Varistor/MOV for UT61e - Page 1
Forum section: electronics / testgear

author: MosherIV
author_rank: Super Contributor
timestamp: July 06, 2016, 02:56:28 pm
content: HiI do not know much about Varistors (or MOVs) apart from reading the specification.I am looking at thesehttp://cpc.farnell.com/epcos/b72210s0381k101/varistor-40-0j-385vac/dp/RE03689?ost=RE03689&selectedCategoryId=&categoryNameResp=All%2BCategories&searchView=table&iscrfnonsku=falseFrom the specification, it look like it clamps at 1025Vstarts conducting at 385VAC and 505VDCSo I have some question for those that know about them if you do not mind, please?Q1. Is this a suitable Varistor to use ?Q2. What happens when the Varistor starts to conduct, does it affect the circuit being measured (data sheet shows 1mA but not sure what that is referring to) ?Q3. In another thread, someone else was asking about Varistors and said they have a current limiting resistor in series with the MOV, I do not see the MOV in the schematics - does it need the current limiting resistor?

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 06, 2016, 04:08:10 pm
content: Quote from: MosherIV on July 06, 2016, 02:56:28 pmq1. depends on the application/circuitq2. a mov always conduct, it is just that it will conduct more the higher the voltage. 1ma means that at a certain applied voltage it will sink 1ma. Of course it will affect the circuit beign measured, the same as a resistor in parallel will doq3. no it doesnt, generally movs are used in inductive loads o prevent over voltages, and you want to dissipate as much energy as possible to avoid the voltage from increasing

author: MosherIV
author_rank: Super Contributor
timestamp: July 06, 2016, 04:24:41 pm
content: Thanks MasterTechQuoteSo, I am talking about there use as protection in multimeters.How does a multimeter use the MOV and not affect the circuit it is measuring?I guess I may have to go and re-watch Dave's video on how multimeter input protection works again.

author: wraper
author_rank: Unknown
timestamp: July 06, 2016, 05:47:53 pm
content: If you attach MOV directly on the input, there will be explosion once clamping voltage is exceeded and DUT can provide enough current. You cannot just put MOV somewhere, it must be designed into the circuit to work properly.

author: MosherIV
author_rank: Super Contributor
timestamp: July 06, 2016, 08:25:32 pm
content: Hi wraperThanks, maybe I should fill in the gaps.The UT61e already has MOVs in the design, there are not fitted to the cheap Chinese versions.See attached picture, labelled SG1, 2, 3 & 4One of my questions was asking if the MOVs I had chosen was right. Having just finished re-watching Dave's mm input protection video, it is now clear to me the answer is no.I was wondering the significance of the turn on voltage verses the clamp voltage. I now have my answer.Since the MOV turn on voltage will affect the DUT measurement, I need to choose MOVs with the turn on voltage for the max voltage I want to measure, the clamp voltage should not be too far off the turn on voltage.Although Dave did not say in his video, I am guessing the input protection works by blowing the PTC and/or the ww resitor when the MOVs blow. So it is not a problem that the MOVs destory themselves, that is their job in the case of an extreme over voltage surge.Thank wraper and masterTech for your explainations.

author: wraper
author_rank: Unknown
timestamp: July 06, 2016, 08:57:57 pm
content: I'd say they need voltage rating of about 1.5 kV DC in total, seems they are in series there.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2016, 01:21:36 am
content: Maybe this would be of some interest.https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/875/

author: MosherIV
author_rank: Super Contributor
timestamp: July 07, 2016, 02:43:31 pm
content: Thanks joeqsmithI had a look through the thread and found the reference to the MOVs from RS. The thread says that they are specified for lower than the meter is capable of reading.In addition, the SG1 -3/4 are not on the schematic so I do not know if they have put them in series or straight across from PTC to GND. I cannot tell from any of the pictures how they are connected.I am still waiting for my UT61e to arrive from China, so I do not know which version of the PCB I will get, one takes 3 MOVs and the other 4 MOVs.When I figure it out, I will post here.

author: highwayman
author_rank: Contributor
timestamp: July 07, 2016, 04:30:51 pm
content: I (vaguely) remember doing a continuity check on these. Don't remember which designator went with which MOV.Only one of the MOVs was connected to the input. The other three were all connected to the other side of that MOV that was connected to the input. Presumably, the other side of those MOVs went to three different input circuits.

author: MosherIV
author_rank: Super Contributor
timestamp: July 08, 2016, 12:19:17 pm
content: Thanks, I will check it out when my UT61E arrives, should be soon nowsince the other stuff I bought has started to arrive, Yipee.

