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### Topic: 12 volt 30 amp automotive relay (Read 2660 times)previous topic - next topic

#### arduinoPi

##### Jun 27, 2013, 12:21 am
I bought some 12 volt 30 amp Automotive relay for controlling large 12 volt loads. I am trying to find what voltage to give the coil of the relay to power it. There are no data sheets to my knowledge but on the back of the package it says the following.

Coil voltage: 12 volts
Pull-in voltage 6 volts
coil resistance 66 ohms
nominal current 160mA
drop-out voltage 3.6 volts

I am confused on some of these terms and what they mean.
Do I just power the coil at 12 volts?
All of my projects --> http://www.frankmagazu.com/

#### pito

#1
##### Jun 27, 2013, 12:34 amLast Edit: Jun 27, 2013, 12:38 am by pito Reason: 1
A relay has different voltages for pull-in and drop-out. You can connect it to 12V (plus a flyback diode). It will draw 180mA when on. So you need a transistor (ie npn with 1k in base) when you want to wire it to arduino.

#### arduinoPi

#2
##### Jun 27, 2013, 12:59 amLast Edit: Jun 27, 2013, 01:01 am by arduinoPi Reason: 1
when I plugged it into 12 volts it used a constant 230 mA. It does get warm to the touch even when there is no load.

I actually have 2 of the same part and the other relay uses 250 mA as well.
my power supply is 12.23 volts
All of my projects --> http://www.frankmagazu.com/

#### pito

#3
##### Jun 27, 2013, 01:00 am
So find a lowest voltage the relay works reliably..

#### craigcurtin

#4
##### Jun 27, 2013, 04:21 am

when I plugged it into 12 volts it used a constant 230 mA. It does get warm to the touch even when there is no load.

I actually have 2 of the same part and the other relay uses 250 mA as well.
my power supply is 12.23 volts

Use a MOSFET to drive it or get a DC SSR

Craig

#### PedroDaGr8

#5
##### Jun 27, 2013, 05:43 am
Hooking up an automotive relay directly to something like an arduino is a bad idea. I've seen a lot of uC killed by automotive relays. When the coil is released, the field collapse can generate a voltage to the tune of 100s of volts (very little amperage but to damage ICs you don't need much). This is the source of "thump" in some car audio systems. You can use a reverse biased diode across the coil to shunt the EMF but even then its not fool proof. Instead, do as someone else mentioned use a MOSFET to switch it.

#### arduinoPi

#6
##### Jun 27, 2013, 06:39 am
I have a MOSFET on it as well as a diode now it seems to be alright. I did measure the high voltage when I was just plugging it in to a battery.
I do have one problem. It is hard to explain so I drew a "schematic".

What happens is when I power on the MOSFET to power the relay to power the charger (and the laptop is off) the relay clicks ~5 times. It is not a loose wire I checked. When the laptop is on the relay will switch on and off continuously. My guess is that there is interference coming from the laptop or the charger but I may be wrong. Anyways here is the terrible drawing (sorry about this I know it is terrible. I think it is time I get some schematic making software)

All of my projects --> http://www.frankmagazu.com/

#### PedroDaGr8

#7
##### Jun 27, 2013, 06:45 amLast Edit: Jun 27, 2013, 06:50 am by PedroDaGr8 Reason: 1

I have a MOSFET on it as well as a diode now it seems to be alright. I did measure the high voltage when I was just plugging it in to a battery.
I do have one problem. It is hard to explain so I drew a "schematic".

What happens is when I power on the MOSFET to power the relay to power the charger (and the laptop is off) the relay clicks ~5 times. It is not a loose wire I checked. When the laptop is on the relay will switch on and off continuously. My guess is that there is interference coming from the laptop or the charger but I may be wrong. Anyways here is the terrible drawing (sorry about this I know it is terrible. I think it is time I get some schematic making software)

This is a common issue with mosfets. Basically, induced charges can build up in the gate of the MOSFET causing the MOSFET to open. Put a resistor on the digital pin and connect it to ground. Somewhere around a 10K-100K resistor should work. Closer to 100K is better because this limits current flow through the resistor to ground. This setup should stop what you are describing.

#### arduinoPi

#8
##### Jun 28, 2013, 08:19 pm
I added the resistor and the problem is still occurring
All of my projects --> http://www.frankmagazu.com/

#### arduinoPi

#9
##### Jun 28, 2013, 09:23 pm
I added a 400 microfarad capacitor across the 12 volt terminals. the relay now clicks continuously but it clicks slower about 5 times a second ti was going much faster before the capacitor. What does this mean?
All of my projects --> http://www.frankmagazu.com/

#### tocpcs

#10
##### Jun 29, 2013, 06:23 am
Test the relay with no loads attached, and that'll rule that out (meter continuity across it if you can't hear the click out of it).

#### jackrae

#11
##### Jul 21, 2013, 10:16 am
If your relay is rated at 12 volts and its coil resistance is 66 ohms then the heat dissipation within the coil is going to be roughly 2.2 watts.  This indeed is going to feel warm after a while - but that's what the relay is designed to do.  It matters not a hoot whether the relay has a load applied to its contacts or not.  The heat comes from the coil.

30A contacts are meaty contacts and to be effectively closed with minimum contact resistance they need to be pulled together fairly strongly.  This is why the coil draws the current it does.  If you start mucking about with coil power (by installing series resistors) then the pull on the contacts is lowered and they will not have the same current carrying capacity that they were designed for.

So saving on the one hand will lead to poor performance on the other.

#### LROBBINS

#12
##### Jul 21, 2013, 10:35 am
I have attached two app notes from Tyco that I think are worth a read.  For high-current DC applications, a diode across the coil will protect your input device BUT can seriously compromise contact life.  Better a pair of diodes (one normal, one Zener), or even a resistor, so that kickback is snubbed, but doesn't fall so quickly that the contacts can't snap open and break the spark.  Ciao, Lenny

#### TomGeorge

#13
##### Jul 21, 2013, 11:24 am
Hi, one big problem here is we do not have your circuit diagram, the one that was supposed to be in a previous message is not there.
I don't think any of us should comment until we see how you have hooked up the relay , mosfet, arduino.
Also I note people keep saying connect a 10k or 100k resistor from the arduino output to ground to make sure that the gate of the MOSFET is held OFF under no signal conditions.
I know it may be semantics but place the resistor AT the MOSFET between gate and source, this way if any circuit mistakes occur around the MOSFET the gate if left open, will be at source potential, that is OFF.
With no resistor between gate and source, just touching the gate terminal can turn it ON.
Also note not all relays are the same as you have found, automotive relays are the prime example, in a car it doesn't matter if  two identical relays draw up to 30% differences in coil current, they still do the job. They are mass produced and if they are cheap, you get what you pay for.
Buy a known product if you want reliable consistent specifications.

def; MUDMAP, hand drawn map, not to scale, not using any recognizable characters or units. LOL

Tom
Everything runs on smoke, let the smoke out, it stops running....

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