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Surge Protection

Started by Chris October 30, 2016
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:sqfd1c5jd42s0m8c4kegt106603p9ct0cg@4ax.com...
> On Sun, 30 Oct 2016 19:52:55 -0700 (PDT), Phil Allison > <pallison49@gmail.com> wrote: > >>Chris wrote: >> >>> >>> >>> > An NTC surge thermistor might help. >>> >>> Seems to be the popular suggestion around here. >>> >> >> ** Not good proof of anything. > > I'v used disc-type NTCs and they worked well, and were reliable.
A popular PTC device for CTV CRT degauss was stated in the datasheet to have a recovery time of 6 minutes - an NTC inrush thermistor of similar size would probably have about the same recovery time. They're usually at least a couple of Ohms at running temperature - so probably better than nothing if you cut power and switch back on before it cools.
<tabbypurr@gmail.com> wrote in message 
news:52c0684a-ee5c-47a0-9c11-49bdda542ba7@googlegroups.com...
> On Monday, 31 October 2016 00:00:54 UTC, tabby wrote: >> On Sunday, 30 October 2016 21:58:12 UTC, Chris wrote: >> > On Sun, 30 Oct 2016 14:00:28 -0700, John Larkin wrote: >> > >> > > Are you building an oscilloscope? The easiest and cheapest thing to >> > > do >> > > these days is buy a switcher that makes, say, 24 volts DC from the AC >> > > line. A board, box, or wall-wart. >> > >> > I want to up-rate and make more robust old scopes and network analyzers >> > made in the 70s and 80s that typically used the first generation of >> > switchers. I find a great many of them show signs of surge damage as it >> > seems the manufacturers didn't fully foresee the cumulative dangers of >> > successive surges back then. >> >> A fixed R would be a fair cheap choice. In series with the existing ntc >> would slow its decay. In old kit you could maybe use a filament lightbulb >> as a long lived NTC R. > > Erm, they're PTC Rs, no use here.
A lot of people still do it though....................................................... It was a big time trick of the service trade to bridge a blown fuse with a 60W bulb and "see what smoked". In reality; the PTC bulb surge current can damage a few semiconductors downstream. Not that long ago; my monitor produced a nasty niff instead of anything on the screen when I plugged it in. More or less resigned to scrapping it, I opened it up to see if there was any worthwhile salvage. The fuse was blown, so I fired up the iron and did a few checks - all the usual suspects were innocent, so I resorted to the bulb trick. A whisp of smoke started rising from the mains in common mode choke.
On Mon, 31 Oct 2016 08:29:49 -0700, John Larkin wrote:

> On Mon, 31 Oct 2016 15:22:57 -0000 (UTC), Cursitor Doom > <curd@notformail.com> wrote: > >>On Mon, 31 Oct 2016 07:34:48 -0700, John Larkin wrote: >> >>> You're in the audio business; there's not a lot of science there. >> >>Oh boy. I smell a flame war in prospect. > > I'm an engineer and I do what works.
I'm sure you do. But Phil doesn't take criticism *at all* well (even if it's sprinkled with fairy dust and castor sugar;-)).
On Mon, 31 Oct 2016 19:31:26 -0000, "Benderthe.evilrobot"
<Benderthe.evilrobot@virginmedia.com> wrote:

> >"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >news:sqfd1c5jd42s0m8c4kegt106603p9ct0cg@4ax.com... >> On Sun, 30 Oct 2016 19:52:55 -0700 (PDT), Phil Allison >> <pallison49@gmail.com> wrote: >> >>>Chris wrote: >>> >>>> >>>> >>>> > An NTC surge thermistor might help. >>>> >>>> Seems to be the popular suggestion around here. >>>> >>> >>> ** Not good proof of anything. >> >> I'v used disc-type NTCs and they worked well, and were reliable. > >A popular PTC device for CTV CRT degauss was stated in the datasheet to have >a recovery time of 6 minutes - an NTC inrush thermistor of similar size >would probably have about the same recovery time. > >They're usually at least a couple of Ohms at running temperature - so >probably better than nothing if you cut power and switch back on before it >cools.
We had one product that had a giant (850 watt) 50/60 Hz power transformer. The combination of capacitor charging and magnetic saturation from random turnoff magnetization tended to weld the power switch. We put a biggish disk NTC in each of the transformer (120/240) primaries, and that fixed it. I tried teasing the switch on/off with various timing, and the NTCs still helped. The switch failures sure stopped. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:rp9f1c9dbgt5qaqau6ji0r1ngktivdkgt6@4ax.com...
> On Mon, 31 Oct 2016 19:31:26 -0000, "Benderthe.evilrobot" > <Benderthe.evilrobot@virginmedia.com> wrote: > >> >>"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >>news:sqfd1c5jd42s0m8c4kegt106603p9ct0cg@4ax.com... >>> On Sun, 30 Oct 2016 19:52:55 -0700 (PDT), Phil Allison >>> <pallison49@gmail.com> wrote: >>> >>>>Chris wrote: >>>> >>>>> >>>>> >>>>> > An NTC surge thermistor might help. >>>>> >>>>> Seems to be the popular suggestion around here. >>>>> >>>> >>>> ** Not good proof of anything. >>> >>> I'v used disc-type NTCs and they worked well, and were reliable. >> >>A popular PTC device for CTV CRT degauss was stated in the datasheet to >>have >>a recovery time of 6 minutes - an NTC inrush thermistor of similar size >>would probably have about the same recovery time. >> >>They're usually at least a couple of Ohms at running temperature - so >>probably better than nothing if you cut power and switch back on before it >>cools. > > We had one product that had a giant (850 watt) 50/60 Hz power > transformer. The combination of capacitor charging and magnetic > saturation from random turnoff magnetization tended to weld the power > switch. We put a biggish disk NTC in each of the transformer (120/240) > primaries, and that fixed it.
When my welding set welded its on/off switch, I couldn't get hold of a big enough NTC. A cooker oven switch did the trick, but I had to remember to only turn the knob far enough to click - the rest of rotation is the simmerstat bit. The place I worked at had almost identical stackable switch units to the original equipment - but they kept them too well guarded.
John Larkin wrote:

