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Topic: Are there 5V to 12V relays with max current ratings of 25A? (Read 9324 times) previous topic - next topic

GHarrington44

I am trying to activate 2 12 VDC motors that each draw currents of 12 A when at full load. I also have a 25 A external power supply that's used for both motors since the arduino can't handle output currents like that.

I realized that I will have to use two relays. I know there are 5V to 12V relays but are there relays that can handle 25A of current like that and are also compatible with the arduino?

If so, can someone link me to the product?

retrolefty


I am trying to activate 2 12 VDC motors that each draw currents of 12 A when at full load. I also have a 25 A external power supply that's used for both motors since the arduino can't handle output currents like that.

I realized that I will have to use two relays. I know there are 5V to 12V relays but are there relays that can handle 25A of current like that and are also compatible with the arduino?

If so, can someone link me to the product?


There are a lot of low cost 12vdc relays designed for high current used in automotive applications for window motors, seat motors, etc. Here is one example:

http://www.allelectronics.com/make-a-store/item/RLY-351/12VDC-SPDT-40A-AUTOMOTIVE-RELAY/1.html

Note that you will have to drive the coil via a switching transistor as the coil draws more current then an arduino output pin can provide directly and also be sure to wire a diode across the relay terminals for spike suppression.

Lefty

dc42

I think you will find some at http://uk.farnell.com/relays_automotive. You will need a transistor to drive one from an Arduino. Alternatively, switch your motors using power mosfets.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

GHarrington44

@Retrolefty

How will I know the specs of the transistor that I need? I assume the base current is the current of the arduino which is 200 mA. If the power supply is 25 A, does that mean the transistor needs to amplify the current from 200 mA to 25 A? I feel like that is a bit large but I am sure I am wrong

GHarrington44

And @dc42, I am looking at the website you gave me. Would the coil voltage be the 5VDC from the arduino and the contact be the 12V needed by the power supply? I am new to all this so bear with me

dc42

If you are powering the Arduino from USB, then choose a coil voltage of 5V and use the Arduino 5V power supply to power the relay. If you are powering the Arduino from an external power supply of more than 12V, then it would probably be better to make that supply a 12V supply and choose a relay 12V coil, to reduce the power dissipation in the 5V regulator.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

retrolefty


@Retrolefty

How will I know the specs of the transistor that I need? I assume the base current is the current of the arduino which is 200 mA. If the power supply is 25 A, does that mean the transistor needs to amplify the current from 200 mA to 25 A? I feel like that is a bit large but I am sure I am wrong


The transistor is only used to power the relay coil on or off, it has nothing to do with the 25 amp current flow you might be using on the relay's contacts. Any small NPN transistor would work as the relay's coil resistance is 88 ohms and as it's a 12vdc coil it will draw less then 60ma when turned on. So a common 2N2222A or similar would work fine. 60 MA is too high to drive directly from an arduino output pin so that is why the switching transistor is required to control the relay's coil. Use a 1k ohm series resistor from the output pin to the base of the transistor, ground the emitter and the collector wires to the coil and the other side of the coil wires to +12vdc and finally add a wire from an arduino ground pin to the ground of the 12vdc voltage source.

Lefty

GHarrington44

@ dc42 what would the coil resistance be? is there a way to determine the minimum current resistance using the specifications i have? Do I just use V=IR? in that case would my V be the 12V and the current would be the 40A?

michinyon

You need maybe 1 Amp to drive the coil of the relay which is going to open and close the contact to transfer the 25 Amps to the motor.

You need a transistor to deliver that 1 Amp current into the relay actuating coil.   You drive the base connection of the transistor using the arduino output.

I am saying 1 Amp here,  but it might only be half an amp.   The point is,  the current required to drive the relay is much LESS than the load current,  but much MORE  than the available Arduino current.

Two other points:

A lot of the "relay modules" available,  have the transistor built into them,  so the control input can be driven directly by the Arduino output.

A relay will only turn a motor ON, or OFF.   For any kind of speed control or direction control,  you don't really want a relay,   you want some kind of motor driving module like an H-bridge or ESC.


michinyon

"dc42 what would the coil resistance be? is there a way to determine the minimum current resistance using the specifications i have? Do I just use V=IR? in that case would my V be the 12V and the current would be the 40A?"

The relay control input is putting current into a solenoid coil which moves a contact to control the output current.    So the ohmic resistance of that relay coil is going to determine the steady state current when the relay coil is activated,    but as it is an inductive load,  there are other considerations.   You need to consider the voltage and current transients when you activate and de-activate the relay coil.

dc42


@ dc42 what would the coil resistance be? is there a way to determine the minimum current resistance using the specifications i have? Do I just use V=IR? in that case would my V be the 12V and the current would be the 40A?


You can get the coil resistance from the relay datasheet. For example this one http://uk.farnell.com/imo-precision-controls/srke-1ct-sl-12vdc/relay-spco-12vdc/dp/1094084 has a 12V coil and 90 ohm coil resistance, so the coil current is 12/90 = 0.133A. You need to drive the relay using a transistor that can comfortably switch that amount of current.

Have you considered driving the motors using power mosfets instead? For that to be feasible, it must be acceptable for the motor power supply and the Arduino to have a common ground.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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