EEVblog #468 - Uni-T UT513 Insulation Resistance Meter Teardown

[EEVblog]

Teardown Tuesday.
Inside the Uni-T UT513 5KV Insulation Resistance meter.
And what happens when you apply 5KV across the range switch of a cheap arse multimeter?

Relay datasheet: http://www.okita.co.jp/home/okita-up/english/products/reed/relay-pdf/lrl-10x-100pcv_eng.pdf

[c4757p]

They used a special power adapter and plug for isolation? God I hate having to keep track of those! Why couldn't they just use a little inverter to provide their own isolation to whatever adapter you use? There can't be much cost difference between the external transformer and an internal one - throw in a couple $0.03 transistors and you have yourself an inverter!

[Psi]

I'm more concerned about why it keeps crashing. 

If it keeps locking up it's not all that useful, even if the pcb is well designed with good attention to detail.

[c4757p]

I bet it has to do with that shielding over the MCU - not enough. With the rise time of those arcs across the range switch, there must be a ton of EMI in that thing.

[EEVblog]

I bet it has to do with that shielding over the MCU - not enough. With the rise time of those arcs across the range switch, there must be a ton of EMI in that thing.

Yep, it's the likely cause.
These things usually aren't used to generate sparks though 

[Rufus]

I doubt the relay switches the output. The biggest danger of insulation testers is leaving capacitance in what you are testing charged up.

The relay probably shorts the multiplier and output through the chain of power resistors above it.

The other relay probably switches the transformer secondary windings to give two output ranges.

The cut outs around the 'earth return' line are not for high voltage. Measuring the current in the 'earth return' is what the whole instrument is about and 1T ohm at 1kV is only 1nA. The cut outs and lack of solder resist will be to reduce board leakage. It would have been interesting to see what that return current fed into.

Is Villard Australian or something? I thought Cockcroft-Walton was more common and Greinacher invented it earlier anyway.

Anyhow thanks for the tear down. I nearly bought one of these for a job where I should have been testing at more than 1kV (it would have identified earlier a clearance screw up which broke down at 1.5kV - lol). I haven't needed more than 1kV since but nice to see what I would get if I do in the future.

edit: Or maybe the relay just shorts the output through the big resistors, the chain with capacitors across them is strange, maybe it is feedback for the switcher and the 200M resistor is just for measurement.

[c4757p]

These things usually aren't used to generate sparks though 

Oh, come on! That's the first thing you do when you have something that puts out 5 kV!

[nitro2k01]

With regard to the crashes, I'm thinking that maybe the lead placements actually matter. As you placed the leads, you've created a nice antenna loop around LV part of the meter. Maybe the crashes would go away if you actually placed the leads and device under test strictly behind the meter.

[NiHaoMike]

Maybe try using it to measure the resistance of a piece of ESD mat as well as common insulators?

I was expecting a TV type flyback with integrated rectifier. I would expect such a thing to be cheaper than a lower voltage transformer and voltage multiplier.

[Involute]

[c4757p]

If you use the subject line like that, people are going to miss the question.

Among other things, it prevents debris from compromising isolation distances.

[oPossum]

The USB chip is made by WinChipHead. The stylized WCH is their logo. They make the horrible CH341 USB to serial chip - possibly the worst chip of it's kind.

I wouldn't be surprised if the USB on that thing has some problems.

[sleemanj]

For more sparkly goodness, forum user Lightages posted this a few days ago:
https://www.eevblog.com/forum/testgear/how-to-let-out-the-magic-smoke-in-a-ut61e/

[Lightages]

Yeah! This insulation tester is lots of fun! The UT71E buzzes really good too, but mine survived 2500V, for how long it would do it I don't know. Would it take 5000V? I don't want to ruin it.

[nitro2k01]

And what happens when you apply 5KV across the range switch of a cheap arse multimeter?

What would happen if you pulled the same stunt on a Fluke?  I went back and reviewed Ep#64 and at around 5:30 you (Dave) were raving about the design of the input section. Would that prevent the arcing you showed the "cheap arse" multimeter suffering? Because to my, admittedly inexpert, eyes the range switch seemed pretty similar as a set of curved tracks on the PCB.

If the input protection works as it should, the MOVs should kick in and be the path of least resistance for the current. The moves are connected through a beefy 1 kOhm power resistor, which will dissipate power while the current flows through it, while clamping the voltage so that there's never potential between any two points that are so close that you would get potential (no pun intended) for arcing.

[baoshi]

The C8051F352 is made by SilLabs. Datasheet here http://www.silabs.com/Support%20Documents/TechnicalDocs/C8051F352-Short.pdf
These are sold as "Mixed Signal MCU"s as they usually come with better Analog circuit. For F352 is has built in 16bit ADC.

[rolycat]

Is Villard Australian or something? I thought Cockcroft-Walton was more common and Greinacher invented it earlier anyway.

Many references on the net identify the Swiss physicist Heinrich Greinacher as the original inventor of the voltage doubler in 1913, and of extending it into a cascade in 1920. The British John Cockroft and the Irish Ernest Walton rediscovered the cascade while working together in 1932 (and won a Nobel prize with it).

Villard apparently invented a more primitive diode clamp circuit. However, none of these references identifies who Villard is or was, or why his (or her) name is so frequently misapplied to the Greinacher cascade.  Can anyone here shed light on this mystery?

[EEVblog]

Villard apparently invented a more primitive diode clamp circuit. However, none of these references identifies who Villard is or was, or why his (or her) name is so frequently misapplied to the Greinacher cascade.  Can anyone here shed light on this mystery?

I was taught that the basic doubler without the filer is the Villard doubler, and once you add the diode/cap filter it becomes a Greinacher doubler / Cockcroft-Walton doubler. Adding more becomes the cascade/multipler. Cockcroft-Walton is probably the most popular, but I have often used the name Villard.
I have no reference for that though. Maybe buried away somewhere...

[M0BSW]

 Dave, does this mean I can send my lame arsed vc99 to you for total destruction

[HKJ]

When I got the UT513 some time ago I wondered why it hat a E-STOP, you could just push the TEST button to turn it off, it is larger and it is illuminated.
First time it arced over, I found out why. The E-STOP is the only button that always works.
It might have a nice hardware design, but I do not like that the microprocessor locks up when it arcs on 5000 volt (I have not seen it on 2500 volt).

[Phantomix]

The C8051F352 is made by SilLabs. Datasheet here http://www.silabs.com/Support%20Documents/TechnicalDocs/C8051F352-Short.pdf
These are sold as "Mixed Signal MCU"s as they usually come with better Analog circuit. For F352 is has built in 16bit ADC.

Thanks, just wanted to post that here
I'm developing software for these Silabs MCUs (that means, I "have to"), they have an crusty old 8051 core tweaked with more peripherals and a higher clock (usually up to about 25 MHz and single cycle instructions)...

Edit: The f352 used in the uni-t can run at 50 MHz

[tom66]

Nothing wrong with Golden Power et.al.
Cheap alkalines often perform equally well (or nearly as well) as branded batteries
http://www.batteryshowdown.com/

[Entropy]

Interesting teardown, they put a lot more stuff into this than in the UT511, which I own. The UT511 only goes up to 1000V and is approx. 80 EUR. Attached is a photo of the main pcb for comparison:

[tom66]

Interesting they still use the mu-metal on the processor, so perhaps they have problems with it crashing at just 1kV too.

[EEVblog]

Interesting teardown, they put a lot more stuff into this than in the UT511, which I own. The UT511 only goes up to 1000V and is approx. 80 EUR. Attached is a photo of the main pcb for comparison:

HUGE difference!, thanks.