Resistors for current measurement

I'm always nervous wiring my good VOM in series into a circuit to measure current. Another way to measure current is to measure voltage across a resistor that is series wired into the circuit in question. This way the VOM remains in high impedance mode and so is less susceptible to damage.

Most circuits will not be adversely affected if the series resistor is very small in comparison to the circuit resistance itself. OTOH, the resistor must be large enough to produce an easily measured voltage when the current is flowing through it.

I acquired three 50 watt, 1% resistors in resistances of 0.1, 1.0, and 10.0 ohms and attached them to a large aluminum plate to act as a heat sink.

Given 50 watt capability, the maximum current and associated voltage for each resistor is...

ohms amps volts
0.1 22.4 2.24
1.0 7.1 7.1
10.0 2.24 22.4

https://s3-us-west-1.amazonaws.com/f...e/128212-1.jpg

https://s3-us-west-1.amazonaws.com/f...e/128212-2.jpg

I'll never do it but it is interesting. Thanks for sharing Marv.

Marv i may be wrong but i think an unpainted aluminum surface, better even with a little thermal compound should work better
Ariscats

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Originally Posted by Ariscats

Marv i may be wrong but i think an unpainted aluminum surface, better even with a little thermal compound should work better

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You're correct, of course, but that bit of aluminum was lying around, I was in a rush, and all my applications involve currents far below the 50 watt limitation.

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Originally Posted by mklotz

You're correct, of course, but that bit of aluminum was lying around, I was in a rush, and all my applications involve currents far below the 50 watt limitation.

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O.K. Marv. I just made a comment.
Ariscats

You made me feel guilty (and I deserve to be) so a few minutes with a sanding drum and we have some nice bare radiative surfaces...

https://s3-us-west-1.amazonaws.com/f...e/128490-1.jpg

The use of resistors in this situation is called a "shunt"
Another method for measuring high current draw is to measure across the supply lead that oft times goes from the power source directly to the load. Disconnect, and, accurately measure the resistance of that lead, and you have a "known" shunt value. Reconnect, and then measure the current draw and you achieve the same result

Cheers Phil

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Originally Posted by mklotz

You made me feel guilty (and I deserve to be) so a few minutes with a sanding drum and we have some nice bare radiative surfaces...

https://s3-us-west-1.amazonaws.com/f...e/128490-1.jpg

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Don't forget the compound. Otherwise! LOL
Ariscats

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After the 1st number of years — and easily as many blown up meters — I discovered I could blow up a $5.oo meter, with about the same efficacy as I could a more expensive model. And since, truth be told, I rarely needed the kind of sensitivity of either a Simpson or a Fluke, I defaulted to always having an extra meter handy. Kinda like having an extra truck key. (And I probably saved a couple thousand dollars, over the last 40 years.)

On another note, I did a helluva lot better, when I started marking the damned dials. So I could clearly tell what settings I had them on. :rolleyes:

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Originally Posted by Scotsman Hosie

After the 1st number of years — and easily as many blown up meters — I discovered I could blow up a $5.oo meter, with about the same efficacy as I could a more expensive model. And since, truth be told, I rarely needed the kind of sensitivity of either a Simpson or a Fluke, I defaulted to always having an extra meter handy. Kinda like having an extra truck key. (And I probably saved a couple thousand dollars, over the last 40 years.)

On another note, I did a helluva lot better, when I started marking the damned dials. So I could clearly tell what settings I had them on. :rolleyes:

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One of Harbor Freight's freebies is a small digital VOM. I've tested a few and they're accurate enough for doing TLAR electrical work.

I might use them as you suggest but when I get them I usually raid them for their nine volt battery to use in in my homemade electronic test gear.

Those are the ones I've been using for the last 15 or 20 years. Always have at least a couple 'ahead' — for when one craps out.

On the other hand, I'm always knocked out by your craftsmanship — and ability to come up with so many useful gadgets. And keep track of them all. How do you keep track of 'em all?

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Originally Posted by Scotsman Hosie

On the other hand, I'm always knocked out by your craftsmanship — and ability to come up with so many useful gadgets. And keep track of them all. How do you keep track of 'em all?

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Thanks much for the compliments.

Well, I'm blessed with (cursed by sometimes) a near eidetic visual memory. [My wife marvels that I can walk into the room where she's viewing a TV movie and, with a casual glance at the screen, remark that I've seen the movie before.]

