I just modded my Basetech BT-305 laboratory power supply to become an IoT device.

Everything works as planned, except the volt display in the multimeter-part of the supply, which is directly shown below. I disassembled the display part and saw a missing resistor, but am unable to find out how many ohms it had, as I am not an electrical engineer, so I hoped you guys could help me.

This is the problematic part:

enter image description here

I also found the circuit diagram PDF on the page I bought this device

Basetech BT-305

I bought the above from Basetech BT-305 Labornetzgerät, einstellbar 0 - 30 V/DC 0 - 5 A 150 W Anzahl Ausgänge 1 x

In case anyone has the same device and has a picture of this part, that would solve the problem of searching for the right resistor immediately.


A high-enough resistor that would fit in there is 0.5 kilo Ohms. This was measured by using a very high potentiometer and slowly decreasing the resistance until it worked as it should. Thanks to William for the idea in the comments below. Picture of the part that was missing:


As there seems to be interest in how i modded this device for iot in the comments.

the goal is to switch the output on/off over the internet (app/http requests)

In short: disassembled the device, added a second AC power plug (female) into the bt-305, so that a 5v wallcharger could be powered in the case. The 5v charger is used to power a "Sonoff SV" which is an esp-based smart switch from itead. The sonoff sv supports a so called "isolated mode", which means it input is 5v, and the 2 output pins just short each other when getting the command to switch, acting like a normal switch on 2 cables. Those output pins are soldered to the pins of the "output" button of bt-305, so sonoff is just simulating a button press. Sonoff has an app to communicate with the switch, or you can connect it to IFTTT and send http requests to a rest-url via web-hooks to set the state of the switch. I generally use the app, as it supports "inching mode" which means the switch-output gets shorted for just 0.5 seconds, which is necessary to simulate a button press.

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    $\begingroup$ I don't think you have attached the correct schematic. It will helpful if you can find the schematic for the display portion of the power supply. $\endgroup$ Mar 12 '19 at 13:29
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    $\begingroup$ If you cannot find the schematic, measure the voltage across the contact points where the missing resistor is located , with power applied to the board Remove power but write down the voltage you measured Solder in a multiturn potentiometer of high value (~ 250K) with the pot set at the highest resistance Now re-apply power to the board and slowly adjust your new pot until your meters come on Power down again remove the pot and measure the resistance across the pot and replace the pot with a fixed resistor of the same value Calculate the wattage for your resistor, done $\endgroup$ Mar 12 '19 at 18:04
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    $\begingroup$ tought of exactly the same thing, now i can be sure it was not a stupid idea :) thanks for your reply, will try it out tomorrow and comment if it worked. $\endgroup$
    – Dogan
    Mar 14 '19 at 0:14
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    $\begingroup$ just measured the voltage between the 2 pins, it is 3.6 Volts AC $\endgroup$
    – Dogan
    Mar 14 '19 at 17:44
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    $\begingroup$ Are you sure it was a resistor and not a missing diode ? Is there any marking on the circuit board where the missing part is ? $\endgroup$ Mar 15 '19 at 7:18

The resistor value can be determined fairly accurately by process of observation, comparison and elimination.

There are two 7 segment LED displays and supporting driver circuits. These include CS7107GN driver IC, a block of identical resistor blocks and similar resistor and capacitor blocks.

Within the Similar resistor and capacitor Circuit block (highlighted in red) has approximately 5 capacitors and about 3 to 4 resistors.

Measure the resistor values in each block. Most likely the values are similar, close or some combination.

Use a potentiometer to determine the appropriate value.

Below is an image pictorially describing the process. Blocks are segment in colours

7 Segment Display PCB


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