Perplexed!

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Harold_V
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Re: Perplexed!

Post by Harold_V »

I usually err on the side of safety, but without a knowledge of a higher potential than the 120 volts, I thought I was in safe territory with my 250 volt selection. I can see the wisdom of a higher setting. No harm comes from that, and it's all too easy to reselect a lower voltage once a range is known.

I wasn't aware that analog meters are most accurate near full scale. I thank you for that.

H
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Charles T. McCullough
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Re: Perplexed!

Post by Charles T. McCullough »

Steggy wrote: Sun Jan 29, 2023 5:34 pm
Harold_V wrote: Sun Jan 29, 2023 4:24 pm Thanks! While the explanation is a bit of an overload for my feeble mind, it does make sense now, and I thank you for that. The condition you described (light load) was precisely the condition (no load at all).

Something I meant to earlier mention was when using an analog meter to check a circuit carrying an unknown voltage, always begin with the meter set to its highest range. Pegging a d’Arsonval meter movement, the type found in most multimeters, may cause unseen mechanical damage that will permanently degrade accuracy. Once you have an idea of the maximum in-circuit voltage, you can safely reduce the meter's range to get a more precise reading. As a general rule, analog meters produce their best accuracy when the pointer is at or near full-scale.

I can attest that sometimes the mechanical damage is not so "unseen"! My son-in-law thought he needed to replace a simple wall switch, but got confused when he removed it and found FOUR wires in the box on a switch with 4 terminals. But it was not on a circuit with any other switches to control the ceiling light and his replacement switch was the simple two terminal type one would expect. To make matters worse, he just pushed the wires out of the way without labeling them as to what terminals they had been on.

OUCH!

He asked my help in troubleshooting it. I got my small, and trusty, Simpson 260 multimeter meter (that I purchased with my very 1st paycheck back in 1963) and went to help. I'll tell you now that we determined it was not the switch but a faulty light socket. Of which I am glad because we never figured out why that electrical box had 4 wires. But it was no small feat to study the bends in the wires to decide which terminals to reattach them to on the original switch.

But in troubleshooting things... it got really confusing and at one point I was measuring resistance between the 4 wires (the fuse removed and no voltage measured anywhere FIRST, before switching my precious meter to Ohms, of course) and I was not getting anywhere. So I went to a wall outlet to see if that socket had power.

Oops!

I forgot to rotate the selector switch to AC Volts.

That poor meter now seems to never know what reading it should show, as the violence it met with bent the needle permanently into the shape of a Question Mark!
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Bill Shields
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Re: Perplexed!

Post by Bill Shields »

In case one subtle point has been missed in this discussion.

A replacement CAP rated for 120 VAC, or even 240 VAC would be headed for trouble.

When replacing a CAP, always pay attention to the old ratings.
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Steggy
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Re: Perplexed!

Post by Steggy »

Bill Shields wrote: Sun Jan 29, 2023 11:28 pm In case one subtle point has been missed in this discussion.

A replacement CAP rated for 120 VAC, or even 240 VAC would be headed for trouble.

When replacing a CAP, always pay attention to the old ratings.

Yep, especially with this type of motor, whose performance is strongly affected by capacitance.
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Steggy
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Re: Perplexed!

Post by Steggy »

Charles T. McCullough wrote: Sun Jan 29, 2023 7:22 pmI can attest that sometimes the mechanical damage is not so "unseen"!...I got my small, and trusty, Simpson 260 multimeter meter...I forgot to rotate the selector switch to AC Volts.

Oh dear! Aside from making a hairpin out of the pointer, you likely burned up a resistor in the meter’s divider network. Those resistors are not off-the-shelf values, so fixing this sort of problem is not going to be easy.

When I was in the Navy, we had a multimeter called the PSM/4 (when “PSM” is pronounced out loud, what you hear resembles something you do after downing three or four beers). It was a Navy-specific version of the Simpson 260 in a watertight case. I never zapped one the way you did, but I can tell you from first-hand experience that the meter, when closed up, could float. :shock:
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Charles T. McCullough
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Re: Perplexed!

Post by Charles T. McCullough »

Actually, my son fixed it for me!

Bought the same model off E-bay and gave it to me!
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