Need help with kilovolt meter resistor divider  - Page 1
Forum section: electronics / projects

author: Kakashi
author_rank: Contributor
timestamp: August 15, 2017, 04:30:24 pm
content: I just bought these two kV meters from a flea market and was wondering if I can make them as bench voltmeters. I want (if it's possible) to add multiple ranges with a rotary switch.Say 0 to 30V,300V,3KV and the full-scale 30kV. Now I need help designing the resistor dividers.Something like this:

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2017, 05:08:47 pm
content: Regardless of what the printed scale says, the basic meter movement responds to current. It doesn't look like there are any multiplier resistors in side those meters, so you'll have to figure out the full scale parameters of the meters (full scale voltage and full scale current). Once you know these, you can then calculate new resistor values for the ranges that you want. However, beware that resistors for the very high ranges (3kV and 30kV) will be *very special* resistors that can withstand these kinds of voltages. Not something to fool around with if you're not familiar with high voltage. Here are two of my videos that might help you out:and one on high voltage probes:

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2017, 05:27:16 pm
content: Measure the coil resistance.Apply a known current to the coil. This can be as simple as connecting the meter to a 9V battery, in series with a resistor. You don't know what the current rating for the coil is, so start with a high value (say 1M), if the needle only moves a tiny bit, then go down to a lower resistance. When you manage to get the needle between a quarter and three quarters of the scale, record the current, by measuring the voltage across the series resistor and applying Ohm's law. The full scale current can then be calculated.It's now easy to work out what value of series resistor is required, to give you the desired full scale value. In your example, the full scale is 1mA and the coil resistance is 500R, so for 1V you need a total resistance of 1k for 1mA to flow, 1000 minus the coil resistance of 500R is 500R. If you wanted the full scale range to be 3V, then it the total resistance would need to be 3k, so the series resistor would need to be 2k5.

author: Kakashi
author_rank: Contributor
timestamp: August 15, 2017, 05:50:16 pm
content: It seems there are some resistors inside that plastic cylinder, also when measuring the resistance in the both meters is 4.99K?. So I think that there is additional "circuitry", and that the terminals aren't connected directly to the coil.

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2017, 08:32:53 pm
content: Quote from: Kakashi on August 15, 2017, 05:50:16 pmOnce you measure the full scale current and voltage, that will tell the story. It is certainly possible that there are resistors contained, but can't tell from your pictures. Measurements will reveal all. You still have some very careful considerations for your >300V ranges. The danger is *real* with these high voltages, don't take the precautions lightly.

author: Vtile
author_rank: Super Contributor
timestamp: August 15, 2017, 09:54:37 pm
content: In those meters they propably have been used with external voltage transformer or indirectly measured in some other route measured the voltage of (distribution) network. That brown thing seems awfully lot a current transformer, which dangerous by nature so read the behaviour if you use it for experiments. Also those meters are AC only (by construction) and the accuracy is classified only to 1.5% (if I see correctly) what I can see from the standard markings on the left side corner. The meter type is something I do not remember in english, but as such it measures RMS by nature of it. The deflection is relative to square of current, but those have some linearisation mechanics build in. It theory the movement should be capable of measuring both AC and DC.moving -Ironsystem is what IEC calls it, but can't recal what it is actually called. But it is not your typical D'Arsonval coil movement. Edit. Took a while to find a online dictionary that had it. Atraction-iron or moving-iron movement is the type of these, not D'Arsonval

author: Kakashi
author_rank: Contributor
timestamp: August 16, 2017, 11:47:34 pm
content: Quote from: w2aew on August 15, 2017, 08:32:53 pmYeah, in that plastic cylinder there are some components but not sure what. I didn't want to cut/ruin it because these are NOS voltmeters. As for the warning I agree, I will add the scales but I don't think I will use it above 300V. Was thinking to put the whole resistance divider in wax/plastic/silicone for more isolation.Quote from: Vtile on August 15, 2017, 09:54:37 pmYour assumption is correct, the far right component is a current transformer. Don't mind it though, bought it along with the voltmeters because the whole "package" was very cheap. Anyways do someone know how much do these cost brand new?