Working Voltage of Test Equipment

[Chrispy57]

This morning, I decided to rearrange a section of the workbench to accommodate my Phillips PM3055 scope that had been in storage for about 5 years. Being rather (over)cautious, I thought I'd run it up slowly on the variac, and was surprised to see it burst into life at around 75Vac!

Checking the rear panel text it shows 100 - 240Vac as the working voltage, I just had never noticed or thought about it being other than simply UK mains voltage.

This got me thinking whether it might be a good idea to run such test equipment from a lower "mains" voltage - perhaps via a 240V to 120V auto-transformer that most of us probably have lying around. Does anyone actually implement this measure to reduce the stress on ageing capacitors etc in their own test equipment?

Cheers
Chris

[factory]

The PM3055 uses a SMPS, probably wasn't a good idea to use a variac, low mains voltage will equal higher current through the switching transistor.
I have one in the repair queue that will be getting replacement caps, we had one at work that PSU failed short, it was in regular use until it died, probably dried out caps.

A lot of my older TE uses transformer power supplies, I often think about the high mains voltage these days (245V-250V) and think it would be better nearer the 220V to 230V that the TE was designed for, no easy way to do this though.

David

[Sideband]

A typical SMPS works harder at low voltage.....think about it....the switching transistor/FET has to stay switched on longer to produce the same output. This is controlled by (usually) an optocoupler and a comparator circuit that controls the mark-space ratio of the switching transistor. Lower voltage increases the 'on' time of the transistor, and conversely a higher voltage reduces the 'on' time. Well designed SMPS should be able to cope with their voltage range with no problems but some are better than others.....

[The Philpott]

Conversations about UK voltage tend to get shut down, but- in addition to the SMPS comments already made, i'll just say that theoretically we are harmonised @ 230v and if this were to happen in practice over time (rather than just being stated) it would be a good thing for some older equipment.
We also have to hope that frequency stability continues to be maintained, but that's another subject.

Dave

[Oldmadham]

Originally Posted by factory

The PM3055 uses a SMPS, probably wasn't a good idea to use a variac, low mains voltage will equal higher current through the switching transistor.
I have one in the repair queue that will be getting replacement caps, we had one at work that PSU failed short, it was in regular use until it died, probably dried out caps.

A lot of my older TE uses transformer power supplies, I often think about the high mains voltage these days (245V-250V) and think it would be better nearer the 220V to 230V that the TE was designed for, no easy way to do this though.

David

In Western Australia, for complex historical reasons, the Mains supply was, for many years standardised at 250v.
There was no obvious effect upon the longevity of Mains operated equipment.

[G6Tanuki]

I wouldn't be paranoid about it.

Most of my gear will probably still be working when I am finally discharged-and-grounded.

Unless you are working in an environment where test gear is powered up 10 hours a day 6 days a week, and where a failure means that you start missing contracted turnround times for repairs...


If you are only using kit for a few hours at a time, on an irregular basis, my greater worry would be about the ingress of damp, spiders and the like.

(those tiny thunder fly things have been a real nuisance this year, they are just small enough to get in between variable capacitor plates but can't handle a few hundred Volts...)

[kalee20]

Unless you know the design of the equipment, it's best to operate it in its rated voltage range.

If it's a wide-ranging SMPS, then as already said, currents will be higher at low mains... Voltage will be higher at high mains, so operate it somewhat lower than the maximum, but away from the bottom end.

If it has a mains transformer, then it'll probably be tapped to suit (maybe several!) mains voltages, in which case as long as you use the appropriate tap, the main equipment circuitry will see the same conditions regardless.

Valve heaters, linear regulators, panel lamps, do have an easier life if voltage is reduced, but they still need enough to do their job!

[Neutrino]

Running a SMPS at 120V instead of 240V would approximately double the current taken from the mains. This would approximately double the ripple current in the input filter and increase heating in the electrolytic capacitors, especially if the ESR has increased due to aging.

As mentioned, it’s best to check the rating plate. The valve heater voltages in a piece of my vintage test equipment were too high. I fitted a small auto transformer inside to reduce the mains voltage from about 245V to the rated 220V.

David

[Herald1360]

A relatively small 20V secondary transformer can be used to "buck" the mains supply either up or down by 20V depending on which way round its secondary is connected in series with the mains live side. A 20V 100VA TX would suffice for an up to 5A mains supply.

[The Philpott]

Yes- Clive covered a similar dodge on youtube using a 12v tx normally used for MR16 GU5.3 halogen downlighters. Dropped 240v to around 228v and able to deal with a reasonable amount of power. The first time he tried it he wired it wrong and got a different result!

Dave