Setting Up For High Voltage Live Testing

[stevekasian]

Hi,

I am looking for some advice:

I need to start testing boards under power that have live high voltage sections on them. Until now, I've only been checking powered boards while installed in their parent housing. But this only works in certain instances. I am going to need the ability to power these boards on a bench, so that I can access both sides of the board with test equipment while it's live.

I'd like to know what is the best and safest way to go about powering an LCD TV power supply board out of it's parent unit.

Thanks!
SK

[Wizard]

Re: Setting Up For High Voltage Live Testing

Isolation tranformer

Quality DMM, scope.

And safety practice enforced all the times.

Cheers, Wizard

[stevekasian]

Re: Setting Up For High Voltage Live Testing

Just Googled Isolation Transformer and learned about it. Is there any particular one anyone recommends that is low cost and reliable?

Not quite ready for the scope yet. But for future reference, can anyone advise on what frequency range will be needed for this type of repair work?

Have the DMM!

Thanks!
SK

[Toasty]

Re: Setting Up For High Voltage Live Testing

Isolation Transformers have never been low cost.

You have to obtain one that will handle the load of what you are testing. I recommend one that has thermal circuit breaker or similar device. Fused outputs are ridiculous as testing equipment with a problem will cost fuses until you find the problem. Something in the range of 150-250 watts for starters.

Try to find one with a varaible output or at least multiple taps. Used in conjunction with a Variac of equal or better rating, it makes a great way to slowly bring voltage up on troublesome equipment.

New, you're looking at near $100 or more in an entry level unit. Used can be had for less, but no warranty or repair if it fails.

Strictly an iso with little or no variability would be best had from B&K Precision. They have 2 units.

http://www.bkprecision.com/products/...nsformers.html

Toast

[severach]

Re: Setting Up For High Voltage Live Testing

If all you want to do is run while the boards are hooked to the monitor then all you need is cardboard, a magazine, or a padded envelope to keep the live backs from touching any nearby metal. If you need to tip a board up for testing any bar clamp such as HF 46808 will do. Once you have the high voltage sections protected from you and everything else it's no big deal.

[Krankshaft]

Re: Setting Up For High Voltage Live Testing

I have this one:

http://www.mcmelectronics.com/product/72-6670

65 bucks it's rated at 450 volt amps continuous 550 VA intermittent. This things a beast 16 pounds of iron and copper . It is fused on the output however the external fuse port could be removed and a thermal breaker could be installed in it's place.

450/120 volts= 3.75 amps.

3.75 amps * 120 volts= 450 watts.

That's how to do volt amp calculations since most isolation transformers won't have an amps or watts rating.

I use their high voltage probe with integrated meter for measuring the anodes of CRTs their equipment seems to be well made.

I did look at the multiple tap isolation transformers when I was buying but if I need to vary line voltage I plug a variac into the transformer.

With a variac you actually vary the turns ratio to get the exact voltage you want on the output despite the input voltage.

On those switchable tap isolation transformers assumptions are made about the incoming line voltage. If the transformer primary is rated for 110, 117, or 120 volts then that's what they based the secondary turns ratio of the transformer taps off of. Unless the input line voltage is at that exact specified primary voltage the voltages from the secondary taps will be slightly higher or lower.

Not that the variations are overly important just something to be aware of if you need exacting control.

[Jack Crow]

Re: Setting Up For High Voltage Live Testing

SK,
Had a similar problem when I was working in Kuwait.
What I needed was not always handy to get.
Had to improvise and build my own tools.

At that time I was fixing electronics used in military generators.

One of the things I made was a 'current limiter'.
On the hot leg of my variac / isolation transformer I had a switch that would bring in and out of circuit a 25 watt lamp. So on first turn on, current was limited. Cuts down on the burning smell if something goes wrong.

In this photo you can see my two variacs (large and small).
The black box on the far right of the bench is a modified 'voltage stablizer'.

A common item in Iraq and Kuwait.

