# DC Motor and Relay in series with each other

First off thank you for any comments or answers, I've been reading quite a bit on here and it's helped me a lot which is very much appreciated.

Secondly, apologies if I've made any obvious mistakes / stupid errors. I'm only about 4 weeks in to playing with electronics and even had to remind myself what Ohms law was at the start!

I'm trying to design a signalling system for a model railway which is a fairly trivial problem except that I don't really want to power the trains with a battery. So while the downstream logic circuits are all fairly simple with relays being fed from a nice clean 12v dc supply, the track circuit which detects the presence of a train and also provides power to the motor is confusing me.

The below diagram is very simply what I have at the moment...

I wasn't sure what the simple for a dc motor was and so I have put an inductor and variable resistor in parallel.

The push switches here just represent the train wheels, so they are always both closed or both open.

The Relay is an Omron G2R-2, 12 V dc relay with a 275 Ohm coil resistance.

The motor is a 6V 0.5amp motor (that's the current plan anyway). This one... http://www.nigellawton009.com/6VMicroMotors.html

And I intend to control it via Radio Control using the RX45d-v5 found here: http://www.micronradiocontrol.co.uk/rx_dt_land_v5.html#dt_rx45_v5

So, my questions...

1. Could anyone kindly please point me in the direction of a good learning resource for dc motors? I'm struggling to find decent information that's not a manufacturer extolling the virtues of their particular brand.

2. When a motor says '6V - 0.5A' is this normally the peak current? i.e. does it guarantee that the inrush current on start up won't exceed the 0.5A value?

3. I'm assuming that the 6V of the motor is the maximum value and the controller is effectively a very complicated variable resistor to control the motor speed by controlling the voltage to the motor. Have I got this wrong? My intention was to have the flyback diode over the relay be a 12V zener and have an 18V power supply to leave 6V for the motor. Would I need another resistor in series (R1) or do motor controllers tend to handle this internally, drawing a constant 6V and only feeding the motor what is required?

With the motor being 0.5 Amps, I've obviously got to find somewhere for this current to go. I'm not worried too much about spending extra on bigger Watt resistors or such and the circuit will only be closed for about 5% of the time. So plenty of time for things to cool down between energisations. However I'm obviously keen not to break things.

The relay is rated at 0.43Amps, but is this just what it draws normally? i.e. 12/275? Would it be able to cope with an additional power load through it and is that even physically possible if I've pegged the voltage at 12V with a zener and the coil is a static 275 ohms?

The zener diodes I have are 1N5349B - 5 Watt - 395mA max current data sheet below:

I honestly have little idea how to read these.

1. Is it saying that at 12V the diode will start to conduct in the reverse direction and start out at approx 1mA and 2.5 Ohm of resistance and then, as the current increases, it will be stable all the way to 395mA at which point the diode will have a resistance of 125 Ohms and then any more current will make it go pop? In my mind 395mA at 125 Ohms is 49V, so i assume I'm missing something here?

In which case, is it a very simple thing to add a resistor in parallel with the relay coil to take up the extra current that can't be fed through the coil and diode combination?

1. Where I plan to use 18V - 12V accross the relay and 6V across the motor (if htis proves to be correct), would it be sensible to use a higher voltage supply and add a current limiting resistor? i.e. 24V supply, 12V across the relay, 6V across the motor and 6V across a 6Watt, 24 Ohm resistor to ensure the current stays at 0.5 Amps and no higher.

Sorry for the very long winded question, any help at all would be very much appreciated. Many thanks.

• Far too wide a scope of questions. Your track circuit doesn't make sense either - what does the relay actually bring to the party? Why should anyone consider powering the trains with a battery anyway? I'm confused. May 6, 2020 at 17:01
• Thanks Andy, I'll refine my questions in the future and keep them more on point. The Track Circuit is in order to detect the presence of a train in the 'block' from the relay attached are a number of logic circuits that drive signals in the reverse of the train. Powering the train from a battery would just alleviate the queries I have above and allow train detection to occur by making an electrical connection to the relay through the train axles without the motor having to be connected in series.
– Ed_B
May 6, 2020 at 17:08

So to be kind ...

1. https://www.allaboutcircuits.com/projects/ is interesting ...

2. motors are (when stationary) bits of wires with lowish ohms... resulting in possibly 10 times or more current than the running current of 0.5 A

3. Using resistors gives you the same control as a milk float ... works but not great. You have the motor in series with the coil of the relay which is effectively another resistor.

The 0.43A of the relay is probably how much the contacts are rated for.

The coil has a separate rating (Volts, Amps and maybe ohms or watts) which is in the manufacturers datasheet.

To turn on a motor you need contacts with approx 10 times or more the motor running current or even better a Motor Rated Relay (related to 2.)

4. Have a look how zeners are used (see 1.)... in this specific case no one is ever likely to do this. Maybe be if you are very very very desperate.

5. Work out or copy a motor speed control system (Hint ... PWM), and then add relays to switch the output of that to the tracks as you require.

So in short you have 12V supply for the Speed Controller (or what ever the Speed Control needs ). If the relay coil voltage is different another supply is needed for the relays.

1. Could anyone kindly please point me in the direction of a good learning resource...?

No. That is not what we do here.

1. When a motor says '6V - 0.5A' is this normally the peak current? i.e. does it guarantee that the inrush current on start up won't exceed the 0.5A value?

Most probably not, but many motors described on the internet are very poorly described. The one linked to the question is a good example of what not to buy because of the poor description.

1. Have I got this wrong?

Yes. Essentially, a permanent magnet DC motor with a commutator behaves like a resistor and inductor in series with an opposing voltage source that is proportional to motor speed.

1. Whatever?

The motor current should go through the motor and power supply. The relay should not be in series.

1. would it be sensible to use a higher voltage supply and add a current limiting resistor?

No. It would be sensible to buy a motor and supply that are more closely matched and use an electronic circuit to control the motor speed by controlling the voltage. Putting anything else in series would increase the speed variation due to load variation.