Point motor CDU is weak

Basic electrical and electronics, such as DC/Analog control.
denfin
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Point motor CDU is weak

Postby denfin » Wed Apr 14, 2021 11:02 pm

Hi!

I have built a CDU following the diagram here: Image

Only difference with the one I made is I used three 4700uf 35v capacitors instead, cus I thought that would make it even more powerful, but it does not seem to work well, and is very weak. I tested a point motor with 16v AC and 19DC, and it seems to work quite good, but when I hook up the CDU to 16v AC or 19v DC, it gets much weaker. it seem to charge up quite slow also, the LED takes some time to light up at full power again. So what am I doing wrong here? should I not have used the bigger capacitors?

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Re: Point motor CDU is weak

Postby End2end » Wed Apr 14, 2021 11:25 pm

I'd wait for someone more knowledgeable on the subject but I think you need to feed the CDU with more than 15V. 19V was recommended to me.
Lower power + bigger capacitors will make them take longer to charge.
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Bufferstop
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Re: Point motor CDU is weak

Postby Bufferstop » Wed Apr 14, 2021 11:47 pm

Without seeing the original cct and your modification this will have to be a guess, so-
There will be some resistance between the input terminal and the capacitor(s), this is to limit the inrush current when first connected. Think of it as a tap only partly turned on. The original resistance allowed the capacitor to charge up to the working voltage in a specific (short) time, like filling a bath. You've increased the size of the capacitor(s), the same flow now has three baths to fill, it's gonna' take a long time.
Go back to the original capacitance, it should work as intended. Having a bigger capacitance fully charged will make the first discharge much larger, does it need to be? If you are trying to fire multiple coils at once, unless you always have the same number to fire you will have too big a charge for a single coil.
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Re: Point motor CDU is weak

Postby Ironduke » Thu Apr 15, 2021 12:18 am

denfin wrote:Hi!

I have built a CDU following the diagram here: Image

Only difference with the one I made is I used three 4700uf 35v capacitors instead, cus I thought that would make it even more powerful, but it does not seem to work well, and is very weak. I tested a point motor with 16v AC and 19DC, and it seems to work quite good, but when I hook up the CDU to 16v AC or 19v DC, it gets much weaker. it seem to charge up quite slow also, the LED takes some time to light up at full power again. So what am I doing wrong here? should I not have used the bigger capacitors?


I'm surprised that image doesn't show up. Are gifs not supported?

For the the interest of others, the project is here http://www.talkingelectronics.com/proje ... rview.html

I think Bufferstop (John) has it. Larger capacitors take longer to charge, but I think it should still work. Did you make the kit or assemble the circuit on veroboard or something? Are you sure it's correct?

Do you have a photo of your circuit?
Regards
Rob

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Re: Point motor CDU is weak

Postby Suzie » Thu Apr 15, 2021 7:42 am

The circuit does not appear to be the finest CDU ever made. I think that it is being very conservative with how much current it draws from the 16V AC supply.

There are a few ways to improve it:-

1. Use a 24V DC switchmode power supply. These are not expensive and 24V is really what you need for a CDU.

2. Get rid of the 1N4004 diode on the input. If using DC you don't need it, and if using AC you need to replace it with a full bridge rectifier (4x 1N4004 will do if you have them). The other diodes don't do anything meaningful so they can go too.

3. The BD679 Darlington used as a current limiter to charge the capacitors at constant current is not as good as a simple transistor - you are losing some voltage. I would suggest replacing it with something like an MJE3055 to give a faster charging rate on the capacitors.

4. Using a transistor in this way does not give the fastest charging you can get, it slows charging considerably as the capacitor gets beyond half charged. It would work a lot better if you got rid of the transistor altogether (and the bank of four resistors) and just used a 20W 12V bulb. The circuit will need a complete redesign to get significantly faster charging with a transistor.

So to summarise, I would recommend that you replace the transistor with a bulb (12V 20W G4 capsule bulb will be good) , lose the diodes, and use a 24V regulated DC supply of at least 4A. You will see a big improvement.

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Re: Point motor CDU is weak

Postby RAF96 » Thu Apr 15, 2021 10:36 am

Increasing the input voltage will give you a bigger kick. The size and number of capacitors is storage capacity not oomph. Larger capacitors as pointed out take longer to recharge.

Water analogy...
Capacity is equivalent to flow rate, say gallons per hour, like a large bore hosepipe as opposed to a small bore hosepipe. Lots of water but no pressure.
Voltage is equivalent to pressure, say 75 bar as opposed to 25 bar. More voltage, more punch.
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Re: Point motor CDU is weak

Postby Flashbang » Thu Apr 15, 2021 12:22 pm

Hi
The diode in the main feed (D1) is there for two good reasons. A) On a DC supply it ensures positive is feed to the capacitors positive thus blocking accidental reverse polarity connection. B) On an AC supply it allows the positive half cycle only to reach the capacitors positive terminals. I would not remove it. But it could be replaced with a bridge rectifier if wished on an AC supply. Note the single diode only reduces the supply volts by 0.7 volt.

Have you tried removing one capacitor to see if the remaining one(s) recharge any quicker?

