Multi Voltage Power Supply Unit

[WhiteRose158]

Good evening all,

 

Having just broken the 16v supply I was using for my point motors, I'm on the lookout for a replacement and possibly something that can provide a solution for a few power-related needs on my layout.

 

I've come across this and wonder if it would be suitable. I see that the max output from it is 15v....would that be sufficient for points motors? Also, having an extra 12v output is always useful, but as I'm using a point motor system whereby I touch a probe to a connected up screw, delivering a momentary jolt of power to send the point motor over, I'm wondering if at the point of executing a point switch that it would affect the other output supplies (e.g. a quick dimming of connected lights). Not sure if that is something that could happen, but I'd rather check for any advice before forking out the money.

 

Any help is much appreciated

 

 

[cliff park]

Two thoughts:- you do realise this is not a mains unit, you would need a transformer as well;- you can get round the instantaneous load problem (which could also cause protection problems) by using a capacitor discharge unit. These also protect point motors.

[WhiteRose158]

41 minutes ago, cliff park said:

Two thoughts:- you do realise this is not a mains unit, you would need a transformer as well;- you can get round the instantaneous load problem (which could also cause protection problems) by using a capacitor discharge unit. These also protect point motors.

Hi, yes, I've sourced a 24v transformer that could be used with it.

 

I've got a CDU in place, which I used with the previous power supply (an old cut Hornby transformer, I think it was).

[steve W]

I have just looked at the picture of the proposed power supply and have some concerns regarding the unit itself.  The specification shows an input voltage (AC or DC) 12 - 24V.  The input circuitry appears (but needs to be confirmed) to be a full wave bridge rectifier (black rectangle at top right) followed by a 4700 uf smoothing capacitor.

A standard wire wound transformer used as a supply will have a no load voltage several volts higher than 24V, making the the peak DC voltage on the capacitor of 32V or higher.  The 25V capacitor at this supply voltage is woefully underated and should be a 50V rated component for the 24V supply.

The effect of point motor operation on other circuits is minimised by using a CDU, but does depend to some extent on the overall scheme including the type of regulators fitted on the power board.

 

[DCB]

I  don't see much point to this unit, 15, 12, 9, and 6 volts aren't that useful    15, 5, and 3 are more useful, though the 1 amp max sounds good.   A computer PSU and a couple of polyswitches would do a better job and they can be obtained for free from scrapped tower computers

[AndyID]

8 hours ago, steve W said:

I have just looked at the picture of the proposed power supply and have some concerns regarding the unit itself.  The specification shows an input voltage (AC or DC) 12 - 24V.  The input circuitry appears (but needs to be confirmed) to be a full wave bridge rectifier (black rectangle at top right) followed by a 4700 uf smoothing capacitor.

A standard wire wound transformer used as a supply will have a no load voltage several volts higher than 24V, making the the peak DC voltage on the capacitor of 32V or higher.  The 25V capacitor at this supply voltage is woefully underated and should be a 50V rated component for the 24V supply.

The effect of point motor operation on other circuits is minimised by using a CDU, but does depend to some extent on the overall scheme including the type of regulators fitted on the power board.

 

 

I agree. A 25V cap is likely to overheat and possibly explode.

[WhiteRose158]

Thanks for all your replies. So my proposed solution wouldn't be as simple as I originally laid out it seems. I'm trying to make everything as simple as possible as I'm pretty low on circuitry knowledge beyond the basics. 

 

I have an NCE Power Cab with which I power the track, I already have a dedicated simple 12v plug that will power my lights (a few LEDs, buffer stops, etc) and just need something simple for the point motors. 

[WhiteRose158]

Would this be a suitable power supply for the point motors (run through a CDU, of course)? It's more or less what I was using before but not for any great length of time, so I'm not sure if it would be unsuitable long term.

 

https://www.actionslotracing.co.uk/products/scalextric-mains-power-supply-transformer-adaptor-c990-16v

[divibandit]

On 28/04/2021 at 04:17, DavidCBroad said:

I  don't see much point to this unit, 15, 12, 9, and 6 volts aren't that useful    15, 5, and 3 are more useful, though the 1 amp max sounds good.   A computer PSU and a couple of polyswitches would do a better job and they can be obtained for free from scrapped tower computers

David,

I have a computer PSU with both 12v and 5v outputs and wondered about using this for model railways but I've been told it's not as simple as it sounds. Can you advise, please?

