Insulation tester

[Refugee]

I began this project about seven years ago and it has been laying about doing nothing for ages.
Inspired by a flee-market find it is back again.
It was going to be a range extender for an AVO 8/9.
It now has its own dial and I have started looking into the regulation for using lower test voltages.
The main unit has an SMPSU chip and LOPT from an old VGA screen.
The output voltage shot up with just 20 volts HT
It is the low end of the focus chain I need to find now.
That voltage needs taming big time.

[Nick]

I began this project about seven years ago and it has been laying about doing nothing for ages.
Inspired by a flee-market find it is back again.
It was going to be a range extender for an AVO 8/9.
It now has its own dial and I have started looking into the regulation for using lower test voltages.
The main unit has an SMPSU chip and LOPT from an old VGA screen.
The output voltage shot up with just 20 volts HT
It is the low end of the focus chain I need to find now.
That voltage needs taming big time.

This is a bit scary - What are you trying to achieve here?

For 17th Edition insulation testing you only need 500V or 1000V in exception cases (for higher voltage systems than domestic). What you seem to be building is a breakdown tester which could destructively test insulation - of course, once insulation has been tested to breakdown, it cannot be reused as it will no longer be to spec - there may well be carbon tracking etc.

I've spent a lot of time messing with HV - up to 500,000 V - Using a LOPT is fun, but difficult to control - if you are testing insulation, you need to control the parameters of that test pretty well in order to get meaningful results. Colour LOPTs can generate up to 25KV, B&W ones somewhat less (a few KV) - they are not current limited and can kill. Safety. Safety. Safety.

Finally, the inside of that box looks highly unsafe - the LOPT and HV side should be physically separated and isolated from the LV side of the circuit and preferably have an earthed screen. Special HV connectors and cable also need to be used...

[pwdrive]

Looks interesting, what voltage are you after?

Lawrence.

[Refugee]

It is not as scary as mains derived HT on a pre-war TV.
The original design was to work with scaled up resistor values produce a high ohms range for an AVO using the 500V, 1000V and 3000V ranges in order to provide extra high ohms ranges without doing any modifications to the AVO.
The fly-back unit is based on an SMPSU chip and currant limiting will be done on the LV side working from the chip data sheet.
The LOPT has a built in regulator chain that also includes the focus and black level pots and will be used as intended by the VGA screen manufacturer to achieve regulation. The original circuit was designed to produce 25KV with regulation of an HT supply that varies from 70 to 90 volts depending on brightness demand using a tap on the low end of the focus chain. This point produces 10uA with an output of 5KV and it is just ohms law from there. The 6KV from 20 volts reading proves that the transformer ratio is correct as intended by the manufacturer.
I may well now look a little further and see if CRT testing can be done too using two audio amplifiers connected to the scan coils and fed with the ramp and sinewave from a sweep generator in order to fully check such a tube.
I am looking into floating the negative rail at up to 500V DC with a neon cut off that will open a fast fusible resistor that will drive the chip to a voltage that will shut the output pulses off. Flat screen TVs use this action to shut down if a capacitor goes high ESR.
This is not as bad as what I did in the 1970s with a solid state vibrator using a 405 LOPT over-wind in an attempt to make an arc welder. It used to draw an inch and a half spark and was powerful enough to melt steel wire