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Old 8th January 2009, 07:11 PM   #1
wicked1 is offline wicked1  United States
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Default Basic testing/meter question

I've got a basic question, but I don't think I want to find out the answer for myself...

Is it safe to test high voltages (b+ voltages) w. a digital meter by touching a point on the circuit, as it is w/ low voltage? I want to take some measurements around my power supply, before and after certain resistors, etc. It would be nice if I could just touch those points and take a reading, but I don't want to see any arcs or hear any pops!!

(and by safe, I dont mean ideal situation safe, or as safe as you'd let your child do, but safe enough that I probably wont kill my amp or myself)

Thanks!
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Old 8th January 2009, 07:34 PM   #2
Zibi is offline Zibi  Poland
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Safety Practices, General and Ultra-High Voltage
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Old 8th January 2009, 07:37 PM   #3
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Hi

Probes are isolated up to certain voltage, which value is most generally engraved in the insulation (should be around 1000VDC). A voltmeter has a high impedanca input (in the megaohm range), so you will not be pulling arcs with it.

Measuring DC in circuits is best down by attaching the probes to the points to be tested, turn on the amp, read the value, shut down the amp, let the capacitors bleed and then test a second point. You can also use multiple DMM's...

...why am I writing all this?? Tubelab has everything described over here

http://www.tubelab.com/MeterUse.htm

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Old 8th January 2009, 08:01 PM   #4
wicked1 is offline wicked1  United States
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Yeah, I know.. but tubelab also has many posts talking about taking point readings while the amp is hot... and a few about frying components while doing it.

So from that, I get the idea that it's somewhat ok to do. I just wanted to see how other people feel about it.

and I fully understand that what someone does on their own might be totally different from what they'd recommend to other people.

I'm not new to electronics.. I've built a modular analog synthesizer over the past 10+ years which takes up an entire wall of my basement. That, however, is a max of ~12v.
I just have little faith, and when it comes to high voltage, which I almost never work with... I think its better to ask!
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Old 8th January 2009, 08:18 PM   #5
Svein_B is offline Svein_B  Norway
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I recommend a good insulated aligator clip for your - test lead. This allows you to use only one hand to poke around among high voltages with the + test lead (or oposite if you are measuring negative voltages).
Some good small test clips for your main test probe are also recommended. Then you can attach the test lead where required and left unattached when you do any adjustments etc.

SveinB.
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Old 8th January 2009, 08:19 PM   #6
wicked1 is offline wicked1  United States
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thanks, yeah, that is no problem. I have plenty of clips, jumpers, etc. It's just a pain.. well, time consuming to power down the amp and let the caps drain after every individual test. Would take me an hour to do what could otherwise be done in a few minutes.
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Old 8th January 2009, 08:43 PM   #7
Jeb-D. is offline Jeb-D.  United States
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Meters usually have the max voltage rating listed where the probe terminals are. If you guesstimate the voltage should be less than the rating on the meter, you should be fine.

You don't have to power it down after every measurement. As Svein said, clip the negative probe to ground on the circuit and poke around using the + probe with one hand to measure voltages. Keep the other hand by your side. Do not wear or touch any kind of grounding items, such as wearing a wrist strap or resting your hand on the chassis. Wrist straps are desirable for low voltage ESD sensitive stuff, but only create safety hazards when working with high voltage.

Just think of yourself as a resistor. If there is a potential across you, current will flow. But if one side floats, no current flows. You will get a jolt if you touch something live(because your feet are on the ground, or your sitting on something that is touching the ground) but you should live to tell about it. The insulation resistance between you and ground is pretty high under most conditions.
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Old 8th January 2009, 08:52 PM   #8
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Quote:
Originally posted by wicked1
thanks, yeah, that is no problem. I have plenty of clips, jumpers, etc. It's just a pain.. well, time consuming to power down the amp and let the caps drain after every individual test. Would take me an hour to do what could otherwise be done in a few minutes.
Hollow state work isn't like solid state, where the voltages tend to run low. You're dealing with some potentially dangerous voltages when it comes to hollow state. If it's gonna take an hour, then so be it. Using those meter probes on a live circuit might save an hour, but dead is forever.

If you want to speed things up a bit, get several of those digital multi-meters and run more than one test at a time.

Even if you survive, ain't no fun hitting a 300Vdc line.

If that probe slips, and makes a short circuit, it'll take at least an hour to replace the poofed parts.

I'd suggest getting a whole bunch of the kinds of test probes shown on the right of This Piccie. These being called "IC test clips". They're also great for clipping onto the pins of those 7 and 9 pin mini sockets. An alligator clip can all too easily bridge pins with those little sockets.
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Old 8th January 2009, 09:08 PM   #9
jitter is offline jitter  Netherlands
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Quote:
Originally posted by Miles Prower
Even if you survive, ain't no fun hitting a 300Vdc line.
If you touch high voltages, your muscles will make a sudden involuntary movement, often resulting in even more damage...

Quote:
If that probe slips, and makes a short circuit, it'll take at least an hour to replace the poofed parts.
True, I once blew all power transistors in one channel of my Marantz PM80SE.
At work I also once blew up a high power (2kW) switch mode power supply. That one was not a single poof but a cascading set of little explosions. Those two seconds felt like a lot longer, I can tell you.

Both were caused by a simple slip of a probe.
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Old 8th January 2009, 09:13 PM   #10
wicked1 is offline wicked1  United States
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Thanks, this is exactly the sort of conversation I was hoping for..
But now I have the classic internet advice dilemma.. Do I choose the advice I wanted to hear, or the more conservative advice, which is Im sure, the proper advice.
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