author: Arhammon
author_rank: Contributor
timestamp: July 08, 2016, 05:34:06 pm
content: Quote from: MosherIV on July 07, 2016, 02:43:31 pmThe input schematic have to look like this:I have placed 8 MOV 10391(320vDC). It's a bad idea! They use too much space.

author: stj
author_rank: Super Contributor
timestamp: July 08, 2016, 10:29:46 pm
content: an engineer would not use the term "SG" for a MOV/VDR, those spaces are for a spark-gap device.

author: highwayman
author_rank: Contributor
timestamp: July 09, 2016, 08:11:14 pm
content: I will throw this out there... NOT as an example, but maybe something to poke holes in. After I added these mods, the UT61E has been demoted to low voltage use. The frequency range of the meter has been adversely affected, so I will likely remove the spark gaps eventually and keep it for low voltage.I picked the Littelfuse GTCA25-401M-R02 based on the 400V and 2.5kA discharge current. The thought is that I would have 800V protection since the SG are in series. The SG are pretty close to each other, so I added the heat shrink to reduce the effective air gap between them.Depending on feedback, I might even test my UT61E on an 1kV/2.5kV Hi-Pot tester....poke away

author: joeqsmith
author_rank: Super Contributor
timestamp: July 09, 2016, 09:49:10 pm
content: Quote from: highwayman on July 09, 2016, 08:11:14 pmFrom what I can see your soldering looks fine.

author: MosherIV
author_rank: Super Contributor
timestamp: July 12, 2016, 10:49:13 am
content: HiGot my UT61E, WoopieI buzzed out the circuit in relation to the input protection, ie PTC and how the SG1, 2, 3 and 4 are connected.I am going with MOVs, Fluke seem to prefer them over spark gaps and UniTrend fit them in the European version.I am going with these :http://cpc.farnell.com/epcos/b72214s0321k101/varistor-84-0j-320vac/dp/RE03698According to the circuit, 2 of them in series will give 840VDC, 640VAC and clamp at 1.68KVIf I use one of these for the common MOVhttp://cpc.farnell.com/epcos/b72205s0381k101/varistor-13-5j-385vac/dp/RE03669Then I get 925VDC, 705VAC, and clamp at 1.865KV

author: nabardwell
author_rank: Newbie
timestamp: July 12, 2016, 11:27:03 pm
content: So here is a very useful link to some one who got the European version it is rated cat3 600v and cat4 300v. It has voltage protection but in the form of a single SG instead of a mov.http://www.mjlorton.com/forum/index.php?topic=140.15

author: joeqsmith
author_rank: Super Contributor
timestamp: July 13, 2016, 04:17:28 am
content: I wonder what adding them does besides fills the holes.

author: MosherIV
author_rank: Super Contributor
timestamp: July 13, 2016, 09:36:11 am
content: QuoteIt completes the input protection for the meter. Some components are left out for cost saving in the China region where there is less need for regulatory compliance.The GD (Gas Discharge) or MOV (Metal Oxide Varistor) provide high voltage/surge protection by conducting above a threshold voltage thereby protecting the input from over voltage.The UT61E input protection is lacking because there is no wire wound resistor in line with the PTC, so when the GD/MOV activate for very high energy over volt only the PTC is there to burn out. Fluke use a wire wound resistor which should burn out.It can also protect the meter from static discharge, some people have reported the UT61E being damaged by static.

author: totalnoob
author_rank: Regular Contributor
timestamp: July 13, 2016, 12:41:14 pm
content: Quote from: MosherIV on July 13, 2016, 09:36:11 amWithout meaning to poo poo what you are doing, but do you have proof that these mods will do what you believe they will do? Also, when you factor in your time as well as the parts, are you really saving anything by buying a cheap meter and performing these mods and getting a meter that may or may not be as safe as you believe, vs. getting a meter that is known to meet the CAT ratings stamped on it?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 13, 2016, 01:00:23 pm
content: Quote from: totalnoob on July 13, 2016, 12:41:14 pmI've looked at a few meters inputs and am still amazed at what prevents damage and what does not. This is why I provided the other 61E link.When you mention the cost saving in the China region where there is less need for regulatory compliance, are you suggesting that the meters that are sold to some areas were actually certified?Quote from: joeqsmith on May 14, 2016, 03:22:14 pm

