Forums

Surge Protection

Started by Chris October 30, 2016
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.
"Chris" <cbx@noreply.com> wrote in message 
news:nv5lbj$3c9$4@dont-email.me...
> 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?
An NTC thermistor in series with the mains in offsets the apparent short circuit of applying mains to a fully discharged rectifier/reservoir electrolytic. The SMPSU may already have soft start after the reservoir cap - even if it doesn't, there may be an unused pin for that on the controller chip. You might need to roll your sleeves up and design an add on circuit that fools the regulator into thinking its doing too much PWM for the first couple of seconds after powering up. The old standard for clamping surge spikes is the MOV - but they take a hit with every event they protect from. AFAIK: there's a semiconductor spike clamp - but its like a diac-zilla and basically latches in short circuit fashion till power is removed. Combine one of those in parallel with a slightly lower voltage MOV - when the MOV deteriorates; it allows the semiconductor device to latch and blow the fuse. Then you know the MOV needs replacing as well.
On Sun, 30 Oct 2016 20:31:47 -0000 (UTC), Chris <cbx@noreply.com>
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.
Old-style offline switchers started with a bridge rectifier and a filter cap, so had huge turn-on surge current. They sometimes had a resistor or an NTC thermistor as a surge limiter. Big ones might have a resistor that is shorted by a relay or a triac once the cap is charged. The resistor had a rough life. Modern PFC switchers can soft-start. 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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:3hnc1c9qvej4qmlan0ot3rbfpa097hsjm5@4ax.com...
> On Sun, 30 Oct 2016 20:31:47 -0000 (UTC), Chris <cbx@noreply.com> > 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. > > Old-style offline switchers started with a bridge rectifier and a > filter cap, so had huge turn-on surge current. They sometimes had a > resistor or an NTC thermistor as a surge limiter. Big ones might have > a resistor that is shorted by a relay or a triac once the cap is > charged. The resistor had a rough life.
AFAICR: - it was mostly big stereo amplifiers that had switched resistors - its all about transient response. TV sets almost always had a basic unswitched surge resistor - if I was repairing the PSU, I'd usually replace the resistor with an anti-surge NTC. Generally if the PSU wasn't broke - I didn't fix it. There was a small number of exceptions that I knew might "bounce" if I didn't add the thermistor.
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.
On Sun, 30 Oct 2016 21:31:30 -0000, "Benderthe.evilrobot"
<Benderthe.evilrobot@virginmedia.com> wrote:

[snip]
> >AFAICR: - it was mostly big stereo amplifiers that had switched resistors - >its all about transient response. >
[snip] On one of the amplifiers I built in the '60's using Motorola RF power transistors I literally used a power-sized Mercury-wetted time-delay relay to disconnect the speakers during power-up to avoid the "cannon-shot" as the bias stabilized >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Den s&oslash;ndag den 30. oktober 2016 kl. 23.58.00 UTC+1 skrev Jim Thompson:
> On Sun, 30 Oct 2016 21:31:30 -0000, "Benderthe.evilrobot" > <Benderthe.evilrobot@virginmedia.com> wrote: > > [snip] > > > >AFAICR: - it was mostly big stereo amplifiers that had switched resistors - > >its all about transient response. > > > [snip] > > On one of the amplifiers I built in the '60's using Motorola RF power > transistors I literally used a power-sized Mercury-wetted time-delay > relay to disconnect the speakers during power-up to avoid the > "cannon-shot" as the bias stabilized >:-} >
I think that is quite common, with the relay doing double duty as a disconnect to protect the speakers in case of DC
On Sun, 30 Oct 2016 21:57:44 -0000 (UTC), Chris <cbx@noreply.com>
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.
An NTC surge thermistor might help. -- John Larkin Highland Technology, Inc lunatic fringe electronics
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. NT
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)?