Overview
In a separate project I am hacking a bug zapper. I need a high voltage probe to use an oscilloscope that is limited to 400V. It is just a simple resistor divider, but I did have one clever idea, so I'm posting it.
As you can see in the picture, it's just a string of resistors, nine 1M, a 909K and 100K. This gives a divider ratio of approximately 100x and 10x.
I used nine individual 1M resistors to spread the power dissipation. I didn't want a large and expensive power resistor. I expect to generate around 5000V and at that voltage a 10M resistor dissipates 2.5W. I am not really comfortable dissipating the full 1/4W rating of the part, but I only expect to use it for short periods.
The clever bit (IMHO) is putting a little loop in the connections between the resistors so I have a good place to grab with my scope probes. I am rather proud of how it turned out (it's the little things that impress me).
Each resistor was twisted to the next resistor and soldered. For the first nine 1M ohm resistors I covered them in clear heat-shrink tubing. I would have preferred Teflon tubing, but didn't have any large enough to fit over the resistors. I wanted clear tubing so I can see if one or more of them gets too hot.
When I was done I measured the actual resistance to the return (blue) terminal with the following results:
Resistance
Divider
Rtotal
10.035M
R10x
1.019M
9.85x
R100X
99.83K
100.5x
I am happy with the results. I am not doing any precision measurement, so the divider ratios are close enough to 10x and 100x for my needs.
I plan to mount the banana jacks in a strip of wood for stability and protection.
High Voltage Resistor Divider
Comments