Insulation Tester

[BILLPOD]

I just bought a cheapy insulation tester, an Aneng MH12.   I was checking it out and connected it to some small carbon or metal film resistors in order to see if resistances are in the ball park.   I set the tester to
50 VDC an put the probes across a 6,8 M ohm resistor and got a reading of 10.3 M ohm.   Then I measured a 22 M ohm resistor and got 32,5 M ohm.
What am I missing here?

[BillyO]

What am I missing here?

A good insulation tester.

These cheapies are ball-park devices at best and are not really intended to test resistors.

[BILLPOD]

Thank you BillyO, for your response.   I would hope my 'cheapy' UM12
tester would be 'in-the-ballpark', but the readings seem to be in a different ballpark.    Would you, or someone else with a GOOD insulation tester check what readings they get on some M ohm resistors to see if there are some other factors at play that would cause such wild results?   Thanks.

[BillyO]

I don't have an insulation tester otherwise I would.

Specs say +/- 5% (of range) and +/- 10 counts.  So yours does seem to be off a little.

[Gyro]

The high voltage source on cheap insulation testers can sag under the load of 'low' value resistors. This causes a lower than expected current through the resistor and correspondingly higher resistance readout. It might read better at 100M+

[555]

I have measured some resistors with my Fluke 1507:
10M: 10,5M @50V - 10,62M @1000V
47M: 47,2M @50V - 47,33M @1000V

[srb1954]

I just bought a cheapy insulation tester, an Aneng MH12.   I was checking it out and connected it to some small carbon or metal film resistors in order to see if resistances are in the ball park.   I set the tester to
50 VDC an put the probes across a 6,8 M ohm resistor and got a reading of 10.3 M ohm.   Then I measured a 22 M ohm resistor and got 32,5 M ohm.
What am I missing here?

Connect a DMM to the insulation tester output and check that it is putting out the correct test voltage. It should be within 20% of the specified level.

Check on other test voltages as well but first check that your DMM is rated for the maximum test voltage. If the test voltage significantly deviates from the specified figure that might affect the accuracy of the resistance readings. In that case I would suggest that you have a a faulty unit and should return it.

The resistance reading when the DMM is connected should be either 10M or 11M depending on the model of your DMM. This reading should not vary significantly across the differently test voltages as the input resistors in a DMM are generally more stable than the average resistor at higher voltages.

[BILLPOD]

A big thank you to all who responded.    I 'think' I've discovered the problem......I put NimH batteries in it.   Even though they were fully charged, (all just over 1.3 VDC), it was not close enough to the 12 VDC
the meter requires.   I don't have any 1.5 VDC primary alkaline cells
because of the leakage thing, so I ordered some lithium primary cells and will re-test when they arrive, and report back here.   Thanks again

[xavier60]

A TEKTROM TTE511 Insulation Tester I bought a month ago runs fine on NiMh cells.
I just did a quick resistance accuracy test by connecting it to my Fluke 87V on DCV range.
The TTE511 displayed 10MΩ +/-1% on each of its voltage settings.
As well as the insulation resistance, it also displays the actual test voltage. Insulation testers try to maintain a constant test voltage up to some current limit.
This is the only meter I could find in the $100 price range that also has a 200Ω continuity range.
My particular meter had a fault causing poor accuracy on the continuity range, a faulty BJT in its 1mA current source.

[BILLPOD]

OOPS.......I forgot about this matter, but here is the ending of my escapade.......
I filled my Aneng MH12 insulation tester with primary lithium batteries, (it takes 8 of them), and it works like a champ.  Even my 40M resistors read real close to spot on.   Now to test all my insulation.

[BeBuLamar]

When I was still working I had a Fluke 1577 and it has the resistance function and insulation test function separately. I never compare the reading between the 2.

[Terry Bites]

Insulation testers do just that. They check you're in range.  They are not intended as high voltage ohm meters.
You can get those of course- expect to haemorrhage (sorry yanks) cash.

[Gyro]

It depends which one you have. My second hand Kewtech achieves 1.5% + 5 digits on the 20M and 200M ranges. Accuracy does drop off to 10% + 3 on the 2G range though. That's with 250V/500V/1kV source maintained to 1mA, ie, down to 1M on the 2G range. You can pick them up for <£30 in the UK if you bide your time. I don't know what sparks in the US use but don't neglect professional used stuff.

[nightfire]

Do the test again with 500VDC- some el cheapos have issues when used below 250VDC.
(Also applies to the more expensive appliance testers from the big brands, that they need to be used with higher voltages to produce correct results.)

Also a good idea as mentioned is to measure the voltage put out with a DMM that can handle that- and within a DMM we know we have some precision resistors.