How does a simple MEGOHMETER works ?

[BGAmodz]

Hi everybody on the forum .

I was recently given a analog MEGohmeter , and the selector gives different choices for resistance measurement , but i was surprised by the high test voltages on the unit going from 500 volts to 1 kv .

I mean how can a unit that works with 4 * 1.5 v batteries deliver such high test voltages ??

Am asking this because am troubleshooting it and i saw a tiny transformer on it , i would like to know if it is the one that transforms the batteries voltage by any chance to the test voltages ???

 

[debe]

The little transformer is part of a HV power supply.

 

[debe]

This is a circuit of a Megger kit I built some years ago, & its still going. It might be of some help in understanding how they work.

 

[tronitech]

Hi everybody on the forum .

I was recently given a analog MEGohmeter , and the selector gives different choices for resistance measurement , but i was surprised by the high test voltages on the unit going from 500 volts to 1 kv .



I mean how can a unit that works with 4 * 1.5 v batteries deliver such high test voltages ??

Am asking this because am troubleshooting it and i saw a tiny transformer on it , i would like to know if it is the one that transforms the batteries voltage by any chance to the test voltages ???

You have to understand that the "Megger" isn't designed to just measure Mohm, a bog standard DMM can do that! It's designed to test insulation resistance on mains powered equipment and installations between the Live and Neutral connected together and earth at high voltage. It's part of general safety testing and you'd commonly be looking for resistances over 1 or 2 Mohm. The high test voltages are designed to stress the device above its normal operating voltage and I know on 240VAC powered devices you'd test at 500V so I'll presume, and somebody may correct me if I'm wrong, you'd use 1000V to test three phase 415VAC systems.

 

[BGAmodz]

You have to understand that the "Megger" isn't designed to just measure Mohm, a bog standard DMM can do that! It's designed to test insulation resistance on mains powered equipment and installations between the Live and Neutral connected together and earth at high voltage. It's part of general safety testing and you'd commonly be looking for resistances over 1 or 2 Mohm. The high test voltages are designed to stress the device above its normal operating voltage and I know on 240VAC powered devices you'd test at 500V so I'll presume, and somebody may correct me if I'm wrong, you'd use 1000V to test three phase 415VAC systems.

Sorry i still don't understand this seriously .

How can this megger deliver up to 1000 volts ? the meter is fed by 6 volts only , it is supposed to discharge really fast .

 

[JimB]

How can this megger deliver up to 1000 volts ? the meter is fed by 6 volts only ,

The six volt batteries provide the power for an oscillator which drives a transformer, which converts the 6 volts up to 1000 volts.
There is no great power in the 1000 volts, it will give you a nasty surprise if you get hold of it, but it is unlikely to damage a healthy person.

In days gone by, insulation testers were powered by a small hand cranked generator.

it is supposed to discharge really fast .

Not sure what you are asking/stating with this comment.

JimB

 

[BGAmodz]

The six volt batteries provide the power for an oscillator which drives a transformer, which converts the 6 volts up to 1000 volts.
There is no great power in the 1000 volts, it will give you a nasty surprise if you get hold of it, but it is unlikely to damage a healthy person.

In days gone by, insulation testers were powered by a small hand cranked generator.



Not sure what you are asking/stating with this comment.

JimB

LOL just ignore that last statement .

So if i understand well the megger creates a U=IR equation , so when it injects that voltage the passing current gives the resistance value ?

 

[Diver300]

Sorry i still don't understand this seriously .

How can this megger deliver up to 1000 volts ? the meter is fed by 6 volts only , it is supposed to discharge really fast .

The power supply circuits in camera flashes do the same thing. They run at 300 V or so, from a 1.5 V battery.

Also car ignition circuits, CRT televisions, inverters for running mains devices from automotive batteries, all step up the voltage from a lower voltage DC supply.

https://en.wikipedia.org/wiki/Joule_thief is another example.

 

[JimB]

So if i understand well the megger creates a U=IR equation , so when it injects that voltage the passing current gives the resistance value ?

Yes, that is about it.

One thing to be aware of, the meter has a high internal resistance so that when the resistance being measured is low, only a small current will flow and the voltage at the terminals of the meter will be reduced from the 1000v generated in the meter.

JimB