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Surge Protection

Started by Chris October 30, 2016
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. NT
On Monday, 31 October 2016 00:26:58 UTC, Chris  wrote:
> On Sun, 30 Oct 2016 16:57:38 -0700, John Larkin wrote: > > > An NTC surge thermistor might help. > > Seems to be the popular suggestion around here. What spec should it have > (cold resistance and whatnot)?
Totally depends on the circuit. Hot R must be low enough, P diss low enough, cold R sufficient. But an NTC thermistor is what you already have IIUC. NT
On 30.10.16 21:31, 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. >
Did that with six computers on a 24v->220v ac convertor, used a small relay and a 10 ohm resistor which was shorted by the relay after halve a second. That took care of the inrush current.
Chris wrote:

> > > > An NTC surge thermistor might help. > > Seems to be the popular suggestion around here. >
** Not good proof of anything.
> 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. 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. .... Phil
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.
> > >> 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.
> >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.
Resistors, even biggish wirewould ones, tend to fail after some thousands of surges. I had to replace some giant expensive metal-case 250 watt WW resistors, in this sort of application, with these: https://dl.dropboxusercontent.com/u/53724080/Parts/Welwyn.JPG They withstand serious abuse. -- John Larkin Highland Technology, Inc lunatic fringe electronics
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.
> > > >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. > > 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. .... Phil
On Sun, 30 Oct 2016 19:52:55 -0700, 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??
On Sun, 30 Oct 2016 21:06:53 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

>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. I don't need to explain the quantum mechanics. You're in the audio business; there's not a lot of science there.
>> > >> >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. >> >> 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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
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.
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics