Slow motion turnout motors on half wave DC

[richbrummitt]

Help please:

 

I have installed and wired 7 cobalt slow motion turnout motors controlled by SPDT microswitches in a lever frame powered from a 16V tap on a Gaugemaster transformer fused at 1A. This tap also feeds five SEEP electromagnets for uncoupling that are as yet unused. These are wired in parallel through individual push to make buttons for each magnet.

 

There is one cobalt that does not operate to the end of the travel in one direction and another that if it moves is incredibly slow. Both can be helped along with a push so the motor is running or trying to run.

 

Both the 16V taps on the transformer measure 17.4V so no problem there. I should have just less than 8.7V (20 ish Volts peak) after the diode. If I run the motors on a lab power supply they work well down to 7V. A little slow maybe, but acceptable and not as bad as the current situation. Disconnecting the magnets makes no difference as you would expect.

 

Is there anything else I might try before resorting to feeding the motors 'proper' DC?

 

If I do move to DC could I use something like a pair of these http://www.microminiatures.co.uk/acatalog/ac.html and have enough power to run the magnets from the 1A supply too? If not I will have to add another power supply into the layout, which I'd rather avoid.

 

Thank you,

 

Richard.

[richbrummitt]

I think you'll find these types of motors to improve their performance considerably if you add a (large) capacitor after the diode. Your calculations are wrong btw, with full bridge rectification a AC will supply 1.4 times the AC voltage as DC (minus 1.4V for the diodes in the rectifier) A single diode offers 0.7x the AC voltage. That is because the AC is a sine, not a square wave. This is the average of course, the top voltage is still 16V

 

Anyway, you'd need to measure the voltage over the terminals of the troublesome motors in situ, and the current they draw. Your power supply may be inadequate or wiring with too much resistance.

Thank you.

 

How large is large for the capacitor? Car audio often use 1Farad and I don't have space for that! I have some spare capacitors labeled 100n+100R x 2. They are quite big?

 

Measured in situ the requirement is 50mA for the pair at 9VDC so matches specification. This is the same as a good pair but the operation of the good pair is better. My meter won't read AC current.

 

Running them from the lab power supply there is no change from connecting directly to the terminals and using all the wiring that isn't common to other motors that run well.

 

I might try jumping directly from the transformer to the motor contacts using a diode as a next step until I have an idea about capacitor size.

[Suzie]

Richard

 

In the first instance you should get a bridge rectifier (four diodes) which will improve things no end. Adding a capacitor is a good idea to to smooth things out a bit, and 2200Uf at 35V should be a good starting point. 25V is too low a voltage rating for the cacacitor, a 25V one will eventually go bang due to over voltage - the voltage will build up to more than 25V when no load is applied.

 

If sticking with half-wave rectification use a 10 000uF capacitor instead.

 

If you do not want to mess about, think about investing in an old laptop power supply instead.

[66C]

We have encountered similar problems with very slow operation of Cobalt motors on a half-wave supply - as described in the Cobalt installation instructions.

 

Richard has used single pole switches in his wiring and so these would need to be replaced and some other re-wiring done to use fully rectified DC.

 

I presume the capacitor is to be wired across the half-wave supply ( in which case two will be needed - one for each output) rather than in series - this is not made clear in any of the postings above.

 

Has anyone actual experience of using the capacitors with half wave?

If so please post your views.

 

Regards.

[66C]

I have been doing some research and found the following on the Circuitron (the Tortoise manufacturer) website:

 

http://www.circuitron.com/index_files/AN/AN-6000-11.pdf

 

This may be the solution to the low voltage issue from half wave.

 

Regards.

[Suzie]

So have you just used one microswitch per lever? Or one switch at each end of the travel?

 

It is a swings and roundabouts situation as to which method you use. The simple switch requires a split rail supply with centre tapped transformer, while the complex switch will work from a simple single rail supply.

 

Here is a diagram showing a split supply:-

 

 

Low current motors like the Tortoise will be OK with half wave rectified, but you will need big capacitors for high current motors which will cost a lot more than the bridge rectifier.

[richbrummitt]

...these are electrolytic capacitors... ...A 1mF capacitor is only about 2-3 cubic inch... ...PS: nano is 0.001 micron, so those aren't big by any standards...

Why are they electrolytic? Must they be so? I think 2-3cu.in. Is quite big for the space I need to get four into. (I have wired each frame separately so I have two diodes in each.) see here http://www.rmweb.co.uk/community/index.php?app=core&module=attach&section=attach&attach_rel_module=blogentry&attach_id=281696

 

Yes nano is small. I wasn't thinking. They are quite chunky because the ones I have are rated for mains voltage.

In the first instance you should get a bridge rectifier (four diodes) which will improve things no end.

I dont see how this will help. If I understand correctly this will give me too high a voltage for the motors? I want around 9V and 12V maximum so I would need to add a further transformer.

 

I have one microswitch per lever and would therefore need two 'Laptop' power supplies?

I have been doing some research and found the following on the Circuitron (the Tortoise manufacturer) website:http://www.circuitron.com/index_files/AN/AN-6000-11.pdfThis may be the solution to the low voltage issue from half wave.Regards.

Thank you. I have seen this previously but it had been forgotten. If I add the capacitors I need to reduce the input voltage for either option though?

 

Some kind of separate DC supply is a fall back option if I cannot use the 16VAC supply I have. If I need to add an additional supply of any kind it might as well be DC. I had a look through my supplies of redundant chargers but they all seem to be around 5v, which is too low.

 

Is there a way to use one DC supply and reverse the polarity using just one SPDT switch? I haven't found one...

[Suzie]

Clearly the problem you have currently is insufficient power to drive your motors. Going from half wave power to full wave should double the power available which should move you from 'inadequate power to move the point' to 'barely adequate power to move the point' which might be just enough. Adding the capacitors will improve things still further if desired. You already have the four diodes so give it a go if you have a centre tapped transformer. If you only have a single winding transformer try the capacitors. You do not need to have separate power supplies for each frame so there is only need for a single pair of capacitors, just see what will fit in and give it a try, it looks like you have enough space for at least 1000uF 35V

 

There is no point in talking about absolute voltage values when you are not using regulated DC supplies. The unregulated DC supplies that are being discussed here have an RMS voltage value which is not particularly easy to measure and therefore hard to compare with other solutions, especially when the circuit is in operation. If you want an easily repeatable result find out what regulated DC voltage the motors run well on and build or buy a suitable regulated power supply to suit. Switch mode DC supplies will probably be smaller than the transformer and save you a bit of weight.

 

You have to remember that model railway equipment designed to run off of a nominal 12V DC will usually expect to see a much higher voltage in reality because the 12V DC output from a controller is normally in the 16-20V DC region.

[B954673]

alternately use a old atx power supply from an pc, go down to your local recycle centre, you'll find plenty to chooses from, don't touch anything that's been modified, or has got wet, ect. Short out the green wire with a black wire. Use blue for a -12 v dc line, and a yellow wire for +12 V, (black for ground, common). You should place a 12v light bulb on one output, (yellow and black) to give the supply some load, simple reliable and free

[richbrummitt]

All but two are fine as things are and one of these almost works. I have concerns that doubling the power is too much and then I will have problems with failures, especially if it is difficult to measure and know what I have. Peak voltage will always be what it is regardless of RMS value. Capacitors in addition will ensure failures because the capacitor will charge to the leak value. Despite documentation saying that Cobalts will run okay with 15V there is conflicting information where it seems this has been retracted and that 12V should be considered a maximum.

[richbrummitt]

An epilogue for anyone who comes to this topic at a later date. It might not be the answer they will be looking for but perhaps one of the solutions provided by other contributors will be helpful in this case.

 

In the end I considered that using the 16VAC supply was not the best answer: I could have fully rectified the signal and then dropped the voltage but that was quite some messing about.

 

Instead I bought two battery chargers that I found reduced in Maplin when looking for 12VDC wall warts on the internet. A quick(?) change to the wiring and they were installed and working, except the cobalts seemed to have problems stalling out! Most clicked one or more times when they reached the end of their travel. Documentation on the internet suggests that 15V should be within specification for Cobalt but certainly 9-12V is recommended in the instructions. I thought that the output might be a little high but having checked the supply and reading 12.3V it is only marginally above this. I calculated that a 27ohm 0.6watt resistor should get me to about 10V and everything is acceptable now these are included. 

 

Thank you to all for input.