Divide And Conquer

With a High Voltage generator to build, I though it might be a reasonably sensible idea to be able to measure that high voltage. Now, apart from my oscilloscope, which I'm not confident of the calibration of, I don't have any test gear for hundreds of volts at a few tens of microamps.

So, a HT potential divider was called for. This is a straightforward device, simply a suitable chain of high value resistors, in series with the multi-meters own internal impedance. But first, I needed to know exactly what that impedance is!

Now, the manual says 11.5MΩ but how far can you trust a Chinese manual? So I decided to measure it. OK, so how do you measure the internal impedance of a voltmeter? Well, by balance of course!

Here's the set-up - a fixed voltage is applied across the meter and a series resistor. I chose 10V as this makes everything really easy. As I expected the impedance to be in the 10 to 12MΩ region, I started with a 10MΩ fixed resistor in series. If the series resistor matches the internal impedance - the meter would read half the voltage, in this case 5V. It actually read a little more. So I added another series resistor, a 1M5, and it read a little below. So, it wasn't 10MΩ and it wasn't 11.5MΩ! But 11MΩ.

Set-up to measure meter inpedance

So I now knew the exact impedance of the meter, and I want to be able to easily display up to 1kV. So making use of the meters ranges I decided on a 10:1 ratio, so that for instance a 500V HT would read 50V on the meter. This would still give me two decimal places, so 100mV accuracy in the HT reading. The exact resistor chain values were worked out using Ohms law, I wont go into them here but its sufficient to say I needed the chain, including the meters impedance, to give a current of 4.5uA.

As I was using whatever I happened to have in stock, this resulted in a chain consisting of 5x 10MΩ, 5x 8M2, 1x 6M8, and a 4M7 preset potentiometer, set up as a variable resistor (wiper and one end tied together). The use of the preset meant I could swing the final resistor value around the expected couple of Megohms, to calibrate the chain and get exactly 10:1. This was all built 'snake-like' on a bit of Perfboard. I then used my bench PSU, which goes up to 32V, to test and calibrate the chain - adjusting the preset to give a 3.2V reading.

High Voltage Divider Chain (and a random SCR!)

I'll post up a photo of the completed device later.


Ive also had a play with the Euromarine Radiofix unit today. A pair of headphones were found that would work with it. The RDF unit has a mono 3.5mm jack, but although this pair of 'phones has a 4-way 'mobile phone' jack, one earpiece works. Radio 4 LW on 198kHz and RTE-1 on 252kHz are both incredibly strong, but even then the nulls in the antenna pattern are sharp and deep enough to get a bearing! Despite the level of local noise, and the incredibly weak signals, Sherburn (2.3miles), Finningley (21miles) and Leeds-Bradford (21.5miles) can all be received and a bearing taken.

Of course, this unit was designed for maritime navigation, when there were still coastal beacons, and so covers 180 to 400kHz. The aeronautical beacon band extends up to 550kHz, so I will consider retuning it to cover from 250 to 550kHz, if possible.