Electronic – How to measure voltage in a PS/2 keyboard cable

circuit analysiskeyboardpower-measurementvoltagevoltage measurement

I have an old PS/2 keyboard, I doing a future project with it, but for now I just want to measure the voltage in the cable. However I don't know how to do it, so let me explain.

It's a PS/2 keyboard, and I have a P2/2 adaptor that transforms it into a micro USB cable that get's connected to the PC.

It has 4 wires:

  • Red: VCC power
  • Black: Ground
  • White: Clock
  • Green: Data

The circuit looks basically like this (apology for my drawing)

enter image description here

The mini USB is connected to the computer, then it's transformed into a PS/2 cable via an adaptor. The adaptor acts as some kind of resistance. Then the wires are connected to the keyboard via a PS/2 port.

I have basically drawn the 4 wires as they are there, I have removed the external insulator cable on that segment in the drawing so that I can measure the voltage.

I have measured the voltages at the points illustrated on the picture.

A1->A2 = 0.6 mV DC

B1->B2 = 0.6 mV DC

C1->C2 = 2.2 V DC

D1->D2 = 5.22 V DC

I have also measured the voltages between the wires, after the adaptor:

A2->B2 = 5.24 V DC

C2->B2 = 5.24 V DC

D2->B2 = 5.24 V DC

So I guess the (black) ground cable is like the return path of the circuit since A2,C2 and D2 have 0 Volts between them, they are only connected with B2 separately.

Meanwhile I have also measured the voltages before the adaptor:

D1 had 0 Volts between all other points

A1->B1 = 5.25 V DC

C1->B1 = 3.05 V DC

A1->C1 = 2.2 V DC


The question is what is the voltage in the "Yellow Area", meaning in the wires that go out of the adaptor and go into the keyboard.

So my goal is to determine how much watts of power the keyboard uses, so I would need to multiply the voltage in each cable "going out" with the amperes going through each wire and add them together.

How to measure exactly the voltage in the yellow area?

So which measurement is the correct measurement from above, in order to determine how many volts there are in the wires in the yellow area. And use that volt to multiply with with the amperes flowing through each cable to determine the watts of current that the keyboard uses.

Best Answer

I don't really care about the battery though, what I am interested in is what is the minimum watts needed to use the keyboard. What other way there is to decrease power consuption?

First thing you need to think about that traditional keyboards were not designed for low-power applications, thus if you think you are critical for power probably you need to use some special input device especially designed for these purposes.

Look here. Power consumption will depend on what keyboard is currently performing. If it is idle there will be small current, if you press 10 buttons on it and have all LEDs turned on, it will be another current and thus another, bigger power consumption.

In my opinion, the only way to properly decrease power consumption is to turn keyboard device off when it is not expected to be used. When operator or application will need to use it, you should refer to maximal device's current per its datasheet.

I just took PS/2 keyboard I have, and on its rear side there's label saying that it requires 5 V DC. No information about current though.

There might be information on the web about maximal PS/2 ratings, I found this one which says max current is 275 mA.

Also please keep in mind that you are currently thinking or talking about your particular case with particular keyboard. Imagine you are designer of consumer product, which can be used virtually with any keyboard, thus you can not know which exact current attached device takes. And thus you should refer to minimal and maximal definitions within the standard, planning your device operating cycle accordingly.