Increasing the supply volts will help too.

I always recommend that all wiring throughout where solinoid point motors are used is in a minimum of 16/0.2mm equipment wire (0.5mm2) or a larger wire size will never hurt. :D

If all else fails a kit self build CDU can be purchased for as little as £4.75 (kit 1 with 2 x 2,200uf) or £4.99 (kit 2 with 2 x 4,700uF capacitors) Other types are of course available. Link to low cost CDU 1 Or Low cost CDU 2 link
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Ironduke
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Re: Point motor CDU is weak

Postby Ironduke » Fri Apr 16, 2021 1:20 am

Suzie wrote:2. Get rid of the 1N4004 diode on the input. If using DC you don't need it, and if using AC you need to replace it with a full bridge rectifier (4x 1N4004 will do if you have them).


A single diode will drop the voltage by .7V. A bridge rectifier will drop the voltage by 1.4V. A bridge rectifier is recommended for circuits that require a smooth supply. This is not one of those circuits.

Suzie wrote:The other diodes don't do anything meaningful so they can go too.


The diode that is reversed across the output is there to sink the "flyback" voltage from the solenoid, which can reach hundreds of volts. It is fairly standard practice to put a diode across a solenoid in a DC circuit to protect the circuit from reverse voltage spikes. I would put the reverse diode directly on the output and not behind another diode (ie one diode not two). You could also put a flyback diode directly on the solenoid so the spike doesn't travel through the wiring.
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Re: Point motor CDU is weak

Postby Suzie » Sat Apr 17, 2021 12:37 pm

Ironduke wrote:
Suzie wrote:2. Get rid of the 1N4004 diode on the input. If using DC you don't need it, and if using AC you need to replace it with a full bridge rectifier (4x 1N4004 will do if you have them).


A single diode will drop the voltage by .7V. A bridge rectifier will drop the voltage by 1.4V. A bridge rectifier is recommended for circuits that require a smooth supply. This is not one of those circuits.


Yes it is. If fed with AC the charging time will double if a bridge is not used, on DC the diode does nothing useful. This is a circuit the requires a bridge or nothing.

Ironduke wrote:
Suzie wrote:The other diodes don't do anything meaningful so they can go too.


The diode that is reversed across the output is there to sink the "flyback" voltage from the solenoid, which can reach hundreds of volts. It is fairly standard practice to put a diode across a solenoid in a DC circuit to protect the circuit from reverse voltage spikes. I would put the reverse diode directly on the output and not behind another diode (ie one diode not two). You could also put a flyback diode directly on the solenoid so the spike doesn't travel through the wiring.


The flyback voltage is not going to reach hundreds of volts at that point. The only place it reaches hundreds of volts is across the coil where it might be a good place to put a diode. Don't forget what causes the high flyback voltage, it is when the switch is opened and therefore when there is no circuit via the CDU.

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Re: Point motor CDU is weak

Postby Ironduke » Sun Apr 18, 2021 12:38 am

Suzie wrote:Yes it is. If fed with AC the charging time will double if a bridge is not used,


ok

Suzie wrote:Don't forget what causes the high flyback voltage, it is when the switch is opened and therefore when there is no circuit via the CDU.


By its nature the the CDU sends a pulse of current which suddenly collapses and this happens while the control switch is closed. Obviously it would be better to have a diode directly on each solenoid but nobody does that. Having the diode on the CDU is a simple protection measure. It may even be redundant but it costs almost nothing.
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Re: Point motor CDU is weak

Postby Suzie » Sun Apr 18, 2021 7:35 am

Suzie wrote:Don't forget what causes the high flyback voltage, it is when the switch is opened and therefore when there is no circuit via the CDU.


By its nature the the CDU sends a pulse of current which suddenly collapses and this happens while the control switch is closed. Obviously it would be better to have a diode directly on each solenoid but nobody does that. Having the diode on the CDU is a simple protection measure. It may even be redundant but it costs almost nothing.[/quote]

Some of us do use a diode for each coil, 1N4148 diodes are literally ten a penny, so no good reason not to do it. The diode is there to protect the switch, since no one deliberately fires the solenoid until the CDU is fully discharged, the whole point of a CDU is to move the motor quickly, and it moves in a tenth of a second.

If you hold the switch until the CDU fully discharges, the current reduces to zero, and so the field in the coil reduces to zero, and there is no field to collapse and cause a back EMF. This really, really is not an issue. You might like to do some experiments - get a storage oscilloscope, have some fun and see what really happens - it is more fun than listening to wives tales.

denfin
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Re: Point motor CDU is weak

Postby denfin » Wed Apr 21, 2021 6:04 am

Thanks all your help! I tried with just one 2200uf capacitor instead (which is suggested on that website the diagram came from) but it still seemd a bit weak. And I need a CDU that is pretty strong that can operate 2 points at once also, seems like this one might not do the trick perhaps. I build it completly myself. Don't have another power supply tho, kinda hoped that I did not need to and this old 19v laptop one that I found for free would work. We will see what I do, maybe I buy some other supply or maybe I just buy a nice CDU instead, those on eBay that flashback linked looked pretty nice.


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