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Multimeter resistance

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grahamrounce
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Username: grahamrounce

Post Number: 81
Registered: 12-2008

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Posted on Saturday, 30 April, 2016 - 10:30 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
I have a 30,000 ohms/volt moving coil meter, and seem to have forgotten how to work out its resistance on say the 100uA range, or come to that, any current or voltage range.
Could someone please remind me?
(And a digital meter? There doesn't seem to be any equivalent parameter quoted. )
Thank-you,
(All my sticks seem to have two wrong ends)
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bruce
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Username: bruce

Post Number: 1125
Registered: 04-2008

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Posted on Saturday, 30 April, 2016 - 12:13 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Graham,
If your meter says 30,000 ohms/volt, then use Ohms Law V = I * R:
V =1 V R = 30K so I ( current ) is about 33uA.
Now, any idiot can shove an equation under your nose and solve it, but what does this all mean?
What this piece of algebra is telling you is that, if you ever have 33uA flowing into your meter, the needle will go to full scale (fsd)

Now you can back-track. If you have a 9V battery, for example, this will give fsd when you use a series resistance of 9V divided by 33uA ( roughly 270K ). If the needle doesn't go to full, your battery is knackered! Usually, you have a set scale of 10 ( 300K ) and mark the scale 0 to 10, which makes life easier.

Now for current: if you want to measure current flowing through the meter, the don't use a series resistance, you have a parallel one ( called a shunt ) because it steals some of the applied current or shunts it away. In your 100uA example, you now know that 33uA will give fsd. So where does the other 67uA go? You put a parallel resistance which accepts the excess. Remember that the voltage across both meter and shunt is the same, so you need to know your meter resistance, so you can work out how much you'll need to shunt it with.
There are ways of measuring meter resistance, but my advice is don't: get it wrong and you could bend the needle. The resistance of yours is going to be around 50K, but maybe anywhere between 25K and 100K, so what to do?
Back to your 100uA. Suppose the meter resistance is 50K. For the shunt to rob 67uA, you need a resistance which is LOWER and by twice as much ( 67 divided by 33! ), so around 25K. If the needle is in the wrong place, tweak it a bit until it's right

Digital meters usually show a 1Meg resistance on any range. One trick might be to measure the meter resistance when the dial is set to 100uA fsd. Call it 'R'. I wont do the sums, but the meter resistance is 3*R.

HTH
Bruce
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grahamrounce
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Username: grahamrounce

Post Number: 82
Registered: 12-2008

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Posted on Saturday, 30 April, 2016 - 12:23 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Brilliant! Thank-you. It's coming back to me now.. It was soooo long ago!
(All my sticks seem to have two wrong ends)
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externet
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Username: externet

Post Number: 217
Registered: 05-2005


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Posted on Tuesday, 17 May, 2016 - 11:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Me too, forgot all about my first multimeter as a teenager.
So the Ohms per Volt is not about the multimeter instrument but about the moving coil-galvanometer movement only ?

----> https://en.wikipedia.org/wiki/Multimeter#/media/File:Multimeter-4269.jpg
Abolish the deciBel !
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rob_guyer
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Username: rob_guyer

Post Number: 199
Registered: 08-2011

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Posted on Wednesday, 18 May, 2016 - 07:27 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Some multimeters use the Ayrton shunt configuration which presents a slightly higher voltage burden than the simple shunt when inserted into a current circuit. If one such multimeter is of the moving coil type, the most sensitive range gives best meter damping during transit.
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rob_guyer
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Username: rob_guyer

Post Number: 200
Registered: 08-2011

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Posted on Thursday, 19 May, 2016 - 01:43 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Off-topic.The best quality electronic multimeters get consistent current readings by using a ganged pair of STDP switches to prevent the main current flow from entering the voltage-measuring circuit. Easily spotted on rotary wafer switches by the insulating washers under the rivet heads holding the leaves on each side of the wafer in the pair.
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rob_guyer
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Username: rob_guyer

Post Number: 201
Registered: 08-2011

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Posted on Friday, 20 May, 2016 - 01:16 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Editorial error--"main current flow"should read "the P.D caused by that part of the main current flow passing through two, (unequal), switch resistances from adding to the PD accross the shunt. Easily.............

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