Modify a digital clamp ammeter ?

[Externet]

Hello.
Have a simple Cen-Tech ampermeter #95652 and the AC scales are too large for my needs : 200 Amperes and 1000 Amperes.
Did not find the schematic for it and would prefer to have a 10A (or 20A) scale instead of the 1000A.

Would you have any surgery suggestions ?

This guts from a very similar unit are shown on the web; there is a couple of trimmers by the sensing coil plug...

 

[Externet]

Found my ammeter to be modified has a 2.7 Ohm resistor in parallel with the sensing coil terminals !
Then the sensing coil signal passes trough a single 1K series trimmer resistor to the rest of the circuitry that should be the A/D converter.
Removed the resistor and without it; a current that previously showed 4 Amperes now displays 84 !
Will tailor its value and the trimmer to bring the reading to proper value. So far, optimistic that can be shifted to a tenth of the original scales of 200 and 1000 Amperes becoming 20 and 100 Amperes. Will see if the decimal point is manageable...

 

[rjenkinsgb]

That's typical for a current transformer setup. The 2.7R would be coarse calibration & the trimmer fine adjustment.

I'd suggest you now add back-to-back fast recovery diodes across the CT output to avoid it frying the meter board it it is ever put on a high current cable, though.

A CT can generate crazy voltages if the load (eg. a preset in that) becomes open circuit; something like a mini tesla coil...

 

[Externet]

Thanks for teaching me something new today.

Edited, added : The original 2.7 Ohm resistor produced a 4 Amperes reading.
A 33 Ohm resistor instead produced a '28.3' reading.
A 56 Ohm resistor produced a '44.6' reading;
A 51 Ohm resistor produced a '40.2' reading; will stick to that one.

With ~4 Amperes flowing,
I get '040' reading on the 1000A scale, should be '04.0' for a now-fake100A scale.
I get '40.2' reading on the 200A scale, should be '4.02' for a now-fake 20A scale.

Will scratch my head with the decimal point, has to shift one position. Unknown 'COB' IC.