# Will relay draw too much current?



## Roy Von Rogers (Mar 21, 2009)

tido said:


> I purchased a cheap 12V 30A relay.
> 
> I have a switch which controls the 12V going to the relay to open or close it.
> The question I have is the following:
> ...


 
If its one of those Bosh types....no. Do you have a meter, if you do you can test to see how much that relay pulls.

And btw a resistor doesnt slow the flow, it just dissipates it in to heat.

Roy


----------



## Ziggythewiz (May 16, 2010)

Most relays take a couple watts or less. Hopefully your DC-DC can handle that. Yes, adding a resistor would slow the flow, but that would prevent the relay from working properly. If you added a resistor the relay would no longer get the voltage or current it is designed for.

As Roy said, if you have a small ammeter you can test and see exactly how much it takes.


----------



## major (Apr 4, 2008)

tido said:


> I purchased a cheap 12V 30A relay.
> 
> I have a switch which controls the 12V going to the relay to open or close it.
> The question I have is the following:
> ...


Hi ti,

Your question is confusing, at least to me  What current are you worried about? The coil current or the contact current? What do you use the relay to connect? Where is it in the circuit? Do you have a 12V battery? What is the DC/DC rating? Got a diagram? Photos?

Regards,

major


----------



## tido (Sep 20, 2011)

Thanks for the answers.

Major, note that it is a very newbie question as I don't know much in electricity. What I have is the relay (simply drawn in the immage in attachement)

The wire in red is where I am questioning. When I close the relay (with the switch), doesn't it create some sort of short circuit? Do I have to do something to prevent a possible problem or it is integrated in the relay?


----------



## major (Apr 4, 2008)

tido said:


> Thanks for the answers.
> 
> Major, note that it is a very newbie question as I don't know much in electricity. What I have is the relay (simply drawn in the immage in attachement)
> 
> The wire in red is where I am questioning. When I close the relay (with the switch), doesn't it create some sort of short circuit? Do I have to do something to prevent a possible problem or it is integrated in the relay?


Hi ti,

Thanks for the effort with the diagram, but I can't quite follow it. Here is one maybe you can try to pick apart. http://www.diyelectriccar.com/forums/showthread.php?t=2372 

To try to answer your question: A basic relay is a 2 part device with 4 terminals. One part is the coil with 2 terminals. The other part is isolated from the coil and has the contacts with 2 terminals, usually larger than the coils terminals.

The coil is the actuation which causes the main contacts to close (conduct) when voltage is applied to the contacts. So your switch is used to apply a voltage to the relay coil. The coil of the relay has sufficient resistance to limit the current which flows when the switch is closed applying the voltage to the coil.

When the coil is energized, the main contacts close and conduct to the high current load, the heater in this case, I guess. Is it the heater element? Or just a heater fan?

A heater element will be practically useless running on 12V. To get sufficient wattage the current will be excessive at such a low voltage and require a huge DC/DC and/or aux battery. Therefore most heaters use the high voltage, as shown in the linked diagram.

I hope this helps,

major


----------



## tido (Sep 20, 2011)

Will a ciramic element designed for 120 VAC work well on 144VDC?

Also I can see that there are small heaters made to work when plugged in the 12V aux of the car. Do these heaters use converters?


----------



## TEV (Nov 25, 2011)

Your 12V/30A can switch just that : a 12V and 30A load. You CAN'T use it on a 144V DC because the contact will fail and usually it's fails ON ( the contacts get welded together )


----------



## major (Apr 4, 2008)

tido said:


> Will a ciramic element designed for 120 VAC work well on 144VDC?
> 
> Also I can see that there are small heaters made to work when plugged in the 12V aux of the car. Do these heaters use converters?


Better see what you can find with a heater search or start a thread about heaters


----------



## TEV (Nov 25, 2011)

The 12V heater it's powered from 12V, on that you can use your relay.
But a 12V heater more likely will be a 14V*15A= 210W of power, not to much use unless you are living far south.


----------



## PStechPaul (May 1, 2012)

If you have a relay like this, it is an automotive type and rated at 40A at 14VDC maximum. Note that it has 5 terminals, two for the coil and three for the SPDT contacts:










Here is a DPDT relay with contacts rated 20A 250VAC ($6). You can usually use AC relays on DC if you wire contacts in series and derate, so this should work on 125VDC and 20A resistive heater load.
http://www.mpja.com/12VDC-DPDT-20A-Aromat-Relay/productinfo/18610+RL/


----------



## tido (Sep 20, 2011)

Ok, thank you all for the information!


----------

