K8055 measuring 5V - showing 1.36V?

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1

I’ve got weird issue. I’m using this board for computer controling for some time, but now i need to measure some analog signals. After starting some measuring i’ve noticed that it measures completely wrong!

I’ve tried measuring alcaline battery voltage. Multimeter shows 1.5V - K8055 board 20 bars so - 0.39V (5V/256=~0.0195V/1bar)…

next i’ve tested with voltage regulator L7805. Multimeter shows 5.01V, K8055 - 70bars (1.367 volts).

Board is assembled without R8 and R9, so the input voltage should not be divided or multiplied…

so fallowing this i can connect 18V to its input to show me 255?

2

Do you get similar reading if you use A1 or A2 input?
Have you turned the trimmer potentiometers to max position clockwise?

3

it’s fixed. i didin’t knew that these potentiometers have any meaning while using external power supply. now it’s reading much better.

4

But does that mean that adjusting potentiometer increases the max input voltage? so if i’d set potentiometer to half then 5V would be 127 bars? and 10V would be 255?

5

[quote]But does that mean that adjusting potentiometer increases the max input voltage? so if i’d set potentiometer to half then 5V would be 127 bars? and 10V would be 255?[/quote]Yes, this is true.

6

Yes.
These pots act like linear attenuators (they are even marked ATT1 and ATT2 on the board) because they are connected as voltage dividers, you can see that in the schematics.
I estimate that the range can be extended up to 100 V, the power dissipation on the pot would then be 100 mW. However, one has to be careful, as shorting the SK2 or SK3 headers with large voltages connected to input would cause large currents flowing into the 5 V bus and may damage the board and the computer.