# how to change the scale of an ammeter(ICL7107)?

Discussion in 'The Projects Forum' started by onlyvinod56, Aug 4, 2011.

1. ### onlyvinod56 Thread Starter Senior Member

Oct 14, 2008
362
1
Hello.
I have a digital DC ammeter (0-2)A. It is a ICL7017 based meter.
I want to convert it (0-20)mA.

I found a schematic (click here). In this, to get a 20mA scale the shunt should be 10 OHM (BOTTOM SEGMENT 9+0.9+0.09+0.01 = 10). And the upper segment resistance is 90+900+470K = 470.990K approximately 471K.

I have another 20mA ammeter. In this ammeter, the SHUNT is 10 OHM. But the upper segment resistance is 220K.

For the same IC 7107 and for the same scale 20mA, how come the two resistors are different.??

How the ammeter is not effecting the difference 471K and 220K??

2. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
102
The 470K or 220K is *NOT* part of the current measuring shunt but connects to the input pin of ICL7107, which has extremely high input impedance. As a result, practically no current flow thus no voltage drop on the resistor.

Any value from 1K to 1000K would not really make any difference.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
To convert the 0-2A meter to 0-20mA you would have to get inside and change the shunt (or more accurately the current sensing resistance).

If the IC requires 0-200mV then you exchange the existing 100mΩ shunt with a 10Ω shunt.

I suspect the high resistance is used in conjunction with the capacitance across the differential inputs to "allow" for the response time of the IC's internal A-D conversion. Smooths the input somewhat and perhaps reduces potentially erratic reading behavior. Not sure though.

4. ### onlyvinod56 Thread Starter Senior Member

Oct 14, 2008
362
1
Hi elbc..
thankyou.

Thankyou t_n_k...
I have converted one 2A ammeter to 20mA ammeter. and another 2A ammeter to 200mA ammeter.
I used 10Ω shunt for 20mA and 1Ω shunt for 200mA.
They are working good.

BUT

the display is fluctuating. I have connected a 1KΩ to 20V DC in series with my modified 20mA ammeter.
The reading is not constant. It is fluctuating from 19.92 to 19.99. Why is that second decimal point is not constant. if it is fixed at 19.92 or something else, i can adjust it to 20mA(19.99mA) by changing the trim pot(connected to pin 36).

Last edited: Aug 5, 2011