> > >> >> > >> >> > >> >> > An NTC surge thermistor might help. > >> >> > >> >> Seems to be the popular suggestion around here. > >> >> > >> > > >> > ** Not good proof of anything. > >> > >> I'v used disc-type NTCs and they worked well, and were reliable. > >> > > > > ** See what I mean ? > > > > > >> > > >> >> What spec should it have > >> >> (cold resistance and whatnot)? > >> > > >> > > >> >** For a low current loads a resistor is all you need and has the advantage that its value does not drop when it heats up - rendering it useless if there is another inrush surge before it cools. > >> > >> Theoretically yes, but they work anyhow. > >> > > > >** Nice example of the head in sand approach to engineering. > > > > I make things that work, and then I sell them. >
** You ought to patent an idea like that.
> You're in the audio business; there's not a lot of science there.
** Fraid I am in the electronics business, while audio is something you are pig ignorant of.
> > >> Resistors, even biggish wirewould ones, tend to fail after some > >> thousands of surges. > >> > > > > ** The example I gave will not. > > > >Keeping the value low and the watts rating high is the trick. > > > Value low makes the surge current higher. The problem is thermal > fatigue from the temperature spikes. >
** No fooling? Who would have thunk. Your lack of experience with equipment you did not design is showing - same problem for most designers. And your asinine attitude. .... Phil
Chris wrote:

> Phil Allison wrote: > > > For a load of say 1A rms, a WW resistor of 4.7 ohms rated 10 watts > > should be fine. Other resistances in the AC circuit and the filter > > electro's ESR will bring the effective value up to 8 to 10 ohms total. > > Well that would be great, as I already have dozens of these type of > resistors in my junk box. I'm just wondering what's providing the > protection here, though: the resistance of 4.7 ohms or the reactance > arising from the inductive nature of WW resistors? Or both?? >
** Any reactance is negligible at the frequencies involved, so the resistance alone is doing the job. You say you have seen "surge damage" to SMPSs in scopes etc - but is that linked to *inrush surges* or something else ? The only components likely to suffer from inrush surges are the rectifiers acting on the incoming AC supply - something designers have know about since Noah was a boy. Very likely, the SMPSs you are dealing with already have inrush surge suppression in the form of WW resistors, NTC thermistors or simply the resistance of common mode chokes used in line filters. It is only necessary to limit the worst case surges to about 30amps peak to protect the sort of diodes and bridges that might be involved and this means the total resistance in series with the AC supply needs to be 10ohms. With no resistance included at all, peak surge currents would be well over 100amps and destroy diodes immediately and/or trip supply circuit breakers at switch on. .... Phil
On Sunday, October 30, 2016 at 4:32:15 PM UTC-4, Chris wrote:
> Hi, > > What is the best choice currently for protecting SMPS circuitry in the > event of surge at mains switch-on? I'm talking about the PSU sections of > oscilloscopes, typically, so under normal working conditions less than > one Amp drawn at 240V (as it is here in europe). I read somewhere that > the front end of an SMPS momentarily looks like close to a short circuit > resulting in very large inrush currents at turn-on. How best to limit > these, please? > > Chris.
Metal oxide varistors(MOV) are very good surge suppressors - we use it for power line surge suppressors, satellite TV surge suppressors, wall plug mount LED lamps etc., Very easy to incorporate in any circuit.
On Tuesday, 1 November 2016 05:15:38 UTC, Phil Allison  wrote:
> Chris wrote: > > > Phil Allison wrote: > > > > > For a load of say 1A rms, a WW resistor of 4.7 ohms rated 10 watts > > > should be fine. Other resistances in the AC circuit and the filter > > > electro's ESR will bring the effective value up to 8 to 10 ohms total. > > > > Well that would be great, as I already have dozens of these type of > > resistors in my junk box. I'm just wondering what's providing the > > protection here, though: the resistance of 4.7 ohms or the reactance > > arising from the inductive nature of WW resistors? Or both?? > > > > ** Any reactance is negligible at the frequencies involved, so the resistance alone is doing the job. > > You say you have seen "surge damage" to SMPSs in scopes etc - but is that linked to *inrush surges* or something else ? > > The only components likely to suffer from inrush surges are the rectifiers acting on the incoming AC supply - something designers have know about since Noah was a boy. Very likely, the SMPSs you are dealing with already have inrush surge suppression in the form of WW resistors, NTC thermistors or simply the resistance of common mode chokes used in line filters. > > It is only necessary to limit the worst case surges to about 30amps peak to protect the sort of diodes and bridges that might be involved and this means the total resistance in series with the AC supply needs to be 10ohms. > > With no resistance included at all, peak surge currents would be well over 100amps and destroy diodes immediately and/or trip supply circuit breakers at switch on. > > .... Phil
The problem is gradual degradation of the NTC Rs. I suppose one could always replace one with 4. NT
On Mon, 31 Oct 2016 23:02:12 -0700, dakupoto wrote:

> Metal oxide varistors(MOV) are very good surge suppressors - we use it > for power line surge suppressors, satellite TV surge suppressors, wall > plug mount LED lamps etc., > Very easy to incorporate in any circuit.
MOVs are subject to cumulative damage, though. They should really be replaced periodically. I would say NTC thermistors, suitably rated, would be a better bet. Or Phil's suggestion of a current limiting resistor.