No colour TV's use an NTC in the degauss .. they use a PTC. As the device heats up, the resistance goes up and the current drops ...
We have sold literally tens of thousands of high power, transformer driven audio amplifiers (multi-kilowatt) with a pair of NTC's in the startup circuit, with no significant issues. Obviously in a transformer design, they are in the primary not the secondary, but an NTC would work fine in the output of the rectifier ... but actually, what we do on later switch mode usits is to use a PTC after the rectifier ... which sounds weird ... the idea is it forms a startup resistance which is shorted out with a relay once working voltage is reached ... but in the even of a fault at startup, it just gets hot, goes high resistance and sits their quietly cooking, the voltage on the caps never rises due to the fault, and the relay never closes, saves a lot of blown tracks! On Wed, 7 Aug 2024 at 05:54, gene heskett <ghesk...@shentel.net> wrote: > On 8/6/24 16:54, Peter Wallace wrote: > > On Tue, 6 Aug 2024, Viesturs L?cis wrote: > > > >> Date: Tue, 6 Aug 2024 23:44:21 +0300 > >> From: "[UTF-8] Viesturs L?cis" <viesturs.la...@gmail.com> > >> Reply-To: "Enhanced Machine Controller (EMC)" > >> <emc-users@lists.sourceforge.net> > >> To: "Enhanced Machine Controller (EMC)" < > emc-users@lists.sourceforge.net> > >> Subject: [Emc-users] Question to electrics gurus: Correct setup for > >> rectifier > >> + capacitor > >> > >> Hello! > >> > >> Recently I have posted some questions about the retrofit that I am > >> working on and now I have encountered an issue that I cannot figure > >> out. > >> The thing is that I am swapping original Yaskawa servo drives with > >> Mesa 8i20. One of the things for that was 3phase rectifier bridge (it > >> takes 230 VAC input and provides 340V output) and 4700 uF capacitor > >> for smoothing the ripple. > >> It took me some time and some patience from PCW to fix the config and > >> in the end I got one 8i20 to move one joint in the machine. So my next > >> step was adding 2 more 8i20 drives for remaining 2 joints. It ended up > >> with a blast in one 8i20. So I replaced it as well as I had to replace > >> rectifier bridge. But now my issue is that protection fuse is tripped > >> as soon as I enable machine power. > >> 1) only rectifier bridge connected - all good > >> 2) rectifier bridge + capacitor - fuse tripped immediately > >> Note that I have not yet connected DC bus to 8i20 drives. > >> I checked capacitor with multimeter, it was showing 180V DC (and it > >> did not decrease in that time that I was holding multemeters pins > >> there). I am not sure if that is some residual charge from previous > >> (which I doubt) or if that was some charge that was acquired before > >> fuse was tripped, but this seems to me like a good capacitor. Is that > >> correct? > >> > >> I do not understand why was everything fine in initial testing - I did > >> turn machine on and off lots of times and capacitor was discharged > >> (and recharged!!!) numerous times. I have swapped that fuse for > >> identical unit from a machine that has yet to go through the retrofit > >> process. And it is the same. > >> > >> So my question to electronics gurus - could capacitor be damaged or > >> was it just a beginners luck that everything worked and do I need to > >> introduce some inductor between rectifier bridge and capacitor to > >> limit the startup current that charges capacitor? > >> > >> Viesturs > >> > > > > Typically a NTC is used to limit the surge current, either in the AC line > > or the DC between the bridge and the filter capacitor. Here is a NTC > > manufacturers page on this usage: > > > From my experience with the ntc being used in the color tv degausing > circuit, the high failure rate of ntc's removes ntc's from > consideration, Peter. The Crydom SSR's and a big power resistor Just Work. > > > > https://product.tdk.com/en/techlibrary/applicationnote/howto_ntc-limiter.html > > > > > > > >> > >> _______________________________________________ > >> Emc-users mailing list > >> Emc-users@lists.sourceforge.net > >> https://lists.sourceforge.net/lists/listinfo/emc-users > >> > > > > Peter Wallace > > Mesa Electronics > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > . > > Cheers, Gene Heskett, CET. > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author, 1940) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
