Measuring circuit current

[Kerrowman]

Hi there,

 
I am trying to measure the current in a circuit by putting a digital ammeter inline, as in the diagram, after removing a connecting link that is also shown.
 

When the connector link is in, the circuit works fine but when the meter is in it doesn't work well at all, implying that there is some significant resistance from the meter.
 
In practice it looks like this:
 

 
 

 
 
In the two photos, first I have have some jumper leads plugged in and the circuit runs normally. Plug in the meter instead and, while there is some current flowing, there is not enough for the system to function properly.
 
As I understand it the ammeter should offer minimal resistance to the circuit and so should not interfere with it during it’s measurement process.
 
Using another meter, when disconnected, the resistance of the red plug through to the black plug via the meter set to 'A' is 0.1 Ohms.
 
I do have a clamp meter but I am expecting to use a desktop digital meter that will take readings automatically at regular intervals without my having to remember to stop and take one.
 
Am I doing something wrong here and how can I obtain a good reading without interfering with the circuit's operation?
 
Thanks

 

[Kerrowman]

Interestingly, if I replace the DVM with an analogue meter it’s all fine. Do DVMs have other input components that can ‘get in the way’?

 

[HarryA1]

My old Scope multimeter has a 0.01 ohm shunt resistor in the 10 ampere circuit going by the schematic. That is not much of an intrusion  into a  circuit.  I would think the analog meter is most likely the most accurate meter in your case. What does the clamp on meter give you?

Knowing the on and off time  of the pulses one could calculate the peak current from the  one ampere reading?

 

[Kerrowman]

The clamp meter gives about 1.1 so close to the analogue meter. The current here is steady and continuous and not pulsed.

The DVM reads about 0.95 so there must be something in the DVM that is getting in the way?

 

[HarryA1]

The meter  needs calibration. My AstroAl M2KOR has for DC current: accuracy +/- ( 1.2% rdg + 5 dgts). Whatever that means.  Also "when large currents are measured the  continuous measurement should not exceed 15 seconds".

 

[Kerrowman]

Measurement accuracy is one issue yes but it does not explain why my device cannot function quite as normal when I’m measuring current with this meter compared to when it’s not in the circuit.