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Old power supplies - are not the best.....


newbryford
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For those thinking about using older power supplies for layouts with delicate electronics.... don't. (When I say older, I'm talking 1960s Hammant & Morgan with selenium plate rectifiers)

 

This is the waveform as seen on an oscilloscope (under 1.8A load)

post-408-0-83693500-1530142473_thumb.jpg

 

The supply was plated at 12vDC 2.5A. The no-load voltage was 17.8v

 

With a 1.8A load, this reduced to a shade under 14v.

 

(and still measured 7v or so with the meter on the AC setting)

 

DC my &rse.........

 

Cheers,

Mick

Edited by newbryford
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For those thinking about using older power supplies for layouts with delicate electronics.... don't. (When I say older, I'm talking 1960s

 

 

Yeah, but the sixties were great, if you can remember them.

 

Anyway, it looks to be DC, very slightly.

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That's quite a low reading, Hornby controllers of the same vintage are around 21 volts off load on minimum speed setting.  The 12 volt un controlled output will also be well over 12 volts of load.   The Duette was a budget controller at a premium price.  Many people carefully calculate the resistors required for LEDs and then find the controller delivers 21 volts not the 12 they were expecting.

Personally I would have the bottom off and check the wiring as the rectifiers fall apart and the 1970s wiring had really bad inulation 

Decent H&M variable transformers, safety Minors etc are around 3 volts off load minimum setting.   

The Duette gives reasonable starting a very notchy crawl and lousy speed control between minimum and flat out, while the Safety Minor isn't great for starting but very good for keeping a constant speed.  Early ones had a boost button giving a burst of 12 volts for starting, excellent when you get the hang of just flicking it to start the motor turning.

Probably best to use a Morley or similar if you want your motors to have a long life.  A Playcraft/ Jouef 1960/70s controller would also be a good choice, diode based voltage control either from batteries or 12 volt 1 amp transformer.

Edited by DavidCBroad
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Me thinks, looking at that waveform, if the controller ever had a capacitor inside, across its rectifier output, it has well and trully dried out and needs replacement..

Edited by TheQ
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Me thinks, looking at that waveform, if the controller ever had a capacitor inside, across its rectifier output, it has well and trully dried out and needs replacement..

Its that long since I opened one up, I can't remember if it even had a capacitor (well 2 really, one for each circuit).

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For those thinking about using older power supplies for layouts with delicate electronics.... don't. (When I say older, I'm talking 1960s Hammant & Morgan with selenium plate rectifiers)

 

This is the waveform as seen on an oscilloscope (under 1.8A load)

attachicon.gif20180627_1223481.jpg

 

The supply was plated at 12vDC 2.5A. The no-load voltage was 17.8v

 

With a 1.8A load, this reduced to a shade under 14v.

 

(and still measured 7v or so with the meter on the AC setting)

 

DC my &rse.........

 

Cheers,

Mick

 

Mick,

Jjust out of curiosity, does it have a center tapped secondary with two rectifiers or a single secondary winding and a bridge rectifier?

Thanks,

Andy

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Mick,

Jjust out of curiosity, does it have a center tapped secondary with two rectifiers or a single secondary winding and a bridge rectifier?

Thanks,

Andy

 

No idea - it wasn't mine. I wasn't opening it up. Suitable advice will be given to the owner of the unit.

 

Cheers,

Mick

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A capacitor wouldn't help. The output is clearly not even rectified.

 

I don't know why I didn't spot that, clearly It is an AC output.    Was the reading taken from the AC output?     I can't think it can be from one of the controlled DC outputs or the uncontrolled DC as I have never known one one of these selenium rectifiers to fail other than open circuit, quite often they fail and only leave a half wave output.

 

These days after getting an electric shock from a light switch in a Pre School class room, I trust no one and wont plug a metal cased controller in to the mains without having the bottom off and making sure there is no live wire about to touch the case.

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I don't know why I didn't spot that, clearly It is an AC output.    Was the reading taken from the AC output?     I can't think it can be from one of the controlled DC outputs or the uncontrolled DC as I have never known one one of these selenium rectifiers to fail other than open circuit, quite often they fail and only leave a half wave output.

 

These days after getting an electric shock from a light switch in a Pre School class room, I trust no one and wont plug a metal cased controller in to the mains without having the bottom off and making sure there is no live wire about to touch the case.

 

As in the OP - the output is plated 12vDC........ Those are the only two connections on the power supply

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As in the OP - the output is plated 12vDC........ Those are the only two connections on the power supply

If it was the 12 Volt DC terminals, then something has clearly failed and not a design fault.

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I don't know why I didn't spot that, clearly It is an AC output.    Was the reading taken from the AC output?

Check where the wave peaks & troughs; about +6 & -9, so a motor would run in reverse but may have a short life.

Not all of us have access to an oscilloscope. I guess we would assume that the waveform should be a lot smoother than this & certainly not alternating, but this is the first time I have ever seen the output from such a device. Without another sample there is no way to tell if this is faulty or normal.

 

I am not keen on older H&M controllers anyway because they have very poor slow speed control. I certainly won't let any of my locos near one now.

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  • 1 month later...

Me thinks, looking at that waveform, if the controller ever had a capacitor inside, across its rectifier output, it has well and trully dried out and needs replacement..

 

I agree. It looks like the DC ripple filter cap has failed or is missing.

Edited by Baby Deltic
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