Adding MOV's to a multimeter for extra protection? (and fuse modification) - Page 1
Forum section: electronics / testgear

author: XOIIO
author_rank: Super Contributor
timestamp: May 01, 2014, 03:00:58 pm
content: Hey all, I have a UNI-T UT71C on it's way in the mail, hopefully it won't take too long.Anyways I looked on youtube and found a review/teardown of the T71D which is a model up, basically just more datalogging space so I imagine it is an identical board.Anyways, in the teardown, there were only a couple PTC's for input protection which was disappointing for a $120 multimeter. Anyways, I'm not very familiar with input protection but I do know from many teardown videos there should be some MOV's in there as well. (If you want to explain the main differences aside from the obvious that one is a resettable fuse and the other just blows like I mini fuse I guess?) but I figure, if MOV's are just put in series with PTC's, could one not just ad an appropriate value one in as a bit of a kludge? One lead of the PTC in one side, one lead of the MOV in the other and connect the MOV and PTC. I'm not sure if they are supposed t be in different parts of the circuit or what sort of MOV is best used, so I figured I'd ask.Also, the use piddly glass fuses in small fuse holders, however on the board revision I saw (mine may be different, I'll have to see) they had the footprint for larger fuse holders as well, and just put the smaller ones in place. I figured you could just take out the small fuse holders, put in the big ones and then you could use some proper HRC fuses, I'm just wondering how one would source those fuse holders, if there is a specific size to look for them with or something like that.At least my $50 multimeter had ceramic fuses in it instead of glass.

author: electronics man
author_rank: Frequent Contributor
timestamp: May 01, 2014, 03:18:34 pm
content: the difference between movs and ptcs are movs are voltgae controlled resistors and PTCs are tenparature/current controlled resistors

author: XOIIO
author_rank: Super Contributor
timestamp: May 01, 2014, 03:25:33 pm
content: Quote from: electronics man on May 01, 2014, 03:18:34 pmAlright, so it basically just covers both your bases.Are they usually just in series on multimeters?

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2014, 03:38:59 pm
content: Add the MOV's there on the spaces left on the PCB, and use 2 275V 10mm diameter ones in series. They typically will clamp at around 750V then. I did the same to my UT61E, and used 4 MOV's to do so. Cheap and easy. MOV's were around $5 for 5 at RS IIRC, I had ordered them as part of another order. If you order 543-5158 you get a 710v unit that will fit right in and will work well. Those are 6c each.

author: DJ
author_rank: Unknown
timestamp: May 02, 2014, 05:03:00 am
content: Adding mov's will add capacitance, so will alter those measurements. Also mov's have a softer clamping characteristic. Might want to consider a low-C tvs or the like.

author: XOIIO
author_rank: Super Contributor
timestamp: May 02, 2014, 09:47:00 am
content: Quote from: DJ on May 02, 2014, 05:03:00 amAh, alright, I don't use capacitance much but I would not really want to do anything that changes the multimeter's measurements.