Author Topic: Is it good idea to protect DMMs with new TVS diodes?  (Read 1562 times)

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Offline 001

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Is it good idea to protect DMMs with new TVS diodes?
« on: October 30, 2017, 07:20:51 pm »
Is it good idea to protect DMMs with new TVS diodes?
TVS diode much faster than varistors etc https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode
Capacitanse also small

Maybe some typical circuit avaliable now?
« Last Edit: October 30, 2017, 07:25:12 pm by 001 »
 

Offline MosherIV

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #1 on: October 30, 2017, 08:25:07 pm »
It is not a good idea to try and improve the input protection of a dmm yourself unless you
A) know what you are doing
B) have the equipement to check the modification has the desired effect (without causing unwanted effects)
(see joeqsmith thread on handheld meter robustness testing )

Have a look at Dave's video on dmm input protection


Good meters (such as Fluke and Keysight) has this kind of protection
 

Offline joeqsmith

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #2 on: October 31, 2017, 09:16:21 am »
It is not a good idea to try and improve the input protection of a dmm yourself unless you ...
Agree.  That said, OP may just be asking why not use them.  They certainly have their place. The cheap ANENG meters I looked at use one. Not that it did them much good. The poor circuit board design did a much better job protecting the electronics.  Funny is some people look at this as a good thing.   

I used them as part of the videos I made showing a highly modified UT61E.  I had several videos showing Dave's pre-production 121GW meter after I damaged it with a fairly low voltage hit.  One of the videos showed the meter after I had repaired it.  I added two different TVSs to the meter along with a few other changes in an attempt to harden it. 

My advice is start reading the datasheets.  There are reasons the various techniques are used. 
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline EEVblog

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #3 on: October 31, 2017, 09:53:07 am »
The 121GW has a TVS in lieu of the usual transistor zener pair primarily due to the unusual high voltage 15V diode mode requirement.
They are pretty sweet
http://www.mouser.com/ds/2/389/sm6t-955193.pdf

One issue is that of leakage, it may upset the calibration of the high resistance ranges.
Not easy to find low leakage types. The lower voltage ones typically have higher leakage.
« Last Edit: October 31, 2017, 09:58:59 am by EEVblog »
 
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Offline 001

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #4 on: October 31, 2017, 12:44:05 pm »
Thanx a lot!  :-+

I`m going to use cheap autoraging DMM (in voltmeter mode + extermnal shunt) as a panel ammeter for my DIY bench AC power supply (variac+iso transformer 220/400V)

If I use cheap DMM (in AC voltmeter mode only, 50/60Hz)  can I use TVS diode across external shunt to safe dmm in the case of shorting load?

PS - What is typical DMM input circuit with TVSs?
« Last Edit: October 31, 2017, 12:46:30 pm by 001 »
 

Offline joeqsmith

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #5 on: November 04, 2017, 04:59:07 pm »
The 121GW has a TVS in lieu of the usual transistor zener pair primarily due to the unusual high voltage 15V diode mode requirement.
They are pretty sweet
http://www.mouser.com/ds/2/389/sm6t-955193.pdf

One issue is that of leakage, it may upset the calibration of the high resistance ranges.
Not easy to find low leakage types. The lower voltage ones typically have higher leakage.

That supply was a weak point.   Was this a new change in attempt to harden that part of the design?   

     
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Online T3sl4co1l

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Re: Is it good idea to protect DMMs with new TVS diodes?
« Reply #6 on: November 04, 2017, 07:25:26 pm »
The 121GW has a TVS in lieu of the usual transistor zener pair primarily due to the unusual high voltage 15V diode mode requirement.
They are pretty sweet
http://www.mouser.com/ds/2/389/sm6t-955193.pdf

Hmm, unusually well specified, I like the clamping curves.

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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