# How to wire an LED switch based on rated voltage and current

• Electrical

## Summary:

how to wire an led switch based on rated voltage and current

i need to connect a 12vdc motor which can draw 16 amps
to a switch that has an led on it. the led is rated at 3v 20mA.
The power supply i am using is rated to put out 12vdc at 30A

I thought of the possibility of adding a resistor to the positive pole of the led.
In this case i saw people using ohms Law
R=(Vsource-Vrequired)/ required amps.

In this case i get(12-3)/0.02= 450 ohms
is this right?
because the amperes of the power source is not factored in at all with this calculation will i then use a 450 ohm resistor for any 12v source regardless of the rated current output of the power source? IE. whether the power supply is 20A 30A or 100A will i still simply attach a 450 ohm resistor to the positive terminal of the led?

berkeman
Mentor
Summary:: how to wire an led switch based on rated voltage and current

i need to connect a 12vdc motor which can draw 16 amps
to a switch that has an led on it. the led is rated at 3v 20mA.
The power supply i am using is rated to put out 12vdc at 30A

I thought of the possibility of adding a resistor to the positive pole of the led.
In this case i saw people using ohms Law
R=(Vsource-Vrequired)/ required amps.

In this case i get(12-3)/0.02= 450 ohms
is this right?
because the amperes of the power source is not factored in at all with this calculation will i then use a 450 ohm resistor for any 12v source regardless of the rated current output of the power source? IE. whether the power supply is 20A 30A or 100A will i still simply attach a 450 ohm resistor to the positive terminal of the led?
Welcome to the PF.

The LED has different leads from the switch, correct? It cannot be in series with the switch. Can you post a datasheet?

The LED indeed has different leads from the switch. in total there are 5 terminals. (COM) (NO) (NC) (+(led))(-(led)).I assume the led and switch itself are in parallel. this is a link to the switch and the relevant data specifications. from here i deduced that the led requires 3v 20mA

i will be wiring the switch in such a way that the led is only activated when the circuit is closed. the switch should be normally open
https://www.aliexpress.com/item/4000027999336.html

berkeman
berkeman
Mentor
The LED indeed has different leads from the switch. in total there are 5 terminals. (COM) (NO) (NC) (+(led))(-(led)).I assume the led and switch itself are in parallel. this is a link to the switch and the relevant data specifications. from here i deduced that the led requires 3v 20mA

i will be wiring the switch in such a way that the led is only activated when the circuit is closed. the switch should be normally open
https://www.aliexpress.com/item/4000027999336.html
Okay, then yes, put the 450 Ohm resistor in series with the LED so that 9V is dropped across the resistor when 20mA is drawn. That will give the 3V @ 20mA to the LED. The resistor+LED are wired basically in parallel with the motor.

So then the 12V power supply will be providing 16A + 20mA when the motor is drawing the full 16A.

So it won't matter whether I use a power supply of any current output. The end result will still be to add a 450 ohm resistor in series with the led?

pbuk
Gold Member
STOP.

According to the information you linked those switches are rated for 1A @12V into an inductive load (like a motor). You either need a bigger switch, a relay or a fire extinguisher.

Rayhaan, hutchphd and berkeman
Pbuk thank you so much. This is the first time im contemplating something like this. From what you're saying I gather the led isn't really my biggest problem here. The switch itself isn't made for direct connection to a powerful motor. Especially one that can pull 16 A. I don't really know anything about relays. Can you explain further on what they are and how to go about choosing one

Update. A relay is used as a means for a low current circuit to control a high current one. It does this by activating a switch when the small circuit is closed. This switch then activates the circuit with higher current. It is electrically safer. Less energy demanding and more economical because you can use thin guage cable for your control buttons which is generally far from your component.

Relays have a voltage required by the coil which connects the low demand circuit and a current and voltage rating for the high current circuit

pbuk
Gold Member
Designing control circuits for high power inductive loads is not for the layman. I suggest you contact the motor supplier and/or manufacturer for their recommendations.

berkeman
Mentor
Relays have a voltage required by the coil which connects the low demand circuit and a current and voltage rating for the high current circuit
You can also look into "Contactors" -- you may not need one at your power level, but they are purpose-built for handling inductive power loads like motors (although usually at AC Mains voltage levels):

https://en.wikipedia.org/wiki/Contactor

Pbuk. I will contact the motor manufacturer for guidelines but I would feel much more comfortable also discussing it with people who understand components like this. I am learning after all and wanted to discuss this matter over a forum as a means to help others with a similar problem.

Berkeman thanks for suggesting contactors or breaker switches but I really would like to use these attractive light up buttons and I'm sure others would also like to as well. Please entertain my problem. I want to learn

please see my attached circuit diagram. i look forward to your criticism. this circuit has 2 push buttons for moving the motor in opposite directions by having opposing polarity from one button circuit to the other

anorlunda
Staff Emeritus
Berkeman thanks for suggesting contactors or breaker switches but I really would like to use these attractive light up buttons and I'm sure others would also like to as well. Please entertain my problem. I want to learn
You're missing the point. Using a switch instead of a contactor or a breaker could result in a very short lifetime for the switch or even a fire.

Last edited by a moderator:
berkeman and pbuk
berkeman
Mentor
Berkeman thanks for suggesting contactors or breaker switches but I really would like to use these attractive light up buttons and I'm sure others would also like to as well.
You use the low-power switches to turn on the contactor. It's done all the time.
please see my attached circuit diagram. i look forward to your criticism. this circuit has 2 push buttons for moving the motor in opposite directions by having opposing polarity from one button circuit to the other
Sorry, I'm not able to understand your drawing yet. And be careful with direction/polarity reversals -- you need to ensure that both directions can't be on at the same time.

You use the low-power switches to turn on the contactor. It's done all the time.

Sorry, I'm not able to understand your drawing yet. And be careful with direction/polarity reversals -- you need to ensure that both directions can't be on at the same time.
My diagram is incorrect. This would short circuit my power supply everytime current is run to the motor. Thanks for spotting this. I think I'll have to look at some way to switch both poles on and off. Maybe something like a dpst relay could work?

berkeman
Mentor
My diagram is incorrect. This would short circuit my power supply everytime current is run to the motor. Thanks for spotting this. I think I'll have to look at some way to switch both poles on and off. Maybe something like a dpst relay could work?
Yes, and you can probably find a contactor that has the configuration you want (DP3T, Break-Before-Make).

pbuk
pbuk
Gold Member
So now you have introduced another requirement: switching from forward to reverse. How are you going to deal with the back EMF when you do that?

berkeman
Mentor
So now you have introduced another requirement: switching from forward to reverse. How are you going to deal with the back EMF when you do that?
He'd need to include transorbs, I would think. Unless he can find a contactor that already has them built-in.

I like how complex a seemingly simple solution can be. Are relays and contactors the same thing? I thought contactors were trip switches. Going to Google back emf

Dale
Mentor
Temporarily closed for moderation

berkeman
Mentor
Thread is re-opened after a bit of cleanup. We will keep a close eye on the safety aspects of this DC motor project.

I will start another thread for this. I'm happy to conclude this thread by saying if you need to use an led switch to control a high demand circuit. You should have the switch on a separate circuit and it should have it's own power source. You do this with contactors and relays

Tom.G and berkeman
pbuk
pbuk