The VC-288 digital volt/ampere-meter is front panel meter for general purpose
The micro-controller works like a PIC and have a build-in 12-bit ADC. The Op-Amp I suppose is configured as two voltage comparators, one for the voltage and one for the amp measuring. The intern clock of the micro-controller is in the range from 10 Khz till 10 Mhz.
Specification:
| Voltage range Current range Supply voltage Supplay current Voltage error Current erro Shunt spec. Refresh rate |
0,0v-100v 0-10 amp 4,5v – 24 v < 20 mA ± o,1 % ± 1 % Build in shunt 300mS |
The module has a built in current shunt (for 10 Amps) which should make the meter easy to use. It has 3 smaller wires for connection to a power-supply (black – read) en for measuring voltage (yellow). The power-supply is in the range 4.5 to 30 V. The black wire is used as common.
The basic drawing are:
The two “thick” red and black wires are used for the current measuring.Beware: The shunt resistor is included in de common line!!
Surprised of the cost of the panel meter I bought 2 of them and t
est them in respect till the given specs.Before I will give the result of the test I have done, I can say that I’m be negative surprised of it. The voltage measure is quit good but the current measure is bad. If you see the shunt and the way it is construct you can imaged that this will not be given a good result.
The shunt resistance has a value around 6.2 mOhm and is visible on the photo. The shunt is made from silver plated copper of 1.5 mm. It’s understandable that a little different in soldering on the PCB will give quite big difference in result of the resistance and of course the drop-voltage over the shunt.
While I’m not be able to present Graphics here I will give some result from measuring the current.
For measuring I have used:
– Power supply LN-303pro 3 Amp/30 volt dual based
– ELW 72812 universal measure instrument used as mVolt instrument
Measure
Vshunt in mV I load in Amp I meter reading
3,2 0,5 0,44
6,2 1,0 0,83
8,8 1,5 1,23
11,5 2,0 1,62
14,2 2,5 2,01
16,9 3,0 2,04
The linearity of the meter is very bad.
Further it is interesting to know that the linearity will be better when the supply voltage is drop down till lower working border of about 4,5 volt of the meter. Later on I will explain wy this happen!!!
Because I want to have a meter in the range from 0 – 1 amp, I have change the shunt in a resistance of 62 mOhm with special resistance wire of 1,5 Ohm/meter. That means a accurate resistance.
The range should now be change from maximum 10 amp till 1 amp, however the non-linearity still be. And still I do not know way but I think it is due to the OpAmp Bias current.
PIC – NE76E003
The meter is based on the processor N76E003 and is used for ADC and for presentation. For ADC it used 2 lines from the PIC. Interesting is the specs of the ADC-range. The Vain (analog input voltage) :: min 0 til Vavdd (ADC operating voltage) with min = 2,7 and max = 5,5 volt.
That means that the ADC range is 2,7 – 5,5 Volt. The ADC power supply current is typical 300 µA.
The OpAmp LM358
The opamp is used as a voltage amplifier. It need to amplify from 62mV full-scale (for 1 amp) to 5,5 volt it means ≈ 88 times. Not very much.But there is something difficulty. I mean the Ibias current of the opamp!! (see E CircuitCentre) This bias current is about 88 nA and, that’s interesting, is being influenced by voltage supply. The measured voltage is very small as also the current. However I still do not know the electrical drawing of the meter, I suppose the current it is in the same order as the measured current with as result non-linearity. Non-linearity from the Ibias (see figure).
I came on this idea while the lower the Vcc, the better the linearity of the meter will be.
What to do about it????
I think try to use the meter with a voltage maximum from 5 Volt. The linearity than will be the best. A other way I do not see.