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...
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.
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?
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.
- 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?
- 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.