author: MosherIV
author_rank: Super Contributor
timestamp: July 13, 2016, 01:16:46 pm
content: QuoteNo, I do not have evidence that it makes the meter safer.Yes, I could have spent the money on a better meter, in fact I did. I just wanted to see what this meter was about and knowing some of the flaws of the China import ones, correct the most obvious flaw - poor over volt protection.No, it was never about making the meter meet the CAT rating. I have a Keysight meter for that.I am just playing around and learning more stuff.QuoteGood point, no there is no evidence that Uni-T certify the meters anywhere in the world that I am aware of. I should have said that Uni-T build them to a higher safety specification for other markets. I have a Tenma branded meter, which is the Farnell own brand of equipment which is rebadged Uni-T, and inspecting the meter, it certainly looks like all the safety elements are there : fuses (though not HRC), clamping diodes, PTCs, MOVs, ww resistor.

author: totalnoob
author_rank: Regular Contributor
timestamp: July 13, 2016, 04:49:00 pm
content: Quote from: MosherIV on July 13, 2016, 01:16:46 pmOK, I can see "playing around".I wasn't sure if you were trying to be more "serious" (i.e. someone trying to save money by buying an inexpensive meter then trying to make it nearly as "safe" as its stamped CAT ratings imply it is).

author: totalnoob
author_rank: Regular Contributor
timestamp: July 13, 2016, 04:58:35 pm
content: Quote from: joeqsmith on July 13, 2016, 01:00:23 pmIf your question was directed @ me (I see Mosher answered it from his perspective) I really had in mind something more like the EEV Brymen meter or some other meter that is really not that expensive (some of the Chinese Flukes I believe fit the bill as well) and comes much closer to meeting the CAT ratings on the meter than the UT61E in any of its variants. It appeared to me that Mosher was more "serious" in trying take a cheap meter and make it try to comply with its CAT ratings as a way to get an inexpensive, relatively accurate and "safe" meter for little money. Since we are talking about his labor and not Chinese factory labor, I would think that if it were cost effective Uni-T would have already made the UT-61E meet its CAT ratings, and factoring in "first world", if you will, labor rates to apply to Mosher's labor to research, order and install the components, would only make the equation more favorable to spending a little more for a meter that already meets the CAT ratings stamped on it.

author: nabardwell
author_rank: Newbie
timestamp: July 13, 2016, 10:38:30 pm
content: The European model is certified and it is rated differently. This certification actually means something and the changes to the meter will offer better input protection against esd and over voltage. Tests have been done on the standard design without movs and it causes catastrophic failure and arching on the selector knob traces. There is also an even newer version posted by some one and it actually uses movs for input protection it looks like they might have finally heard everyone complaining about the lack of input protection. This is a ut61d but it has the same layout as the 61ehttps://www.eevblog.com/forum/testgear/uni-t-ut61d-new-version!/

author: nabardwell
author_rank: Newbie
timestamp: July 13, 2016, 10:48:00 pm
content: I dont get it. People it is certified in Europe is has GS certification a real certification unlike the UL certification. "The Geprüfte Sicherheit ("Tested Safety") or GS mark is a voluntary certification mark for technical equipment. It indicates that the equipment meets German and, if available, European safety requirements for such devices." the European model has this

author: nabardwell
author_rank: Newbie
timestamp: July 13, 2016, 11:13:21 pm
content: Also i apologize for misspeaking I meant the CE mark not the UL certification. Both UL and GS require independent verification by a certified test facility CE does not.

author: nabardwell
author_rank: Newbie
timestamp: July 14, 2016, 12:01:00 am
content: Here is the newest German 61e with input protection the layout is exactly the same as the 61e that i have but with movs. The place i found it is a German form i asked them to take more pictures of the movs and ptcs so we can see the values.http://www.mikrocontroller.net/topic/358485

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2016, 01:42:56 am
content: Would be fun to see if it fairs any better than the other flavors of UT61Es out their. The 181A is certified but dies with very little effort. I guess it dies safely so it's ok.Does the GS mark require any sort of EMC testing?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2016, 01:45:11 am
content: Quote from: totalnoob on July 13, 2016, 04:58:35 pmOnly in that I agree with your post. Rest was for OP.

