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

[Honduras]

I'm experimenting wit LM317Ts. I'm using some fairly large capacitors because of the AC power source. I would really like to protect the circuit from ANYTHING unexpected.

Soft Start seems to be s good idea, but, when I Google it, I end up with a thousand options that are unsuitable, and none that are.

Does anyone have a schematic for a legacy soft starter? Or something new that doesn't use ICs?

I have no real probem with ICs except with finding datasheets for them and deciphering the data that I can find.

 

[Cicero]

So you want to basically reduce the inrush, and maybe add some protection?

An easy approach for reducing in-rush is using an NTC thermistor in line. Then for overvoltage protection add in some MOV's. Polyfuses are good options for overcurrent protection as well.

 

[Honduras]

So you want to basically reduce the inrush, and maybe add some protection?

An easy approach for reducing in-rush is using an NTC thermistor in line. Then for overvoltage protection add in some MOV's. Polyfuses are good options for overcurrent protection as well.

TYVM.

 

[Wade Hassler]

How about this? (Suitably changed for your current/voltage/timing, of course. The devil is in etc.)

 

[Honduras]

How about this? (Suitably changed for your current/voltage/timing, of course. The devil is in etc.)

So USB Vbus would be Vin, PWREN would be ground, and VCCswitched would be Vout?

 

[MikeMl]

What are you trying to protect:

Transformer?
Rectifier?
Regulator?
Load?

 

[KeepItSimpleStupid]

Define "fairly large". My amp has 40,000 uf of capacitance at 50V, That's large. Yes, I designed a soft-start circuit for it, but it wasn't foolproof. A resistor pop (intentionally) if something is seriously wrong like I switch the output transistors.

For reliability, you can do two things:
1. The working voltage is at least 2x of what will be expected.
2. Place MOV's on the caps

1&2 won't be soft-start, but it's well worth doing.

 

[Honduras]

What are you trying to protect:

Transformer?
Rectifier?
Regulator?
Load?

Just the power supply IC. When I get something that works I'll add protection for the load.

 

[Nigel Goodwin]

Just the power supply IC. When I get something that works I'll add protection for the load.

You appear a little confused?, a soft start circuit is used for VERY high switch-on surges (think 1000W RMS power amplifier with toroidal mains transformer), in order to prevent intermittent blowing of the mains fuse.

It doesn't protect the PSU in any way, and certainly not the IC?

Certainly nothing as puny as an LM317 is going to benefit in any way from a soft-start circuit.

 

[Honduras]

You appear a little confused?, a soft start circuit is used for VERY high switch-on surges (think 1000W RMS power amplifier with toroidal mains transformer), in order to prevent intermittent blowing of the mains fuse.

It doesn't protect the PSU in any way, and certainly not the IC?

Certainly nothing as puny as an LM317 is going to benefit in any way from a soft-start circuit.

It's a matter of scale. High voltage/current circuits use large surge protection circuits, but the small stuff suffers from surges too, and are less able to deal with them. When the module is turned on it acts as a short circuit for a short time. Short circuit = infinite current, or at leaast as much as the source can provide.

Think about it. It doesn't matter how large or small the circuit is, over-current damages parts. The part may not blow on the 1st or 10,000th turn-on, but the damage is incremental. Sometimes I wish that the component did blow on the first overload. The options are decreased performance and reliability.

Manufacturers don't put inrush protection in small stuff for the simple and sole reason that it decreases profit.

 

[dr pepper]

A 10 ohm prc thermistor is often found in smps's that otherwise would have a big inrush.

Theres always a 555, relay and power resistor, I'm sure you could cope with a 555.

 

[Nigel Goodwin]

It's a matter of scale. High voltage/current circuits use large surge protection circuits, but the small stuff suffers from surges too, and are less able to deal with them. When the module is turned on it acts as a short circuit for a short time. Short circuit = infinite current, or at leaast as much as the source can provide.

Think about it. It doesn't matter how large or small the circuit is, over-current damages parts. The part may not blow on the 1st or 10,000th turn-on, but the damage is incremental. Sometimes I wish that the component did blow on the first overload. The options are decreased performance and reliability.

Manufacturers don't put inrush protection in small stuff for the simple and sole reason that it decreases profit.

Not at all, they don't because there's absolutely no point as it does no damage whatsoever, incremental or otherwise.

As I said previously the ONLY reason they fit surge limiters on larger items is to prevent intermittent mains fuse failure, nothing whatsoever to do with supposedly 'protecting' the equipment.

Where did you get your strange ideas from?.

 

[MikeMl]

Actually, the LM317 does include inrush protection for itself, and for the downstream load. Study the data sheet.

 

[dr pepper]

Seems I got what you are doing wrong too.

As mentioned the mians fuse is one of the main reasons to reduce inrush, a power transformer isnt really affected much, a large smps with high value smoothing caps might blow a bridge rectifier at switch on without protection, but even then the plug top fuse is probably more prone to failing.

 

[Honduras]

A 10 ohm prc thermistor is often found in smps's that otherwise would have a big inrush.

Theres always a 555, relay and power resistor, I'm sure you could cope with a 555.

I was thinking of something more like a transistor MOSFET pair.

Not at all, they don't because there's absolutely no point as it does no damage whatsoever, incremental or otherwise.

As I said previously the ONLY reason they fit surge limiters on larger items is to prevent intermittent mains fuse failure, nothing whatsoever to do with supposedly 'protecting' the equipment.

Where did you get your strange ideas from?.

Ummm, electronics training?

I have to say that I am immediately suspicious of people who tell me. "No. It's not possible or relevant."

But, honestly, my strange ideas come from experience, and research. Sometimes it is expedient to ignore the facts, but I prefer not to do that.

 

[Honduras]

Actually, the LM317 does include inrush protection for itself, and for the downstream load. Study the data sheet.

You're right. Most of my experience with Linear regulators have been with the LM78xx series.

 

[ronv]

I'm experimenting wit LM317Ts. I'm using some fairly large capacitors because of the AC power source. I would really like to protect the circuit from ANYTHING unexpected.

Soft Start seems to be s good idea, but, when I Google it, I end up with a thousand options that are unsuitable, and none that are.

Does anyone have a schematic for a legacy soft starter? Or something new that doesn't use ICs?

I have no real probem with ICs except with finding datasheets for them and deciphering the data that I can find.

Maybe a schematic is in order with some info on the AC source and the size of the capacitors on the input output and adjust pins.

 

[Honduras]

Maybe a schematic is in order with some info on the AC source and the size of the capacitors on the input output and adjust pins.

No one has been able to tell me how to upload a JPEG yet, so that causes a problem.

The capacitors are a 3300 uF in parallel with a 0.1 uF from the input to ground. There is a 1 uF cap between the output and ground. I don't have a cap connected to the ADJ pin.

The AC source is a wall wart that turns 120 VAC into approximately 15 VAC

 

[Nigel Goodwin]

Ummm, electronics training?

Perhaps you should ask for a refund?

I have to say that I am immediately suspicious of people who tell me. "No. It's not possible or relevant."

So almost all electronic equipment prove it's not relevant - but you still imagine it is?.

But, honestly, my strange ideas come from experience, and research. Sometimes it is expedient to ignore the facts, but I prefer not to do that.

Perhaps you could mention what this 'experience' is and what 'research' has shown?, certainly any facts disagree with you.

 

[KeepItSimpleStupid]

Honduras:

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