Volt meter on Lab Power Supply

[DavidDLC]

A friend on mine gave me a used TekPower HY1803D lab power supply. This is a 18 V 3 A power supply

What really annoy me is the fact that the voltmeter on the unit is set to the 200 range volt.

What should I do to change the range to 20 Volts ?

I have an idea about changing some resistors to change the decimal point, but I'm wondering what to do for the input resistance.

Please give some advice.

[alm]

Guess that the same meter is also used for 30V models, where 20V would not be enough. There's probably a resistive divider at the input to divide it down to 0-200mV or so. Just change the divider, keeping the total resistance equal, although it's unlikely to matter as long as it's high enough. Use precision (probably 1% for this kind of meter) resistors. The decimal point would be a separate issue, it's probably easy to change, but you can always use a marker to draw it on the display . It's a single range anyway.

[NiHaoMike]

What are the buttons under the meters for? I would guess to toggle the backlight or check the voltage or current limit setting, but it might be scale adjustment. I would expect scale adjustment to be automatic, though.

[DavidDLC]

NiHaoMike, those are not buttons  , I know they look pretty similar, that is just part of the cover design.

There is nothing hidden or to adjust on that part of the power supply I already took apart the circuit board containing the LCD meters to fix the LED backlight, and checked this out.

[saturation]

Can you photograph the PCB and components, it would be good to see who made this PSU?

Those "buttons" are sometimes easy punchable holes to allow access to an adjust screw, had this PSU had analog meters.

I've seen several of these PSU, and often they use an ICL7106 powered board.  Its a DVM, and if you can get the schematic or trace the PCB you may locate the line than can be jumpered to give another range.  But warning, if its not autoranging you'll just get "OL" once its over 1.999 Vdc, it probably why they made it 100V instead resolution, so you can view the range of output voltage while making the DVM cheaper.

[DavidDLC]

Sorry I didn't have time to post pictures, I was busy preparing the trip to China I had.

Well tonight I was able to make the modification to this power supply, two easy steps:

- The meter has four 1 Meg resistors, and four bridges connected to them for configuration, with the combination of those you select the decimal point for the range 20 or 200 and V or A for the display.

- I changed the input resistor from original 100 ohms to 1KOhm to change the range measurement.

Thanks everybody for your inputs.

[DavidDLC]

Here is a picture of the power supply after the changes.