Simple Schematic - but components may not even exist! [600VDC]

[Luke Pickles]

Hello All,

I'm trying to develop a special earth leakage tester which requires few parts.

However, because the tester may operate up to 600VDC I'm not sure if I can source the following parts:

Ammeter (range 0 - 100mA)
Resistor (2.6k ohms)
Variable Resistor (0 - 20k ohms)

The schematic is here: http://i61.tinypic.com/244y2k4.png
(The tester I'm trying to make is coloured black).

If anyone can give me a direction I'd be forever grateful.

Thanks!

 

[Laplace]

The circuit diagram shows a 2600Ω resistor possibly driven by 600 VDC depending on the connected load. Power dissipation in the resistor will be V²/R = 138 watts. Maximum current will be I=V/R 600/2600=230mA, well within the rating of the fuse. Consider that the resistor should consist of a string of individual wirewound resistors, thirteen 20Ω, each with a 10 watt power rating. But finding a variable resistor capable of the requisite power dissipation will be problematic.

You may need to rethink just how much leakage current this tester will be able to measure.

 

[Luke Pickles]

Hi Laplace,

Thanks so much for the feedback.

Using multiple 20ohm/ 10watt resistors instead of a single 2.6K ohm/ 138watt resistor is a great suggestion. A quick search reveals that this should be easy to source

au.element14.com/vishay-dale/rs01020r00fe12/wirewound-resistor-20-ohm-10w/dp/1185016


Ideally, the leakage current would be as little as 30mA.

So, the fixed 2.6k resistor (or series equivalent) will work with an 80v supply (~30mA).

Any ideas on how to address the variable resistor problem?

Is there some way I could series together resistors and "tap" the various resistive points?

 

[Laplace]

Connecting to a string of resistors using a multi-tap rotary switch would be good, and actually being able to find a rotary switch rated for 100mA at 600VDC would be even better. It may be necessary to build a custom rotary switch as part of your device. Look at how laboratory switches were constructed circa 1900. One may find to it possible to re-purpose the electrical contacts from modern switches to save some time & effort.

 

[Luke Pickles]

I've had a look online for multi-tap rotary switches but can't seem to find something that suits this application.

Unfortunately I don't have enough experience with them to be authorative on this.

Is there anything you might be aware of that would do the job (notwithstanding the issues around still needing high resistance!)

 

[Colin Mitchell]

That is an absurd circuit. What are you trying to do???

 

[Alec_t]

From a quick look at potentiometer specs, the usual max working voltage seems to be 350VDC. Didn't spot anything above that. A transistor rated >600V might be a better/easier-to-source current control device.