Relays: can 12v be used for 24v

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.....

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)

Oops.

P = I^R

Appologies

The last scotch is taking hold.

Redeye

Or Garry From NSW and QLD

see above entry...

P = I^2 R
P = I V
("^2" = squared)

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

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

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

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

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

You COULD put an appropriate resistor in series with the relay coils.
Not ideal, but possible.