Any Suggestions On Input Soft Start For A Linear Power Supply.

[Cicero]

Cicero:

I thin the Surge Stopper Devices https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB8QFjAA&url=https://www.linear.com/docs/40955&ei=czPwVI2YOYvlsAS3v4H4Aw&usg=AFQjCNEPgX0-Y6NsfPSuHXTjExaBnytS8A&sig2=IKNZiT6ZOX1el2UNCPMk2A&bvm=bv.87269000,d.cWc&cad=rja from Linear technology is more appropriate for these low power devices.

PTC's and even NTC's are very finicky to work with.
In a Fire Alarm panel I purchased each Class two output was protected with a PTC. You had a choice of PTC's or fuses.
Because of inrush currents some strobes would not operate., The manufacturer fixed the inrush problem in some devices, I had some of each. Class II supplies are around 100 W.

PTC's for motors work nicely because the overload characteristics are well known.

NTC;s were used in tube televisions. PTC's were used to run the degaussing coils.

In-rush current limiting and surge suppression are not quite the same and I had to use both. A Y-Delta motor starter is sort of a soft start.

For sure, I can definitely see the potential in using both an NTC and PTC together in series, NTC for initial startup inrush, allowing a smoother ramp in current, and a PTC for overcurrent/fuse action.

 

[Nigel Goodwin]

NTC;s were used in tube televisions.

Only in some, many just used a surge limiting resistor instead (often 4.7 ohm 4W or 7W) - but again, it's purpose was to prevent intermittent failure of the mains fuse (commonly the fuse in the plug, as the sets fuse is anti-surge). Even with an anti-surge fuse in the set, it was normal to require a 13A fuse in the plug, as otherwise it would tend to fail very occasionally if it was only 5A.

 

[ronv]

The datasheets use 240 ohms for the more expensive LM117, not for the LM317. They don't even show an LM317.
The graphs on a datasheet are for a "typical" device that you cannot buy. If you get one with a higher maximum minimum load current then IT MIGHT NOT WORK.
Don't you want ALL your circuits to work properly or only some of them? What if the manufacturer had a poor but passable yield so that all of them have the maximum minimum load current? Then ALL of your circuits will not work.

Funny, Here is a whole data sheet full of 240 ohm circuits. While I agree at 40 volts Vin to Vout it can be 10 ma. at 150C, I see no problem with 16 volts and a heat sink.

https://www.mouser.com/ds/2/308/LM317-D-111941.pdf

 

[ronv]

It doesn't pass through the bridge either, as it doesn't exist

Doesn't the first cycle of AC pass thru the bridge to charge the cap?
Transformer losses limit it, but it can still be quite high. I think that's why they have single cycle current ratings.

 

[Nigel Goodwin]

Doesn't the first cycle of AC pass thru the bridge to charge the cap?

Nothing like 50A does

The project under discussion is a tiny low power PSU, using an LM317 - where is 50A going to come from?.

 

[ronv]

No of course it is not 50 amps, but it could easily be 10 or 20 depending on the transformer.

 

[Nigel Goodwin]

No of course it is not 50 amps, but it could easily be 10 or 20 depending on the transformer.

I'm VERY doubtful of that

Assuming of course that the transformer is suitably sized - and not massively over rated. The resistance/impedance of the transformer windings, wiring, rectifier, and capacitor will limit the initial surge.

As I've repeatedly said, it's completely pointless in a low current circuit - I'm in two minds whether or not this entire thread is yet another incarnation of our reoccurring troll?.

 

[ronv]

Nice write up.

**broken link removed**

But yes, a waste of time and money to try to eliminate it.

 

[Honduras]

The datasheets use 240 ohms for the more expensive LM117, not for the LM317. They don't even show an LM317.
The graphs on a datasheet are for a "typical" device that you cannot buy. If you get one with a higher maximum minimum load current then IT MIGHT NOT WORK.
Don't you want ALL your circuits to work properly or only some of them? What if the manufacturer had a poor but passable yield so that all of them have the maximum minimum load current? Then ALL of your circuits will not work.

Have you ever tried it? I don't think so. Recommended is 100 to 1000 Ohms. RTFM. Then drop the subject. Your LM117 tirade is getting old.

 

[audioguru]

No, I have never used an expensive LM117 that uses a 240 ohm resistor that I have also never used. All my LM317 circuits with a 120 ohm resistor work perfectly as guaranteed by the manufacturer of the LM317.

 

[ronv]

Have you ever tried it? I don't think so. Recommended is 100 to 1000 Ohms. RTFM. Then drop the subject. Your LM117 tirade is getting old.

Don't let him get you down. There is an error term in the regulators that is proportional to load current, so they had to pick a number to be able to write all the other specs with regard to that error.
To be sure all the other specs are met (temperature, maximum voltage etc.) you may need 10 ma.. But the regulator will work quite well with 5 ma.

 

[Tony Stewart]

They are polymer metal oxide parts called ICL at Digikey for Inrush Current Limiters

 

[gary350]

We had a soft start computer at work that took about 10 seconds for the power supply to come up to full power. Power supply used a current liming resistor to charge the capacitors bank slow. Once the cap bank was up to full charge the computer worked fine. The power supply was 300 watts and the computer was 300 watts. I was told the cap bank was over sized about 10 times that is why it took so long for the caps to charge up to 5 volts. Once the caps were charged they had no trouble staying charged.

 

[Tony Stewart]

Usually storage Caps are chosen to deliver a "HoldUp time" of 1 cycle min. or 20ms.
The long the holdup time does nothing for power capacity but does improve peak load currents but degrades chargeup time. 30s seems like poor performance.