Controlling current through a relay coil using resistors

[Nigel1500]

Hello all,

I am new here and have a specific issue that I'd like to clear up. I run an N scale model railway and would like to use relays to switch sections of track on and off through a digital decoder. I expect to use this decoder: Digitrax.com: TF4 Quad DCC Function Decoder which has 4x 125mA (250mA peak) outputs.

My power supply will be via the track power, which is 13.8V DC 20A: https://www.electro-tech-online.com/custompdfs/2010/10/ps2012.pdf.

I am wanting to minimise the current going to the relays, so I was going to go with a 5V relay with the following characteristics:
Max pick up voltage: 3.75V DC
Min drop out voltage: 0.25V DC
Coil resistance: 69 Ohm
Nominal operating power: 0.36W

That's the plan. Here's the problem:

I don't know what resistors to use to minimise the current going to the relay so that I don't burn out the decoder. Can anyone explain how to calculate that?

Cheers

Nige'

 

[Reloadron]

You are using DC coil relays with a 5 volt rating and the coil resistance is 69Ω so the coil current is 69 / 5 = .072 amp. Your source voltage is 13.8 volts. So we have 13.8 - 5.0 = 8.8 volts and that gets us to 8.8 volts / .072 amp = 122Ω.

You want to place a 120 Ohm (you won't find 122Ω) resistor in series with your relays. The power is voltage * current so 8.8 * .072 = .634 watt. I would use 120 Ohm 1 Watt resistors in series with the relay coils.

Ron

 

[Nigel1500]

Thanks Ron,

But don't I need 2 resistors to set up a voltage splitter to get from 13.8 down to 3.75?

Therefore, V1 = 13.8, V2 = 3.75, R2 = 69 Ohms ... calculating R1 = 180 Ohms ???

Geez, I get confused by this when different voltages are involved.

Is that right?

Cheers

N.

 

[ronv]

Another idea: If you have found a relay you like why not get one with a 12 volt coil? It will probalby take only about .03 amps to do the job and you won't waste all the power in the resistor.
PS. only one resistor. The relay coil is the 2nd one.

 

[Reloadron]

Thanks Ron,

But don't I need 2 resistors to set up a voltage splitter to get from 13.8 down to 3.75?

Therefore, V1 = 13.8, V2 = 3.75, R2 = 69 Ohms ... calculating R1 = 180 Ohms ???

Geez, I get confused by this when different voltages are involved.

Is that right?

Cheers

N.

No just a single 120 Ohm resistor in series with each relay. Also your relays are 5 volt with a minimum pull in of 3.75 V so you figure your relay coil as 5 volt and not the minimum pull in.

Also, I thought about what ronv mentioned. Why not use 12 volt relay coils? Less current and less waste.

Ron

 

[crutschow]

Perhaps the op thought that a 5V relay took less power than a 12V relay.

 

[Reloadron]

Perhaps the op thought that a 5V relay took less power than a 12V relay.

That's a good possible. I should have mentioned it but ran the post from work sort of on the fly this morning. That plus the 5 volt coil relay had all the specs. Anyway, given a choice I would run with a 12 volt coil and less current and power consumption all around.

Ron