Using a relay to power a relay?

alloy

alloy

Dan, Retired old fart
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Ok cooling fans for the 55 chevy. Have 2 fans, larger one for the radiator, smaller one for trans cooler.

I'd like to have both fans come on at the same time, hence the reason for the 75 amp relay.

So I'm thinking a 12v key on source to turn on a 75 amp bosch relay that is supplied from terminal block directly from the battery. This would supply power to a second relay when the ignition is on.

Second relay is fed power from the first relay and used to turn the fans on, and is controlled by an adjustable temp switch.

Reason I'd like to use 2 relays is so that the fans operate only when the key is on.

I know it will work, but using a relay to power relay? Just seems weird to me for some reason.
 
So it isn't clear what you mean: Do you mean "use the output of Relay 1 to trigger Relay 2", or do you mean: "Use the output of Relay 1 to provide the power to Relay 2", each triggered separately (off the key source)?

The former is odd, the latter is not uncommon. Its not much different than powering both off the same battery source and triggering individually, except 1 relay failure can now take out both fans.
 
I do not see the reasoning for the use of two relays. The fan relay could be fused and that directly connected to the battery terminal. The fan relay coil would be connected to the ignition/car power (key or relay) this would be connected to the fan relay through a thermal sensor in the coil power circuit. These fan relays typically have a coil resistance of ≤80 ohms. So at 12vdc, minimum to trip would be 150ma with a power consumption of 1.8 Watts. It would probably have an inrush current as high as 200ma though for a few milliseconds. If run through a thermal sensor, the key would not carry any high switching current when the coil is triggered, this would be through the thermal sensor.

Relays are often used to power other relays subsystems, so rather than the startup load going through a switch a relay/contactor handles the load and/or remote location to power up a system. Not sure how it works in cars these days, but my radio/amplifier in my truck has a heavy gauge fused wire that goes to directly to the battery terminal, it then uses a separate trigger circuit that turns on amplifier when the radio turns on.
 
Nothing wrong with using a pilot relay (taken from pilot valves in hydraulics). Sounds like you are controlling a sub-grid with one relay to pass power elsewhere. Most cars have this layout in practice, it's called a master power relay, and it must be active before power is supplied to downstream controls. So relays can control relays that control relays. Use good relays, no china junk. Nobody wants to get stuck on the side of the road in the rain with a dying flashlight clenched between their teeth with a multimeter playing an impromptu game of find the bad component.
 
I did not read fine details.

Use one relay to control the fans. This relay can have the coil connected to the ground and your controlled power ounce, keyswitch.

Step 1.

Now, you want to control fans via temperature switch.

Most engine switches screw into the engine and provide switched ground when active. Others can be ground until hot.

These are usually not adjustable. There are adjustable types with remote sensors, and the switch is up to you, but use the switch to control the ground to relay in step 1.




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Ok, I did my best to explain what I'm thinking.

Lets try it again.

#1 relay triggered by ignition to turn on battery power to #2 relay.
Ignition is NOT connected to #2 relay.

#2 relay gets 12v power from relay #1 Relay is triggered by thermal switch and relay turns on fans.

Yes each relay has the ability and capacity power both fans.

The idea is to have the entire circuit dead when ignition is off. Power for the fans goes from the starter battery terminal to the right fender, under the right fender in flexible conduit up to and under the radiator support to hide the wires.


Have been thinking about another configuration, but this would not have the circuit dead with the ignition off.

Single relay.

Battery power to relay through a circuit breaker, hot all the time.

Ignition to pin #86.

Battery power to #30

Fans (load) on #87

With #86 pin not grounded the relay will not turn on.

Adjustable thermal switch from ground to #86 that will allow relay to turn on.

But to me the downfall on this configuration is the circuit (30 amps) is hot all the time and just has a circuit breaker protecting the circuit.


On this circuit (and any other important circuit) I will not use Chinese relays. Bosch is my go to brand. Not cheap, but a tow truck is way more expensive if a Chinese relay fails. Been there, done that.

Adjustable temp switch

relay
 
You are using relay #1 as a master power relay to feed relay #2, which has a thermal switch to ground. That's a sane and logical arrangement, and is consistent with most of the cars on the road. The cost is a teensy bit of heat from the relay coil holding closed, some milliwatts of power lost. The relays used for master power are heavy duty, and more often nowadays they are solid state. It's their job to hold closed under power reliably while the vehicle is in operation, so your use is as designed.

Other times where cascaded relays are useful is when you want to use a very feathery switch to control a high current circuit. Just like a piloted spool valve in a hydraulic ram, smaller amperage relays can drive larger relays to do bigger tasks.

@alloy, I think you're right on target.
 
I've had some experience with relays, but all I can tell you is use GENUINE BOSCH. Anything else is garbage.
 
What you are doing is common.

My post explains it.

Controlled power is the output of your ignition relay.

Nothing complicated.

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