Varistor/MOV for UT61e

[MosherIV]

Hi

I do not know much about Varistors (or MOVs) apart from reading the specification.

I am looking at these
http://cpc.farnell.com/epcos/b72210s0381k101/varistor-40-0j-385vac/dp/RE03689?ost=RE03689&selectedCategoryId=&categoryNameResp=All%2BCategories&searchView=table&iscrfnonsku=false

From the specification, it look like it clamps at 1025V
starts conducting at 385VAC and 505VDC

So 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?

[PartialDischarge]

Hi

I do not know much about Varistors (or MOVs) apart from reading the specification.

I am looking at these
http://cpc.farnell.com/epcos/b72210s0381k101/varistor-40-0j-385vac/dp/RE03689?ost=RE03689&selectedCategoryId=&categoryNameResp=All%2BCategories&searchView=table&iscrfnonsku=false

From the specification, it look like it clamps at 1025V
starts conducting at 385VAC and 505VDC

So 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?

q1. depends on the application/circuit
q2. 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 do
q3. 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

[MosherIV]

Thanks MasterTech

q2. 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 do

So, 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.

[wraper]

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.

[MosherIV]

Hi wraper

Thanks, 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 & 4

One 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.

[wraper]

I'd say they need voltage rating of about 1.5 kV DC in total, seems they are in series there.

[joeqsmith]

Maybe this would be of some interest.

https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/875/

[MosherIV]

Thanks joeqsmith

I 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.

[highwayman]

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.

[MosherIV]

Thanks, I will check it out when my UT61E arrives, should be soon now  since the other stuff I bought has started to arrive, Yipee.

[Arhammon]

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.

The input schematic have to look like this:

I have placed 8 MOV 10391(320vDC). It's a bad idea! They use too much space.

[stj]

an engineer would not use the term "SG" for a MOV/VDR, those spaces are for a spark-gap device.

[highwayman]

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

[joeqsmith]

I will throw this out there... 
...poke away

From what I can see your soldering looks fine.

[MosherIV]

Hi

Got my UT61E, Woopie

I 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/RE03698

According to the circuit, 2 of them in series will give 840VDC, 640VAC and clamp at 1.68KV

If I use one of these for the common MOV
http://cpc.farnell.com/epcos/b72205s0381k101/varistor-13-5j-385vac/dp/RE03669

Then I get 925VDC, 705VAC, and clamp at 1.865KV

[nabardwell]

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.
[url]http://www.mjlorton.com/forum/index.php?topic=140.15[url]

[joeqsmith]

I wonder what adding them does besides fills the holes. 

[MosherIV]

I wonder what adding them does besides fills the holes.

It 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.

[totalnoob]

I wonder what adding them does besides fills the holes.

It 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.

Without 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?

[joeqsmith]

I wonder what adding them does besides fills the holes.

It 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.

Without 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?

 

I'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? 

I am interested in knowing more about how people who have recommended changing the meter have validated their changes.

Just a little thought on this....   I ran both the 61D/E and they failed with a basic ESD pulse.    I also ran a UT181A that failed with a single ESD event.  The UT181A has PTC, MOVs and secondary clamps.       

..............

In the case of the UT181A, ....  the layout has a few problems that needed to be addressed.  A little Dremel, X-acto and dope.    And no, adding a clamp to the output of the regulator was not one of them.   

With the recommendation to add MOVs, I am curious what the goal is?  Safety, robustness?  I would like to know how this was validated?   Do we even know if there was ANY improvement?  Worse, did adding them make the meter less safe?      I can see someone getting a false sense of added safety.   

It could very well be that UNI-T actually tested the meter with and without the additional parts and had the same performance and decided to save some money.  I have no idea.
I don't know about the safety side at all.   I just don't throw that sort of energy at them.  People recommending to add parts should have some data based on fact.   I gave up on Santa Claus and the Tooth Fairy years ago.   

.............

The 181A fully populated with all of it's fancy protection failed with just one hit of a very low ESD event.  Had they removed the parts and saved that money, it would have failed just the same..   On the flip side, had I received the meter without the protection circuitry populated and then correctly guessed what the parts were and added them, it would have still failed.     The layout is important and can't be ignored. 

What you have suggested may not be a bad idea.  If we forget about safety, I could run your meter to see if there was an improvement in robustness but you may not like the end results.   What's that meter cost?  When I made changes to the UT181A, I could have just said I fixed it or just reapplied a single ESD transient and left it at that.   I thought it was better to throw everything I could at it and leave no question the meter's robustness had indeed been improved.
 

How will you know if there is an improvement?

I guess you would not know and you'd better not find out the hard way.
I will send it to you to throw some kVs and see what happens...   

But seriously, if there is a transient and the MOVs are working, wouldn't they clamp it down
safely?   Before them there are two PTCs as well. 

[MosherIV]

Without 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?

No, 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.

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?

Good 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.

[totalnoob]

Without 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?

No, 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.

OK, 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).

[totalnoob]

I wonder what adding them does besides fills the holes.

It 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.

Without 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?

 

I'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? 

If 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.

[nabardwell]

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 61e
https://www.eevblog.com/forum/testgear/uni-t-ut61d-new-version!/

[nabardwell]

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