author: MosherIV
author_rank: Super Contributor
timestamp: July 14, 2016, 07:04:20 am
content: I stand corrected. I was not aware that the European version had been certified. CoolWorth putting the MOVs in then(Yes, I know it does not make the Chinese model GS certified but does improve the input protection, at very least from static)

author: ProBang2
author_rank: Frequent Contributor
timestamp: July 14, 2016, 09:20:47 am
content: Quote from: joeqsmith on July 14, 2016, 01:42:56 amThe UT61e is certified to the standard of 2006 (valid at the time of the certification), so it has to survive ESD up to 4 kV.The standard has changed IIRC in 2014 (valid from 2015) to ESD up to 8 kV, but that is not important for the UT61E.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 15, 2016, 02:31:12 am
content: Quote from: ProBang2 on July 14, 2016, 09:20:47 amSo I looked up what I could find out on the GS mark. Over and over they talk about it being a safety mark. I found a document that shows how they refer to the IEC standards. In the case of multimeters, looks like they follow 61010. Imagine that. What I did not find was any reference to 61326. Can you point me some sort of reference that shows how the GS mark is calling out the ESD testing?I would like to have a 61E GS to repeat the tests on. Wonder if it really would fair any better. Maybe one of the GS owners would be willing to risk their meter with the same grill starter I use.https://www.amazon.com/MHP-Piezo-Push-Button-Ignitor/dp/B00VN1JBSW/ref=sr_1_8?ie=UTF8&qid=1468549797&sr=8-8&keywords=grill+ignitor+piezo

author: ProBang2
author_rank: Frequent Contributor
timestamp: July 15, 2016, 11:43:48 am
content: Quote from: joeqsmith on July 15, 2016, 02:31:12 amReference:http://www.certipedia.com/certificates/50172953?locale=en&page_number=1

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2016, 10:56:22 pm
content: Quote from: ProBang2 on July 15, 2016, 11:43:48 amThank you very much for finding this. I would really like to run one of these. It would be funny if the grill starter still took it out. I'll look into it.

author: MosherIV
author_rank: Super Contributor
timestamp: September 04, 2016, 04:10:19 pm
content: HiI finaly got round to ordering and installing the MOVs.See attached pictures.I went with these in the endhttp://cpc.farnell.com/epcos/b72210s0381k101/varistor-40-0j-385vac/dp/RE03689These turn in at 505V DC385V ACand clamp at 1025VMeaning that the input protection kicks in at 1010V DC and 770V AC.I have no proof od this, and I do not have the equipment to test this.I did a check on the capacitance measurement before and after and yes the MOVs do affect the capacitance measurement. It adds around 300nF to the measurement but if the delta (or relaitve) mode is used this removes the stray capacitance.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 04, 2016, 07:25:29 pm
content: Looks good.I took another look at the layout of the 61E I just bought, what people have done and the GS marked meter. Would really like to know what was used in the GS marked meters.What's your plan now?

author: SeanB
author_rank: Super Contributor
timestamp: September 04, 2016, 08:21:23 pm
content: Thereare lower capacitance MOV's which will do the same job, albeit with lower power handling ability, but those used will work well enough.Joe, you might want to buy a pack of them and add to any meter that has the pads, and compare to another identical meter without them. Should at least survive the test a few times.

author: MosherIV
author_rank: Super Contributor
timestamp: September 04, 2016, 08:33:45 pm
content: QuoteErm...........Just use the meterI do not have access to any test gear to check the input protection.If I have access to PSU that can reach 1000v then I will try the meter on it.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 04, 2016, 10:47:15 pm
content: I already know where this particular rev of the board fails from the factory. It's pretty much like many of the UNI-T's. Was really hoping everyone who recommend a change would have had some data to back it up.I would guess that the GS meter would do better. At least survive long enough to get to the transient generator anyway.Let me look into it a bit and see what I can come up with. You can bet after I mod it, I will go ahead and test it so at lease we can see if there was any sort of gain. The PCB layout is better and I don't think I will need to chop it up like the 181A.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 04, 2016, 11:30:53 pm
content: Mosher, did you look to see if there was an adjustment for capacitance? Actually, it may help to have some sort of full alignment procedure for it.

author: MosherIV
author_rank: Super Contributor
timestamp: September 05, 2016, 10:32:35 am
content: QuoteI believe there is but I am not planning on adjusting it.Using the 'Rel' or Delta mode seems to remove the added capacitance.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 06, 2016, 04:13:46 am
content: Did find a schematic for an older version of the meter. Still may be helpful. Most trimmers appear to be marked on the board so shouldn't be too much of a problem.

author: MosherIV
author_rank: Super Contributor
timestamp: September 06, 2016, 10:05:15 pm
content: HiYeah got that schematic. I think I will leave my meter as is now.I have a Fluke 187 on the way, new toy to play withI might have a go at building a voltage multiplier to reach 1KV in the future.Thanks.