ed. devotes 5 pages to the difference between Potential Difference and Electromotive Force including a fairly complex circuit which requires Kirchhoff to analyse the current flow. Essentially Scroggie says emf cannot be measured as such but may only be expressed in terms of the Potential Difference (measured in volts) which it can overcome.

H Wardle - Fundamentals of Electrotechnology, states clearly that emf and PD are separate entities and gives an example of the internal resistance of a battery causing a difference in their values.

So in my opinion it should be I = V/R although even Scroggie uses both forms of the expression but I have not seem E used in this context for years and in 30 years in the design business have never, before!, needed to understand the difference.

I cannot help but feel we have drifted a little from 4WDs, camping etc :)

Mike Harding

mike_harding@fastmail.fm
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FollowupID: 436249

Follow Up By: agsmky - Friday, Jun 23, 2006 at 14:58

Friday, Jun 23, 2006 at 14:58
oh, that's right....this is a 4wd forum, my apologies to all :-)

This might explain why i am not an (HV) electrician anymore ;-) BTW just googled E=IR and was surprised with the results! ;-)

Andrew
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FollowupID: 436279

Reply By: revhead307 - Thursday, Jun 22, 2006 at 20:43

Thursday, Jun 22, 2006 at 20:43
Thanks all,

yes, 24v spotties on import patrol.

I'll do the right thing then and buy some 24v relays :-)

melted plastic isnt one of my favourite smells.

Kind Regards

Rev

AnswerID: 179947

Follow Up By: Flash - Friday, Jun 23, 2006 at 13:16

Friday, Jun 23, 2006 at 13:16
You COULD put an appropriate resistor in series with the relay coils.
Not ideal, but possible.
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FollowupID: 436259

Reply By: rod2101 - Friday, Jun 23, 2006 at 20:56

Friday, Jun 23, 2006 at 20:56
Put two relay coils in series and use one relay for each spot light!
AnswerID: 180138

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