How to use 25 amps ammeter?

Thread Starter

MoberkBleich

Joined Mar 21, 2020
10
Hi,

I am doing a computer vision project that reads analog meters. I have analog ammeter, which has a max value of 25 amps (60 mV). I just need to make the pointer move, but not sure, how to do it (connect it) effectively, because of the current scale is that big.

So can you tell me, what do I need (and be safe)? I am sorry, this may be stupid question, but I'm a programmer :D.

ana_den1.png
 

MrChips

Joined Oct 2, 2009
30,720
Most meter movements will respond to microamps.
I can see 15mA printed on the scale. I don't know if this means anything.
In any case, if you are allowed to modify/destroy the meter, you can open the meter and remove the shut resistor that is in parallel with the meter. Then put a large resistance (10k-100kΩ) in series with the meter connections. Experiment until you get the desired deflection.
 

Thread Starter

MoberkBleich

Joined Mar 21, 2020
10
In that case you will have to find another meter with lower FSD (full scale deflection) current or voltage.
Would it be possible to take 18V battery and really small load (for example 1 ohm rezistor/shunt) and connect it in series with ammeter?
 

AlbertHall

Joined Jun 4, 2014
12,345
If you have a 1k resistor, connect the meter to an AA battery with the 1k in series. If the pointer doesn't move then the shunt is internal and you will have to either pass 25A through the meter or dismantle the meter to remove the shunt.
 

Thread Starter

MoberkBleich

Joined Mar 21, 2020
10
If you have a 1k resistor, connect the meter to an AA battery with the 1k in series. If the pointer doesn't move then the shunt is internal and you will have to either pass 25A through the meter or dismantle the meter to remove the shunt.
I have bunch of 4.7k, so i will connect them in parallel and I will see :)
 

Reloadron

Joined Jan 15, 2015
7,501
The meter very likely has an internal shunt. Normally the true full scale current is printed in the small print on the meter face. I can't tell from your image. If you remove all the nuts on the rear terminals and snap the face plate off the meter movement, including shunt should fall out of the plastic housing. The shunt resistor should be across the terminals and for a 25 amp meter should be plainly visible.

Ron
 

Thread Starter

MoberkBleich

Joined Mar 21, 2020
10
The meter very likely has an internal shunt. Normally the true full scale current is printed in the small print on the meter face. I can't tell from your image. If you remove all the nuts on the rear terminals and snap the face plate off the meter movement, including shunt should fall out of the plastic housing. The shunt resistor should be across the terminals and for a 25 amp meter should be plainly visible.

Ron
The true full scale current is 15 mA if that is the number (left bottom corner) :)
 

Reloadron

Joined Jan 15, 2015
7,501
The true full scale current is 15 mA if that is the number (left bottom corner) :)
OK then the actual shunt still needs removed. If FS = 15 mA and the meter is 25 Amps the shunt is handling 24.085 amps. You should be able to remove the face plate cover as I mentioned and take the meter apart.

Ron
 

vu2nan

Joined Sep 11, 2014
345
It appears to be a moving coil meter, with a full scale deflection of 15mA or 60mV, requiring a 25A 60mV external shunt.

It may be tested using an analogue multimeter, set to measure Ohms, with the probe current in the selected range much less than 15mA (multiplier / probe current marked on the multimeter).

Nandu.
 
Last edited:

Thread Starter

MoberkBleich

Joined Mar 21, 2020
10
It appears to be a moving coil meter, with a full scale deflection of 15mA or 60mV, requiring a 25A 60mV external shunt.

It may be tested using an analogue multimeter, set to measure Ohms, with the probe current in the selected range much less than 15mA (multiplier / probe current marked on the multimeter).

Regards,

Nandu.
Thank you, for the reply.

I tried to connect it to AA 1.5V battery and 1k rezistor and the pointer moved to the value of 3. It seems like it really requieres external shunt, which is awesome :) Btw, in the bottom left corner, there is written: 15 mA TIII. Doesn't that mean, that the pointer will move to the III. (third) scale mark if the current is 15mA? I know you said its full scale, but 1.5/1000 gives 15 mA which moved the pointer to the third mark.
 

MrChips

Joined Oct 2, 2009
30,720
1.5V / 1000Ω = 1.5mA

Reading 3 on 25A FSD means 3A.
Your 1.5V AA battery is putting out 3A to the meter, only 1.5mA through a 1kΩ resistor in parallel.
 
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