Ammeter scaling

Piwoslaw · 11-04-11, 02:11 PM

I'm thinking about getting a cheap ammeter to measure my DC loads.

I'm not sure what size I should get: On one hand, I'd like it to have good resolution in the 0-1A range, but on the other hand I often need to check stuff which more than 1A.
So the question is: Can a 1A ammeter be scaled (hooked up parallel/series) to measure a higher current, like 5A or 20A?
Question #2: Can a 10A ammeter be scaled to show better resolution in the 0-1A or 0-2A range?

Daox · 11-04-11, 02:14 PM

Buy one with a digital display.

Piwoslaw · 11-04-11, 02:20 PM

I already have a digital multimeter, but it has a 150mA max and I keep blowing its fuse. If I upgrade to a new multi it'll be a clamp style model, but I'm not yet ready for that investment. Unless I turn my eyes towards Santa Claus

Daox · 11-04-11, 02:50 PM

I mean get an ammeter with a digital display. They're pretty cheap on ebay if you can wait for shipping from China.

Xringer · 11-04-11, 07:55 PM

Quote:

Originally Posted by Piwoslaw

I'm thinking about getting a cheap ammeter to measure my DC loads.

I'm not sure what size I should get: On one hand, I'd like it to have good resolution in the 0-1A range, but on the other hand I often need to check stuff which more than 1A.
So the question is: Can a 1A ammeter be scaled (hooked up parallel/series) to measure a higher current, like 5A or 20A?
Question #2: Can a 10A ammeter be scaled to show better resolution in the 0-1A or 0-2A range?

I once had a 0 to 5 ma DC amp meter and hacked it to read out higher amperage,
by adding a few feet of 14g wire in parallel with the terminals.

When a few amps of current was in the wire, a small voltage drop(loss)
develops across the 14g loop, making the meter defect.

I shortened the wire until I got 25A to move the meter to about mid-scale.
I quit right there and called it a 50 amp meter..

Piwoslaw · 11-05-11, 12:36 AM

Quote:

Originally Posted by Xringer

I once had a 0 to 5 ma DC amp meter and hacked it to read out higher amperage,
by adding a few feet of 14g wire in parallel with the terminals.

When a few amps of current was in the wire, a small voltage drop(loss)
develops across the 14g loop, making the meter defect.

I shortened the wire until I got 25A to move the meter to about mid-scale.
I quit right there and called it a 50 amp meter..

That's pretty much what I had in mind. How would I calculate the resistance needed in the parallel circuit, depending on the scale I'd need?

Xringer · 11-05-11, 07:49 AM

I don't recall doing any calculations (it was 40 years ago), but I don't think it's
real difficult to find information on the topic today..

Here's some info for using 500 ohm 1 ma FS meters. (FS=Full Scale. Meter at max).
Ammeter design : DC METERING CIRCUITS
I have a couple of these meters in the basement, I might have to dig around in my old resistor box..

For a high amperage meter, wire will work as a shunt, if the meter is pretty sensitive.
If you had 20-30 feet of bare wire, you could connect using an alligator clip
on one side of the meter to adjust the length.

Connect a power supply (w/ off-on switch). Use a known load and
calculate what the current will be. Target Current=Voltage/Ohms.

Using the clip, Connect 1 foot of the shunt wire across the meter,
and turn on the power supply. Check the meter. Turn off the PS.
(If you forget to turn off the PS, you will smoke the meter).

If the reading was too low, move the clip to make the shunt line longer.
At some point, the meter will display your target current.
If the shunt wire gets too long, your wire is too fat. Try a smaller gauge wire.
And start off with a short shunt, then make it longer.

Your results will depend on your meter. Trying to make a 2A meter
work at 20A might not as easy, as getting a 1ma 500ohm meter to work at 20A.

One meter that I got working at high amperage had white tape,
over the top of it's face where I re-labeled the amperage.

Xringer · 11-05-11, 07:57 AM

500 ohms x 0.001 amps = 0.5v (for FS).

So, if you get one those little un-shunted 1ma-500ohm meters, be extra careful not to put
more than 1/2 volt on it..

You could use a good voltmeter with a PS & load, to find an approximate shunt wire length,
before connecting it to your new meter.

11-04-11, 02:11 PM	#1
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 961 Thanks: 188 Thanked 110 Times in 86 Posts	Ammeter scaling I'm thinking about getting a cheap ammeter to measure my DC loads. I'm not sure what size I should get: On one hand, I'd like it to have good resolution in the 0-1A range, but on the other hand I often need to check stuff which more than 1A. So the question is: Can a 1A ammeter be scaled (hooked up parallel/series) to measure a higher current, like 5A or 20A? Question #2: Can a 10A ammeter be scaled to show better resolution in the 0-1A or 0-2A range? Attached Thumbnails __________________ Ecorenovation - the bottomless piggy bank that tries to tame the energy hog.

11-04-11, 02:20 PM	#3
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 961 Thanks: 188 Thanked 110 Times in 86 Posts	I already have a digital multimeter, but it has a 150mA max and I keep blowing its fuse. If I upgrade to a new multi it'll be a clamp style model, but I'm not yet ready for that investment. Unless I turn my eyes towards Santa Claus __________________ Ecorenovation - the bottomless piggy bank that tries to tame the energy hog.

11-04-11, 02:50 PM	#4
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I mean get an ammeter with a digital display. They're pretty cheap on ebay if you can wait for shipping from China. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-05-11, 07:49 AM	#7
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Tape I don't recall doing any calculations (it was 40 years ago), but I don't think it's real difficult to find information on the topic today.. Here's some info for using 500 ohm 1 ma FS meters. (FS=Full Scale. Meter at max). Ammeter design : DC METERING CIRCUITS I have a couple of these meters in the basement, I might have to dig around in my old resistor box.. For a high amperage meter, wire will work as a shunt, if the meter is pretty sensitive. If you had 20-30 feet of bare wire, you could connect using an alligator clip on one side of the meter to adjust the length. Connect a power supply (w/ off-on switch). Use a known load and calculate what the current will be. Target Current=Voltage/Ohms. Using the clip, Connect 1 foot of the shunt wire across the meter, and turn on the power supply. Check the meter. Turn off the PS. (If you forget to turn off the PS, you will smoke the meter). If the reading was too low, move the clip to make the shunt line longer. At some point, the meter will display your target current. If the shunt wire gets too long, your wire is too fat. Try a smaller gauge wire. And start off with a short shunt, then make it longer. Your results will depend on your meter. Trying to make a 2A meter work at 20A might not as easy, as getting a 1ma 500ohm meter to work at 20A. One meter that I got working at high amperage had white tape, over the top of it's face where I re-labeled the amperage.

11-04-11, 02:14 PM	#2
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Buy one with a digital display. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-05-11, 07:57 AM	#8
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	500 ohms x 0.001 amps = 0.5v (for FS). So, if you get one those little un-shunted 1ma-500ohm meters, be extra careful not to put more than 1/2 volt on it.. You could use a good voltmeter with a PS & load, to find an approximate shunt wire length, before connecting it to your new meter.