Megger
A Megger can be bought when needed, but for the average DIYer the expense is hard to justify. This article shows how to make a basic megger at a cost of almost nothing.
This megger is very simple, and works similarly to old hand cranked meggers.
Components
- Microwave oven turntable motor from a scrap machine
- Bridge rectifier 1kV rated (or 4 diodes)
- 2.2uF 1kV capacitor
- 5x 1Megohm leakage resistors
- A case for the parts
- Output connectors of any convenient well insulated type
How it works
This is a hand cranked micropower dc high voltage generator, and is used with 2 moving coil multimeters to read voltage and current.
Meters
Moving coil meters are used because they handle unsteady readings well. Digital meters aren't recommended as they don't behave well with unsteady readings.
Operation
Connect the voltmeter across the generator's output. Connect current meter and UUT in series, and across the generator output.
___ _
___| +|-----+----(_)----o L&N -----+
_/ |~ | | |
(_) | | (_) Unit Under Test
\___|~ _| | |
|___|-----+-----------o E -------+
Gen BR V A
Note how the current flowing through the voltmeter does not also flow through the ammeter. If connected wrong, the ammeter will read current wrongly.
Turn the knob on the motor at the speed that gives the desired test voltage. 500v is enough for most jobs. Read the leakage current.
Do not crank the motor to above 1kV. Doing so, even briefly, is liable to destroy the various components.
Safety
Never construct or use this machine unless you are competent to work on live electrics. Turntable motors can produce around 1,000v output.
Although output voltage is high, current output is very low, limiting the risks involved in high voltage work. However such machines are capable of inflicting serious damage in foolish hands, so should be secured away from children and grown ups that lack understanding of the risks present.
Fit a shorting strip when stored to prevent any external voltage resulting from accidental handle movement.
Accuracy
These meters can not be considered accurate. The difficulty of maintaining a constant and precise crank speed means that output voltage varies fairly quickly in practice. However they're plenty accurate enough to detect insulation breakdown problems, enable checking of repairs, and test one's own appliances for insulation failure.
Accuracy can be improved significantly by adding a crank handle to the motor, enbling continuous and more constant speed and thus output voltage.
These meggers don't meet BS and aren't suitable for use at workplaces.