How did I just destroy my multimeter? And my power strip?

[Zben3129]

So, this involves tube amp voltages, pretty good at destroying DMM's I guess

What I was trying to do was measure the secondary of an unknown power transformer from a salvage amp. It was specced at 380-0-380, but I'm pretty sure it was designed around 110v line voltage, not 120. Anyways, the primary was listed as blue-blue, the heater secondary as purple-purple, and the HV secondary as Red - Red/Yellow - Red.

Enter colorblindness. These were the old cloth covered wires, and the blue and purple looked very similar, except I was 99% sure I was holding the blue's

So, I hooked up the primary to the 'blues' and hooked up my DMM, set for 750vAC unfused range, across a Red and Red/Yellow.


I then flipped the switch and all hell broke loose. I heard sizzling behind me, turned around and saw a large spark come from either the wall outlet or the powerstrip I was plugged into. Also, I think my multimeter made a bad noise. Immediate power down.

Now how did that cause the death of my multimeter and surge protector? Shouldn't there have been 0v on the HV secondaries as I was not applying any voltage to the primary?

I think because the 120v on the 6.3v secondary had "nowhere to go" to speak it just shorted, therefore shorting the line voltage from wall. How does this fry a surge protector?

And for the multimeter, now all it does is read "1888" no matter what range it is on, and shows a low battery symbol. Also, it only stays on for abnout 10 seconds before fading out. Have tried changing batteries.


PS the transformer is fine, go figure


Please help me figure some of this out, so as to not make mistakes like this again

[beans_amps]

Any winding can be used as a primary or power input.  When I say that I mena that theoretically the transfomer does not care. 

What happened in your case was that when you put 120 Volts on the 6.3 Volt winding, The 380 volt secondary then generated the output voltage of the new transfomer ratio of 6.3:120 (17 to one) instead of the expected 120:380  (6 to one) .  That means the secondary tried to generate 12kV or so.  This voltage will arc about an inch with no problems.  12kV will also easily kill the input of most any meter. 

Sean

[Jered]

  I don't understand why the reset on your power strip didn't trip?

[davent]

I recall reading of people using a wallwart outputting a low AC voltage to power unknown transformers while testing, rather then the full wall output, to avoid extreme surprises!

dave

[Zben3129]

I recall reading of people using a wallwart outputting a low AC voltage to power unknown transformers while testing, rather then the full wall output, to avoid extreme surprises!

dave

I almost always do this too  The one time I don't it bites me. Except using my usual method of a 24v transformer would have still resulted in 6kV, still way too high.

Also, I knew that you could use a transformer backwards using secondary as primary, but I never knew that applying voltage to one secondary generated voltage on the other secondary.


And the reset button doesn't do anything, I somehow destroyed the strip.

Dead meter or something I can replace?


Thanks

Zach

[Paul Perry (Frostwave)]

Whenever you are testing an old unknown transformer, don't connect it to the mains.
Connect it to a 6.3V Ac source (from an AC output plugpack or transformer).
Then you can measure the voltages on the other windings & get the ratios.

Another hint: if you put a multimeter (with a very high imput impedance) across tow transformer outputs that aren't even connected, you will get some kind of reading because of capacitive effects. So use the resistance scales to check what is what, before applying voltage anywhere..

[George Giblet]

Paul's right on the money.  The worst thing you can do is start plugging unknowns into the mains - it doesn't matter what it is, don't so it.  For the transformers, once you have a good idea what the windings are *then* you connect it to the mains and measure the precise voltages.

Regarding your meter, the 1888 thing looks like something is really fried.  I have my doubts it is fixable.  That's not to say it isn't fixable but it's going to take you a fair amount of investigative skills and perseverance to get any idea at all..   The best hope is to find someone who has posted something on the web for you model or brand.  You might be able to send it back to the manufacturer, they might even swap it - who knows, you can be lucky.

[Paul Marossy]

  I don't understand why the reset on your power strip didn't trip?

I don't think that the reaction time on those is quick enough to do anything in a case like this one.

[GibsonGM]

I had that happen with, luckily, a 120/9V transformer.  It was trying to give me 1.6kV...luckily I wasn't metering it, and it only browned my lights out a bit  (had no load on it, only magnetizing current was flowing).  A good lesson there; I like Paul's idea of apply 6.3VAC to an unknown before regular mains...

[Zben3129]

Thanks for the info guys,

I had already measured it with a 12.6v transformer and saw 45v 0 45v on the HV secondary and .4v on the heater secondary. Only problem was I mixed up the colors due to colorblindness when I was switching from the transformer to wall voltage.


I'll just buy a new meter, this was only a 4 dollar futurlec one luckily (or was it harborfreight?).

And do surge protectors prevent an appliance from drawing too much current? Possibly the surge they refer to is a surge in your mains line and keeping that out of your appliances, like if your house gets hit by lightning for example.


Zach

[George Giblet]

> And do surge protectors prevent an appliance from drawing too much current? Possibly the surge they refer to is a surge in your mains line and keeping that out of your appliances, like if your house gets hit by lightning for example.

I suspect most don't but some may.

Without getting complicated, with mains you basically have two options outright fuses, circuit beakers, and PTC type resettable fuses.   There's may be laws in your country regarding the use of, or type of,  PTC fuses on mains.

Fuses and circuit breakers are often slow to act.

[Ronsonic]

Here's another tip and a worthy project for any bench that works on line powered stuff - use a safe light.

Wire up an electrical box with an ordinary household light switch and a pair of outlets wired so that the two outlets are in series. Wire the switch to bypass one of the outlets. The outlet with the bypass switch gets a 40, 60 or 100 watt bulb plugged into it. The other outlet is for the device under test. With the bulb in series it will act as a current limiter and a diagnostic aid. Bypass the lamp with the switch for normal operation.

These are great for powering up old gear with suspect caps and parts or running up new builds for the first time.  Very useful for solid state power amps so you don't sacrifice parts in testing. Highly recommended for any operations involving anything that could be called a "death cord."