It has a motor driven variac, so when the voltage sags or surges, the motor drives the variac wiper arm up and down to a preset output voltage.
It was in this unit I put my current limiter bulb after the sensing point.
http://www.flickr.com/photos/9010884...7604808002766/

Another tool I had was an AC Power Supply. It would make a regulated AC voltage and frequency, for testing things. Keep in mind my shop was a military repair facility, and I had to support 400hz power systems as well as the more normal 50 and 60hz items.
http://www.flickr.com/photos/9010884...7604808002766/
Sometimes you get luckey and find one cheap on Ebay.

Unless your going to do power supply units for a living and have to test them across the whole range of input, this is clearly over kill for a home shop. For what I was doing, it was needed. I think it was good for about 200 watts.
At this time I still do not own one.

This is a Military 400hz AC voltage regulator
http://www.flickr.com/photos/9010884...7604808002766/
See how small the main transformer is.

Similar unit for a Military 50/60hz generator of the same size (15 and 30kw)
http://www.flickr.com/photos/9010884...7604808002766/
Big heavy chunky transformer.

I have worked for places that made PAV's (Phase Angle Volt Meters) and they used big audio amps and an HP signal generator as exciter voltage.
From that all the benches had access to 200 VAC at 400hz. Not too much current if something shorted or a finger got across the line. A short on the output would not kill the big Macintosh tube amp that was supplying the grunt.

The items I repaired on this bench (none visible at the moment) were motor controllers used in Army air conditioners. The input to the analog board was 248vDC. The heathkit regulated power supply gave just enough DC to make the switching supply on the board work properly.

http://www.flickr.com/photos/9010884...7604808002766/

For those boards I had to use portable (aka battery powered) test instruments to avoid shorting the low side to ground.
So battery operated scopes and hand held DMM's were the rule here.

So what you want can be done, just need to be good at it.

Depending on your budget there is a neat tool made my Sencor. It's a variac, isolation xfmr, and electrical leakage tester. It was made for the TV repair business to test that sets w/o transformers did not zap clients. It has a circuit breaker on the front in the event of over load. Still looking for one.

Be well there and much luck.

Jack Crow aka Radio Mike
Herndon VA

[stevekasian]

Re: Setting Up For High Voltage Live Testing

Hi Jack Crow,

I've been away for quite some time here and am just getting back to the world of repair. I wanted to thank you for taking the time to post this detailed and informative entry. Sorry I haven't been around to acknowledge your help until now, but it is appreciated and I will no doubt find it quite useful in the near future. ;-)

Thanks again,
SK

[KD3RF]

Re: Setting Up For High Voltage Live Testing

<On the hot leg of my variac / isolation transformer I had a switch that would bring in and out of circuit a 25 watt lamp. So on first turn on, current was limited. Cuts down on the burning smell if something goes wrong.>

An old time Ham operator taught me a similar trick when troubleshooting an unknown piece of equipment.

He wired a 100W light bulb in series with the AC line between the isolation transformer and the item under test.

Even if there was a dead short to ground, all that would happen is the light bulb would light-up. Simple trick saved many a fuse and blown circuit breaker that way.

Initial in-surge would light the bulk momentarily, and then it would gradually dim-down as the device under test came alive.

[Th3_uN1Qu3]

Re: Setting Up For High Voltage Live Testing

A 100W bulb is always in series with my mains when i'm testing a SMPS. 100W is still a bit of power and it'll still let out the magic smoke if there's something wrong and you don't spot it, but the optical indication of the bulb is enough to keep that from happening.

It gives you enough power for the PSU to come up fully if it's working right, so it's great for troubleshooting a unit that intermittently blows fuses. Or transistors for that matter, since in most SMPS the transistors die to protect the fuse...

[kc8adu]

Re: Setting Up For High Voltage Live Testing

called a dim bulb tester.saved a bunch of parts that way.even when firing up antique radios and vintage ham gear that has been sleeping 50+ years.