High voltage display.

Sometimes it becomes necessary to have a display for high DC voltage. An analog meter is always a possible solution, but the resolution and dial accuracy are usually quite modest unless you are willing to pay tens of Euros for a high quality meter.

I recently ordered from Banggood a digital display for 70 to 500 VAC. I needed to have something to indicate the output voltage of my Variac transformer. I soon realized that the fact that it could display voltages up to 500 units, could be used to hack the meter for general use. In principle it could be used to indicate up to 500 volts, milliamperes, or even degrees of temperature just to name a few.

The meter is based on a microcontroller incorporating AD converter and three-digit 7 segment driver. No published specifications can be found. The AD conversion is most probably programmable to enable the same chip to be used for different display ranges. Apparently the ADC resolution is always fixed (10 bits?).

There are numerous versions of the same panel meter available. Some are even equipped with current shunt resistor to display DC currents. This particular model, has the type number DSN-DVM-568AC printed on the PCB and it seems to be the most versatile for its display range of 500 units and autoranging capability.

One interesting feature is the way the 7 segment display is multiplexed. Instead of the normal  strategy to address segments and digits with specific control signals, each individual segment and decimal point (24 in total) is addressed using a 5 by 5 line array.

Is It Safe to Use?

Of course the first thing with Chinese electronics is to study its safety and reverse engineer the schematics to see what is needed to hack it. This is something I have learned from the superb YouTube videos of DiodeGoneWild.

The schematic above is drawn to roughly present the components in the actual PCB layout.

As usual, the high voltage safety leaves lots to hope for. The operating voltage for the chip is created from the input AC through a capacitive dropper and a bridge rectifier. Although the meter can display AC up to 500 volts the dropper cap has only 630 VAC spec. The peak voltage of 500 VAC is over 700 volts 

The SMD resistors in the input chain cannot be considered safe for such high voltages. The input leads are marked L and N (signifying live and neutral, respectively). Yet it might be difficult to construct a device where neutral input can be always guaranteed when non-polarized power plugs are used. It is true that since there are only two input leads, the risk is small if the installation is properly isolated.

The measured voltage is also used to power up the meter as well. Therefore, the display does not wake up until the input voltage is about 60 to 70 volts AC. However, if the microcontroller chip power is supplied separately, the display starts from 0.00 volts. So this would be the strategy for this hack.

The DC Hack.

The following image shows the components which are not needed if using this meter for 500 VDC display:

Depending on the required voltage range it might be advisable to replace all the voltage divider components with good quality resistors. The input pin (#13) of the microcontroller chip has a maximum voltage of 2.0 volts, which produces the maximum reading of 500 units. Consequently, you need to calculate the voltage divider to output 2.00 volts from input of 500 volts. The existing 10 kOhm trimmer pot on the PCB can be useful to adjust the reading, so leave it in place.

To reuse the original components as much as possible you can connect the hot part of the voltage divider to the cathode pad of the original input diode (just left of the microcontroller chip in the image below).

For 500 VDC use the required additional resistance is 605 kOhms, which might be constructed using 560 and 47 kOhm resistors and compensating the 2 kOhm excess with the trimmer. You may need to select resistors which read a little low for their face value.

The existing capacitors are not required but leaving them in place is also OK. They do not cause any problems and may even help to reduce noise at the input. The PCB could look like this after the clean-up of the AC input components.

The voltage for the electronics cannot be tapped from the measured DC voltage. The heat dissipation would be just huge. Therefore a suitable DC source should be available. There are several components on the PCB to regulate the 5 volt input and all of them are not necessary for low voltage input. 

It is expected that the current consumption of the microcontroller and display will be about 15-20 mA. Therefore, the power dissipation of the 78L05 regulator IC is not too high. The  6.8 V Zener diode protecting the input of the regulator creates a current sink, we must get rid of it. If you have a supply of 7 to 12 volts DC, you can connect it directly to the input pin of the 78L05. 

Higher voltages may be used with the help of the voltage dropping resistor on the right. Its value is just too small now that the Zener is removed. You can replace it with another which can consume the better part of the voltage difference and let the 78L05 run cooler.  A suitable value for 12 VDC input supply would be 330 Ohm 1/2 W.

Just remember that there is common ground for both supply and measurement zeroes. This is necessary to observe if you e.g. make the DC supply from tube radio filament transformer and try to measure the B+ of the same equipment. Common ground is not usually tolerated in tube amplifiers.