Well that lasted a long time. Just blew up my UT-61E - Page 1
Forum section: electronics / testgear

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 02, 2016, 09:43:08 am
content: I'm building the front end (minus the feedback loop) of a simple boost converter to kick out 30V at ~500mA. Schematic here:Power supply is a TTI PL330 and the pulse source is a 4060 RC oscillator. 22R series resistor is to stop the inductor exceeding its current limit.I connected the output across the cap across the UT-61E and after 6 seconds, I get an OL reading. It hit about 70v as expected before the DMM blew up.No fuse blown, nothing. Can't see any damage on the board. Just doesn't read a damn thing. I suspect this might be switching transient pulses off the inductor popped something. The thing is only 2-3 months old.Gah Chinese crap. Back to the knackered old Fluke 76.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 02, 2016, 12:53:13 pm
content: Quote from: MrSlack on March 02, 2016, 09:43:08 amA new transient generator is born!

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 02, 2016, 01:55:14 pm
content: Also found out that the 1N5819 now conducts in both directions just as well. Something serious went wrong here.

author: macboy
author_rank: Super Contributor
timestamp: March 02, 2016, 02:29:49 pm
content: Quote from: MrSlack on March 02, 2016, 09:43:08 amYou built yourself a little high voltage generator there. With no load to absorb energy, all the energy pumped from the inductor every cycle will just increase and increase the voltage on the cap until bad things happen.I like to put a MOV across the output of such things when mucking around. Any jellybean MOV pulled from an old power supply. It tends to save MOSFETs, diodes, and meters.If the meter is really less than 3 months old, it may be covered under warranty or by credit card insurance if you used a card to buy it. Many credit cards have 90 days of theft, loss, or damage coverage for almost anything you buy. Many will also double the warranty up to an extra year.p.s. I simulated your circuit exactly as posted. It behaves poorly. I would not have taken it from paper to metal just yet.

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 02, 2016, 02:45:51 pm
content: That SPICE simulation was only the basis of the circuit. The built version does have a load on it (1K resistor). I knocked up a peak meter circuit to measure the pulse voltage when the inductor field collapses and it only tops at 75v unloaded. Sticking it on the Fluke I'm getting a peak voltage before it rolls off of 34v which is within my expectations.A MOV isn't going to do much because it breaks down at ~200v which is far away from this.I suspect this was a faulty meter to start with. It just hadn't blown up yet.I tend to hit reality quickly. I only trust SPICE to show that the idea is feasible and then develop it further from there.Now for the control loop...

author: Monkeh
author_rank: Super Contributor
timestamp: March 02, 2016, 02:59:54 pm
content: Quote from: MrSlack on March 02, 2016, 02:45:51 pm200V isn't going to break a UT61E.Also, MOVs come down to tens of volts clamping voltage, and go up to thousands. They're not all identical..

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:24:24 pm
content: Fair points. I've got a few MOVs here and they are all 200v. Poor assumption on my part.

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:30:06 pm
content: Found the issue and I look like a right idiot. I carefully examined the meter and it turns out the battery is reading 7.6v. I replaced it snd it springs back into life as normal.I think this may have just been a massive coincidence and the battery meter on the UT-61 is s**t and I'm an idiot.

author: Performa01
author_rank: Super Contributor
timestamp: March 02, 2016, 04:35:12 pm
content: Well, things like this happen to all of us from time to time, don't they?Glad you posted the resolution of that puzzle though. I really couldn't believe a circuit like this would break a meter - particularly without blowing the output cap first...

author: djQUAN
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:41:33 pm
content: Quote from: MrSlack on March 02, 2016, 01:55:14 pmIIRC, you peaked 75V. 1N5819 is rated at 40V peak reverse voltage.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2016, 03:20:53 am
content: Quote from: MrSlack on March 02, 2016, 04:30:06 pmEarlier UNI-T meters started talking crap long before the low battery indicator came up.That's why I've never been fond of them.At one job,an incorrect value of resistor had been fitted to a unit,causing it to malfunction.(it should have been 15k)One of the "heroes" looked at it with a UNI-T,which read 13k---near enough,seeing it was "in circuit".It was hard to see,but the colour code didn't look like 15k to me,so I hung my old Fluke 77 across it,amidst much jeering from the aforesaid group.Sure enough,---25k!Fitted a new battery to the UNI-T,& it also read 25k.The resistor turned out to be nominally 27k.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 03, 2016, 03:28:01 am
content: That was an easy fix. Good to hear that's all it was.

author: 3141592
author_rank: Regular Contributor
timestamp: March 03, 2016, 07:39:02 am
content: Quote from: MrSlack on March 02, 2016, 04:30:06 pmYou were about to scare me away from purchasing a 61E for myself, I'm glad to hear 70V doesn't kill it

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:46:22 am
content: Indeed. I've got a UT33A that is like that. Now delegated to continuity testing in the car.Quote from: djQUAN on March 02, 2016, 04:41:33 pmI'm using two in series in the actual circuit while I wait for something a little heftier to arrive.On a positive note, I've managed to bodge a feedback loop into this and replaced the oscillator with a ramp generator as per a recent w2aew video using just transistors, swap out the inductor for a larger one, used a logic level MOSFET instead and did some tuning with a bit of experimentation (bode plots and transfer functions - can't be bothered) and it's knocking out a stable 30v at 300mA which is a little under expected but OK. It doesn't drive a capacitive load well though as there is some oscillation and if you short it, bad noisy things happen so I will do some more thinking.This is the first non custom IC based converter I've built so not too terrible so far.

author: Kilrah
author_rank: Unknown
timestamp: March 03, 2016, 07:56:48 am
content: Quote from: MrSlack on March 02, 2016, 04:30:06 pmThe other day I was at work and the A/C stopped at literally exactly the same time as the screen of the computer in front of me went dark. Instantly went "oh joy, a power cut"... but no, just a perfect coincidence of the computer's power save and the temperature reaching their set point at the same millisecond or so

author: mnementh
author_rank: Super Contributor
timestamp: March 10, 2016, 02:32:04 am
content: *Pokes Kilrah*Boo!Shoulda known I'd find you spoiling the neighborhood if I lurked here long enough...mnem*tzzzzzt*

author: Helix70
author_rank: Unknown
timestamp: March 10, 2016, 10:45:49 am
content: Quote from: 3141592 on March 03, 2016, 07:39:02 amYou might have missed the moral of the story here.

author: mos6502
author_rank: Frequent Contributor
timestamp: March 10, 2016, 10:53:52 am
content: That's the trouble with Wun Hung Lo meters ... weird issues that can cause huge headaches. See my experience with the UT139C.By the way, cheap Chinese calipers do a similar thing ... no low battery warning, they just start to give false readings.