 

Steve

[melmerby]

On 27/04/2021 at 20:36, steve W said:

I have just looked at the picture of the proposed power supply and have some concerns regarding the unit itself.  The specification shows an input voltage (AC or DC) 12 - 24V.  The input circuitry appears (but needs to be confirmed) to be a full wave bridge rectifier (black rectangle at top right) followed by a 4700 uf smoothing capacitor.

A standard wire wound transformer used as a supply will have a no load voltage several volts higher than 24V, making the the peak DC voltage on the capacitor of 32V or higher.  The 25V capacitor at this supply voltage is woefully underated and should be a 50V rated component for the 24V supply.

The effect of point motor operation on other circuits is minimised by using a CDU, but does depend to some extent on the overall scheme including the type of regulators fitted on the power board.

 

Coming back to this.

It would be a whole lot more than 32v if the input is from a nominal 24v transformer

If a transformer is rated at 24v on load it could be at least 20% higher off load =28.8v

RMS peak = 28.8 x 1.414 = 40.7v

Less diode drop of about 1.4  = 39.3v

A 25v electrolytic will fail possibly explosively at that voltage.

 

IMHO that unit is designed by someone who doesn't know what they are doing.

 

[Michael Hodgson]

45 minutes ago, melmerby said:

IMHO that unit is designed by someone who doesn't know what they are doing.

 

It's probably just about OK for what it does provided the input voltage is only 16v.

 

I don't like the narrative description.  The output to terminal 5 seems to be simply the AC or DC input, though it says the output is "protected on the positive pin by diode 5".  There isn't a positive if it's AC and I don't see 5 diodes (counting bridge rectifier as Nos 1-4), since the red thing at the left looks like an LED to show that the device has power.   Interestingly the "wiring instructions" show the bridge rectifier as 4 separate diodes.

[Michael Hodgson]

On 01/05/2021 at 08:28, WhiteRose158 said:

Would this be a suitable power supply for the point motors (run through a CDU, of course)? It's more or less what I was using before but not for any great length of time, so I'm not sure if it would be unsuitable long term.

 

https://www.actionslotracing.co.uk/products/scalextric-mains-power-supply-transformer-adaptor-c990-16v

That seems to be a simple transformer - AC in, AC out with the secondary rated at 800mA.

Whether that is suitable for your CDU will depend on whether your CDU accepts AC input or not.

[melmerby]

26 minutes ago, Michael Hodgson said:

It's probably just about OK for what it does provided the input voltage is only 16v.

 

I don't like the narrative description.  The output to terminal 5 seems to be simply the AC or DC input, though it says the output is "protected on the positive pin by diode 5".  There isn't a positive if it's AC and I don't see 5 diodes (counting bridge rectifier as Nos 1-4), since the red thing at the left looks like an LED to show that the device has power.   Interestingly the "wiring instructions" show the bridge rectifier as 4 separate diodes.

Even with 16v AC you are close to the peak voltage topping the cap rating. Needs a 35v rating with 16v AC, needs a 50v rating with 24v AC

 

The diagram also has a diode to protect the straight through connection!

 

I'm sorry, I wouldnt trust something so basically "wrong" in design.

 

 

[kevinlms]

2 hours ago, melmerby said:

Even with 16v AC you are close to the peak voltage topping the cap rating. Needs a 35v rating with 16v AC, needs a 50v rating with 24v AC

 

The diagram also has a diode to protect the straight through connection!

 

I'm sorry, I wouldnt trust something so basically "wrong" in design.

 

 

I'm also curious as to how it steps down the other outputs.

It says that each output can have a maximum of 1 Amp at it's rated voltage, with an AC input capacity of 4 Amp.

There is only a single resistor on the whole circuit board.

[AndyID]

That thing should not be for sale and I don't think it's a cheap import either. Looks like it's from the UK.

[melmerby]

42 minutes ago, kevinlms said:

I'm also curious as to how it steps down the other outputs.

It says that each output can have a maximum of 1 Amp at it's rated voltage, with an AC input capacity of 4 Amp.

There is only a single resistor on the whole circuit board.

And some 78XX regulators which will do the fixed voltages.

There are no heat sinks and if you have a rectified DC voltage of 25 (at least) and draw 1A from the 6v output, that regulator needs to drop 19v at 1A which equates to 19W. (enough to make the regulator glow!)

The whole think is poorly designed.

34 minutes ago, AndyID said:

That thing should not be for sale and I don't think it's a cheap import either. Looks like it's from the UK.

Agreed

[melmerby]

This so called "Multi Voltage PSU" shows why it is dangerous for those with little relevant knowledge to trust what they see on the 'net.