With this kind of memory, I can mentally review in detail all the things I've built. It's especially helpful when I wish to design something new. I seldom put plans on paper and do all the design work in my head. Occasionally I'll make small working sketches to work out complex dimensions.

It also helps in finding stuff. Most times I can picture which side of which drawer or cupboard an item of interest is located. I reinforce this by always storing something for the first time in the first sensible place that occurs to me knowing full well that that place will pop to mind when I go to find it in the future.

It appears to be an inheritable trait. Like me, my oldest daughter can instantly tell you whether a picture/drawing/map was located on the left side page or right in an opened book.

Neither of us are totally eidetic. We can't visualize text we've read and 'reread' it unless the text happens to be part of a pictorial representation.

Another surprising thing is the fact that my general memory deteriorates with age (now 77) normally; most days I can't tell you what I had for dinner yesterday. Yet the eidetic thing doesn't seem to deteriorate. It's probably housed in some part of the brain distinct from where 'normal' memory is kept.

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Originally Posted by 12bolts

The use of resistors in this situation is called a "shunt"
Another method for measuring high current draw is to measure across the supply lead that oft times goes from the power source directly to the load. Disconnect, and, accurately measure the resistance of that lead, and you have a "known" shunt value. Reconnect, and then measure the current draw and you achieve the same result

Cheers Phil

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Shunt. Yep, will be inquiring further on both selecting and finding those.
Here, at least three heavier machines will enjoy ammeters to gauge motor loads while cutting.

Already know, there is a narrow threshold between TLAR and opening a can of Lucas Smoke. Color me disappointed, positive I had a meme for that.
yup
https://brandonmgclub.com/store/3030...LucasSmoke.jpg

Quote:

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Originally Posted by Scotsman Hosie

After the 1st number of years — and easily as many blown up meters — I discovered I could blow up a $5.oo meter, with about the same efficacy as I could a more expensive model. And since, truth be told, I rarely needed the kind of sensitivity of either a Simpson or a Fluke, I defaulted to always having an extra meter handy. Kinda like having an extra truck key. (And I probably saved a couple thousand dollars, over the last 40 years.)

On another note, I did a helluva lot better, when I started marking the damned dials. So I could clearly tell what settings I had them on. :rolleyes:

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I generally have no fewer than 3 of the very cheapest DVM's Harbor Freight carries. There is supposed to be one for each vehicle, plus the house, but they seem to vanish sometimes. I've also got at least 3 of their more expensive DVM's, and a couple for Radio Shack (tells you about how long I've had them) and I think I still have a VTVM stashed in a cabinet somewhere... :dance:\

With all that, I still had to go buy a new one, for $7.99!!! a few weeks ago. Could not find a DVM anywhere in my house!!! Also found one of their fancier ones on sale open box for $63, and grabbed that, too. Measures all the usual stuff, plus light and sound... Could be handy! If I can find it again!

Bill

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Originally Posted by mklotz

<<<<<<snip>>>>>> It also helps in finding stuff. Most times I can picture which side of which drawer or cupboard an item of interest is located. I reinforce this by always storing something for the first time in the first sensible place that occurs to me knowing full well that that place will pop to mind when I go to find it in the future.

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When elected President, shall I make that law?
I have at least 3 dozen constituents....

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Originally Posted by Toolmaker51

When elected President, shall I make that law?
I have at least 3 dozen constituents....

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When you win, I'd like to the secretary of war.

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Originally Posted by Frank S

When you win, I'd like to the secretary of war.

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Deal; but I'll have to rename it accordingly. Defense is a little too apathetic.
You are a shoe-in for Department of Transportation, by the way.

Quote:

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Originally Posted by Toolmaker51

When elected President, shall I make that law?
I have at least 3 dozen constituents....

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You can be president but I'm holding out for king ! :-) It's going to take some draconian changes to sort out the current mess and I don't want any damned liberal parliament getting in my way.

Occurs now to me, about getting first things first; starting out with a wise king might be the answer.

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Originally Posted by Toolmaker51

Occurs now to me, about getting first things first; starting out with a wise king might be the answer.

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Ah yes, the benevolent autocrat; the perfect solution to the problem but, like a hammer with a loose head, never to be trusted on the job.

To work well, he would need an incorruptible ombudsman but such is as unlikely as a benevolent autocrat.

Vive la RESISTANCE!!
Ariscats

Ohmmmm...
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Originally Posted by IntheGroove

Ohmmmm...
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Teehee, guffaw...
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