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 Post subject: Meter shunt value quesiion.
PostPosted: Nov Sat 25, 2017 9:58 am 
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I'm going to apologize if this is such a very basic question, but for some reason I'm not getting this one straight in my head.

I'm building a variac with a 2.5 amp rating. When I asked about finding panel meters, it was suggested that I use a 500 mA meter, and have a shunt on a switch so the meter could then read say 3 amps. I'm assuming that would be in series with the meter, but how would I calculate the shunt value that I need? I get that it would be a voltage divider (the shunt and the meter) but I'm not sure what to go on for the value of the meter.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Sat 25, 2017 11:55 am 
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Hi Michael,

No, a current shunt is in parallel with the meter.
Part of the load current flows through the meter, the rest flows around the meter.

In your case you want 500mA through the meter and 2500mA around it (3 amps FS).

You need to know the resistance of the meter. That's on the datasheet.
For example, the Simpson Wide-Vue 500mA (AC) meter's resistance is 0.9 ohms.

It's easier to work this as a voltage problem.
The voltage across that meter = R * I = 0.9 ohms * 0.5 amps = 0.45 volts.
That same voltage appears across the shunt.

Since R = E / I, the shunt resistor = 0.45V / 2.5A = 0.180 ohms.
The power dissipation = E * I = 0.45 * 2.5 = 1.125 watts. I would use a 5-watt shunt.

Use a good quality commercial spring-return toggle switch like Carling or C-H.
Wire it so the shunt is connected when the switch is idle, disconnected when pressed.

- Leigh

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Sun 26, 2017 12:32 am 
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If you don't have the data sheet for your meter, you can measure its resistance with an ohmmeter. If it's a DMM, just remember to subtract the test lead resistance from the reading, as the test leads are about the same order of magnitude as the resistances you want to measure.

The switch and wires leading to it need to be very heavy duty and oversized for the amount of current that will be handled. Their resistances are in series with the shunt and will affect its calibration. Per Leigh's calculations above, if you need a shunt resistance of 180 milliohms, a switch and wires might contribute about 40 milliohms, for example. In that case the shunt itself would only need be 140 milliohms. Easiest way to deal with this is to make the shunt out of a coil of wire that is longer than needed, then adjust it by cutting back and soldering until the proper length is found with the switch and wiring that will be used in the project. Note that if the resistance of the shunt resistance is too high the meter will come to full scale before the desired current is reached, so one should start out by seeing how the meter responds on low currents first. Don't throw the whole three amps across it right off the bat or you could peg the meter.

From the copper wire tables, #22 AWG wire has a resistance of 16.14 ohms per 1,000 feet, or 0.01614 ohms per foot. To make 140 milliohms, one would divide 0.14/0.01614 = 8.7 feet of wire. Some may object to the use of copper since its temperature coefficient (change in reistance with temperature) is fairly large, so if you have constantin or monel metal wire available, by all means use it! The calibration won't be affected as long as the temperature doesn't change appreciably, so making sure the shunt is well ventilated will help. #22 AWG is rated for 7 amps in "chassis wiring" applications which means it cannot be in a bundle or in conduit. One could also use heavier copper wire but then longer lengths will be needed since it will have less resistance per unit length.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Sun 26, 2017 5:50 am 
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Michael

One thing to be careful of is to make sure that meter does not require an external shunt to work for the current shown on the meter face.

If you have trouble reading the meter resistance, and chances are you test probes, unless they are Kelvin clips, will introduce a lot of error.

I'd suggest running enough current through the meter to give you a full scale reading, and them measuring the AC voltage across the meter terminals. You can then use Ohm's Law to calculate the meter's internal resistance, and then use the data provided by Chris and Leigh to figure out your shunt resistor value.

pete

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Mon 27, 2017 7:22 am 
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It took me a minute after reading this to finally have the DUH moment and I see where i was thinking wrong.

So that leads to a point that Chris brought up, and that being that the switch would add some resistance to the shunt, and throw off the calibration. So the thought came to mind if I could run both through the switch, that would even the score. And what I came up with was a DPDT switch.

Looking at the switch as two DPST switches linked together, the center of both sides would be connected together, and that would be the input. The outputs of one side (let's call that switch 1) would be connected together to go to the meter. So it's connected to the meter in either position. But the other half (switch 2 so to speak) would have the shunt on one selection, and nothing on the other. This way, when it flipped to no shunt, the two sides are separated, so current only goes to the meter, and when flipped the other way, current goes to the meter and the shunt. But since both sides now go through the switch, we now have the same additional resistance to the meter as we do the shunt.

I know that there might be some very slight difference between the contacts on both sides of the switch, but I would think it would be so minor of a difference that it wouldn't be noticeable enough.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Mon 27, 2017 3:20 pm 
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TPAairman wrote:
I know that there might be some very slight difference between the contacts on both sides of the switch, but I would think it would be so minor of a difference that it wouldn't be noticeable enough.
The ratio of resistances should be the same as the ratio of meter and shunt resistances.
So the resistance of the shunt leg should be 1/5th that of the meter leg.

My earlier suggestions were based on an accuracy of about 5%, which should be fine.
If you want precise measurements, there are many small factors that must be considered.

- Leigh

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Nov Tue 28, 2017 2:01 am 
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Put a fuse in series with your ammeter. This will prevent you from blowing
the meter.