Fortunately it is not a piece of mains equipment, unlike the belt sander being discussed elsewhere that had a dangerous & illegal mains lead.

[DCB]

17 hours ago, divibandit said:

David,

I have a computer PSU with both 12v and 5v outputs and wondered about using this for model railways but I've been told it's not as simple as it sounds. Can you advise, please?

 

Steve

Most important thing is the Computer PSU can deliver a lot more amps than is healthy for model railway use.   Polyswitches are cheap and nasty  overload cut out devices, they stop conducting when overloaded and start again when the  overload is removed.  Many off the shelf model railway power units use them, as they cost about 20p where a decent re settable overload must be about a tenner.   If you use 1 amp polyswitches in circuit you should be fine on 12 or 5 volt circuits.

The other issue is the 12 volt output is 12 volt as measured  on a digital voltmeter, whether on load or off load. Most model railway 12 volt equipment is nearer 16 volts, some is over 20 volts off load so trains run slower and CDUs charge slower and to a lower state of charge.   But PSU's Voltage doesn't drop when you draw a couple of amps, in the way model railway units drop from 21 to 7 volts when you draw a couple of amps, They  are designed to work 24/7, they have cooling fans and as long you are careful can supply auxiliary power inexpensively, but use overload protection, don't just connect the wires direct, unless you are doing a bit of welding.

[kevinlms]

2 hours ago, DavidCBroad said:

 But PSU's Voltage doesn't drop when you draw a couple of amps, in the way model railway units drop from 21 to 7 volts when you draw a couple of amps, They  are designed to work 24/7, they have cooling fans and as long you are careful can supply auxiliary power inexpensively, but use overload protection, don't just connect the wires direct, unless you are doing a bit of welding.

Nothing unique about computer PS, any properly regulated power supply will maintain the voltage, up to the rated current of them.

[melmerby]

4 hours ago, kevinlms said:

Nothing unique about computer PS, any properly regulated power supply will maintain the voltage, up to the rated current of them.

I've got several of the wall mount supplies surplus after the equipment they were made for died, or was junked.

They are very stable, being Switched Mode Power Supplies (SMPS)

Some useful voltages as well - a couple of 12v DC, a 5v DC but also a 22v DC!

[kevinlms]

6 hours ago, melmerby said:

I've got several of the wall mount supplies surplus after the equipment they were made for died, or was junked.

They are very stable, being Switched Mode Power Supplies (SMPS)

Some useful voltages as well - a couple of 12v DC, a 5v DC but also a 22v DC!

Yes, it's amazing the variety of voltages and ratings, for devices that do the same job.

 

You have to be careful about the ratings and check properly.

 

I was called back to a customer, because allegedly the work I did for her didn't work and she had 2 replacement modems sent out by her service provider. Both with the same non result - the lights came on, but no internet. So it had to be my cabling, which was working fine when I left.

 

On re-attendance I picked up one of the new modem boxes and it was heavy, because it still had the power supply in it. I checked its ratings and what was on the modem rating plate and the plug pack being used. Hmmm.

 

Yes, while both fitted and looked identical and were 12 Volts DC, but the old one was rated at 1 Amp and the later one at 1 1/2 Amp. So plugged in the new one and Bingo, it works perfectly.

Apparently, it had taken over two weeks of no internet & countless hours on the phone to her provider, before she asked me!

I guess the fact all the lights came on correctly, but no one home fooled everyone!

 

 

[divibandit]

On 11/05/2021 at 02:51, DavidCBroad said:

Most important thing is the Computer PSU can deliver a lot more amps than is healthy for model railway use.   Polyswitches are cheap and nasty  overload cut out devices, they stop conducting when overloaded and start again when the  overload is removed.  Many off the shelf model railway power units use them, as they cost about 20p where a decent re settable overload must be about a tenner.   If you use 1 amp polyswitches in circuit you should be fine on 12 or 5 volt circuits.

The other issue is the 12 volt output is 12 volt as measured  on a digital voltmeter, whether on load or off load. Most model railway 12 volt equipment is nearer 16 volts, some is over 20 volts off load so trains run slower and CDUs charge slower and to a lower state of charge.   But PSU's Voltage doesn't drop when you draw a couple of amps, in the way model railway units drop from 21 to 7 volts when you draw a couple of amps, They  are designed to work 24/7, they have cooling fans and as long you are careful can supply auxiliary power inexpensively, but use overload protection, don't just connect the wires direct, unless you are doing a bit of welding.

Thanks David,

I'm not planning to run diesel locos fast anyway so that's not a problem.

 

Steve