Scales - Old Style

If you own a Shumatech DRO-350 or 550 and unplug the unit periodically, you may notice that one or more scales lock up. A fix that works for me is to put a 120 ohm resistor across the scale power leads. This has two effects. It dumps the voltage on the capacitors connected across the scale at power down and causes the same supply voltage rise at each power up. That, so far, has eliminated my lock up problem.

The resistor may also eliminate the 0.200 jump problem. In most cases, this jump occurs very rarely so it is very hand to trouble shoot. However, my theory is that noise is getting into the power wires going to the scale. These wires are connected to a low resistance at the DRO end but have no resistive termination at the scale end. Since caps have had no effect on my 0.200 jump problem, I must conclude that they can't stop the transient. The low resistance value at the scale will give me a that termination.

 If you have the jump problem on just one scale, then I suggest you put a resistor just across it. The value is not that critical and something between 100 and 200 ohms should be fine. If you plan to put resistors across all 3 scales, then I recommend you put about 300 to 600 ohms across each one. This will have the equivalent load on the scale power supply. The power being dissipated here is around 22.5 mW max so 1/8W resistors would be fine. You are drawing about 15 mA from the supply. This is a huge increase over the few microamps being drawn by the scales but is tiny compared to the DRO.