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.
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.
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
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.
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 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
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I'm looking for work... see my website.
s -
I think that is quite common, with the relay doing double duty as a disconnect to protect the speakers in case of DC
An NTC surge thermistor might help.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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
Seems to be the popular suggestion around here. What spec should it have (cold resistance and whatnot)?
Erm, they're PTC Rs, no use here.
NT
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
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.
** Not good proof of anything.
** 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
I'v used disc-type NTCs and they worked well, and were reliable.
Theoretically yes, but they work anyhow.
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:
They withstand serious abuse.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
** See what I mean ?
** Nice example of the head in sand approach to engineering.
** The example I gave will not.
Keeping the value low and the watts rating high is the trick.
.... Phil
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??
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.
Value low makes the surge current higher. The problem is thermal fatigue from the temperature spikes.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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
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