Attachment:
AC  ammeter fuse.jpg
AC ammeter fuse.jpg [ 69.9 KiB | Viewed 306 times ]
Attachment:
Ammeter multiplier constant current.jpg
Ammeter multiplier constant current.jpg [ 18.94 KiB | Viewed 306 times ]
Attachment:
Triplett 330 GP  Ammeter.jpg
Triplett 330 GP Ammeter.jpg [ 75.01 KiB | Viewed 306 times ]

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 12:06 am 
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An excellent set of youtube videos about this subject were posted about a year ago on the W2AEW youtube channel.

One here: https://www.youtube.com/watch?v=wbRx5cQZ8Ts

and one here: https://www.youtube.com/watch?v=4U-nxdp-LDw

Alan posts tremendous tutorials on many aspects of electronics and radio.

Terry

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 1:27 am 
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The videos are well done for DC meters. Applying shunts
to AC meters gets into grave safety issues if someone
in an industrial setting decides use things seen on the internet.

Industrial settings globally are strange birds. Entire manufacturing
systems have been crated up and shipped to third world locations.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 4:11 am 
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I was going to wait until I was done to post an update, but I'll do it now anyway. I found a 1 amp meter. When it arrived, I noticed the cover of the guts in the back aren't the standard round cover. I'll have to post a picture, but it's like a cap, and it wasn't on tight. I noticed a resistor in there, so I took the cover off and found a few other parts. Here's a link to a diagram that I made of it. Everything, including the transformer is in the back cover of this meter.

https://easyeda.com/tpaairman/Meter-d5ab7d3300214e9d9d95f94ea3be22cf

I put this on my circuit simulator, and it looks like I can put a 3K resistor in place of the line from R2 to the junction of R3 and 4 to let the meter read 2.5 amps which is the rating of both my Isolation transformer and variac.

What I'm going to do, is have a DPDT switch. One half will select between the 3K resistor and a wire. So 3 amp and 1 amp. On the other half of the switch, the input will go to the common of the switch, the 3 amp selection will be a wire, the 1 amp selection will have a 1 amp fuse. So if it's set on 1 amp and you go over, it will blow the fuse.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 7:20 am 
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Concerns: T Will the new current rating flow in the primary of the meter's
transformer?

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 3:31 pm 
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That's a fairly conventional meter bridge circuit similar to what is done in many VOMs. It can give fairly accurate results if the scale is custom drawn. The diodes have to be closely matched for forward current and temperature tracking.

But unless you know that the current transformer can handle three amps without saturating or overheating, you're back where you started from. You'd still have to put a low resistance shunt across the meter terminals to increase the current range.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 7:18 pm 
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Chris108 wrote:
But unless you know that the current transformer can handle three amps without saturating or overheating, you're back where you started from. You'd still have to put a low resistance shunt across the meter terminals to increase the current range.


That could be an issue, but I really only have one way to find out. If I fry the transformer then I'm out $12 and start over. Or I might be able to re-wind it, but I think I'd still just get a new one. I hate winding coils.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Tue 05, 2017 10:30 pm 
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Do add a fuse in the wires to the ammeter. One 'aw shucks' may melt or even set the
meter on fire.

shucks---- got a bushel of 'em, some were hard to hide... :D

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 12:12 am 
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radiotechnician wrote:
Do add a fuse in the wires to the ammeter.

A fuse won't open nearly as fast as the wire in the meter movement.

Unless you use an instrumentation fuse.
They cost as much as the ammeter.

- Leigh

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 1:39 am 
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As important as a variac is, if you are serious in this hobby, it seems to me simpler and ultimately cheaper in the long run, to buy the proper meter. They aren't THAT expensive and you will probably, sometime, find a use for the other meter.

Charlie


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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 1:49 am 
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tinwhisker wrote:
As important as a variac is, if you are serious in this hobby, it seems to me simpler and ultimately cheaper in the long run, to buy the proper meter.

What does a Variac have to do with the current meter???

- Leigh

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 8:22 am 
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tinwhisker wrote:
As important as a variac is, if you are serious in this hobby, it seems to me simpler and ultimately cheaper in the long run, to buy the proper meter. They aren't THAT expensive and you will probably, sometime, find a use for the other meter.

Charlie


The problem is that I would need a 3 amp meter. A 2.5 amp is not exactly common. With a 3 amp meter, the -0-1 amp section is not very big, meaning that the 100's of mA can be a very small section. So doing it this way, I have a higher resolution to my meter.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 8:24 am 
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Leigh wrote:
tinwhisker wrote:
As important as a variac is, if you are serious in this hobby, it seems to me simpler and ultimately cheaper in the long run, to buy the proper meter.

What does a Variac have to do with the current meter???

- Leigh


This is for the variac/ISO transformer that I'm building. He's probably referring to that.

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 Post subject: Re: Meter shunt value quesiion.
PostPosted: Dec Wed 06, 2017 9:06 am 
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Leigh wrote:
radiotechnician wrote:
Do add a fuse in the wires to the ammeter.

A fuse won't open nearly as fast as the wire in the meter movement.

Unless you use an instrumentation fuse.
They cost as much as the ammeter.


- Leigh


The 1 amp Tripplet model 330 G/P that I posted the picture of is protected by a Buss AGX 2 fuse.

I have blown that fuse before. Those meters are 'student meters'

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