Inrush Current-How to limit

[Overclocked]

How can I passivly limit inrush current on a linear type power supply?

Ive heard that there is a special type of thermistor for inrush current, Is this what I can use to limit it ?

 

[zevon8]

Yes, a NTC ( Negative Temperature Coefficient ) thermistor is a common method of limiting inrush current on electronic devices. Switching supplies often have these devices installed.

For a linear supply, you need to be carefull of the instaneous current draw on power up, so that the thermistor does not become a "fuse", especially if there are really large capacitors or a huge transformer. Often linear supplies have a longer inrush period than switchers, so you will have to check the design.

Here is a link to one maker, there is some useful information regarding their use and selection:

https://www.ametherm.com/Inrush_Current/inrush_current_faq.html

 

[Mike - K8LH]

Check out NTC Thermistors... I have one of their CL30 8-Amp 2.5-ohm devices waiting for installation on the filiment winding of my SB-200 kilowatt amplifier... Not sure if you could use something similar on the primary winding of a high voltage supply... Please let us know what you discover?

Regards, Mike

 

[mstechca]

I haven't heard of inrush current before, but what I do know is that according to ohms law, a resistor can limit current.
The only way it can't is if you don't use one with a high enough wattage rating.

 

[zachtheterrible]

I'm assuming that inrush current would happen when the supply is turned on and a large capacitor gets charged, like when you turn on a vaccum and the lights dim because the motor takes more current to start . . .?

If this is the case, a simple choke coil would do the trick I would imagine.

 

[Russlk]

In the old days, we used an inductor but since they are large and expensive, most people look for something else. A resistor works well enough, it you can stand the power loss.

 

[k7elp60]

On linear powersupplys one thing that limits the inrush current is the DC resistance of the transformer secondary winding. It that is not enough
resistance then one way to go is to use NTC(negative temperature coefficient) thermistors as mentioned before.
The CL30 that was mentioned has a steady state maximum current of 8 amps. Its cold resistance is 2.5 ohms, but under load the resistance can drop to 0.06 ohms.
As a side note, I have used the CL50 as inrush current limiter for RV 12 volt lamps.

 

[Overclocked]

The transformer Im using provided 12V@14Amps. The caps I am using are pretty big.

But where would I put it? In parallel with the primary or secondary? What if I were to use a Resistor in parallel with the transformer?

 

[Nigel Goodwin]

The transformer Im using provided 12V@14Amps. The caps I am using are pretty big.

But where would I put it? In parallel with the primary or secondary? What if I were to use a Resistor in parallel with the transformer?

If you put it in parallel you're INCREASING the inrush current.

MY first thought is WHY do you feel you need something to limit the inrush current?, do you have a specific problem which you suspect is caused by it? - usually it's occasional failure of the mains fuse.

As already suggested, you can buy NTC thermistors, specifically designed for this type of operation, which would be one solution (if required at all?).

However, the usual requirement for inrush limiting is in BIG audio amplifiers (PA ones), and those that use a torodial mains transformer - toroids give a MUCH larger inrush current. It's only usually found in larger amplifiers, something like 300W RMS per channel upwards, and only usually those with toroidal transformers.

The method is to have a large wattage resistor in the incoming mains supply, with it's value chosen to limit the inrush to less than the mains fuse rating. You then have a timer circuit, that turns a TRIAC ON, that shorts out the resistor once the inrush current has finished - essentially it's a 'soft start' circuit.

But your transformer doesn't sound large enough to require this type of circuit?.

 

[Dean Huster]

The current rating for the rectifier is another reason for limiting inrush current.

 

[Overclocked]

Well Ive heard its a problem in large linear powersupplies

But wait a second maybe you are right since in a transformer pin=pout, correct?

So if I know Pout, I could easily find how much current the primary requires at stable operation

So Let V=13, and I=14Amps P=182 Watts (using p=IE) So if we know V and P, then lets find I

182Watts/120=1.51667 Amps. But thats at NORMAL operation. So maybe a 2 Amp fuse on the Primary would be suffciant?

 

[Nigel Goodwin]

You might need a larger fuse than that, but you could try it and see - however, make sure that any fuses you do try are anti-surge ones!.

 

[Overclocked]

Isnt that what MOVs are for? To protect against a surge?

 

[Nigel Goodwin]

Isnt that what MOVs are for? To protect against a surge?

That's for voltage spikes (or surges if you like), this thread is about the high initial current surge when a circuit if turned on - particularly those with large (multi hundred watt) toroidal transformers.

 

[Overclocked]

I have a Iron core (the box Type, err regular type) Transformer. So I dont need to worry about inrush current?

 

[Nigel Goodwin]

I have a Iron core (the box Type, err regular type) Transformer. So I dont need to worry about inrush current?

I wouldn't have said so, the inrush for a 'normal' transformer is far less than a toroidal one - and even with a toroid your transformer is too small for it to be a problem.

 

[Overclocked]

For the transformer the outputs are (secondary)
12 V 9 A,
12 V 3.2 A, 12 V 2 amps,
44 V 2.5 amps

For the 9amp Line I am using a 10 Amp fuse, for the 2Amp line, a 3Amp Fuse, and for the 3.2 Amp Line a 4 Amp Fuse.

I havent deciced if I should fuse the primary, If So, I might use a 3 Amp Fuse.

Would Those Ratings for the fuse be good enough?

 

[Nigel Goodwin]

The most important fuse would be the primary one! - and as we've been talking about inrush current, that's where it occurs.

 

[Styx]

In the old days, we used an inductor but since they are large and expensive, most people look for something else. A resistor works well enough, it you can stand the power loss.

old days? i still use chokes to limit inrush current.
Admiddily it is only used if I need to ensure continuos current and the choke is needed

Otherwise it is a resitor that gets shorted out by a thyristor after a set time/voltage

but since a passive means is needed...

 

[Overclocked]

The most important fuse would be the primary one! - and as we've been talking about inrush current, that's where it occurs.

Well Following my math above, I'll use a 3 Amp Fuse, instead of a 2.

But isnt a Fuse's job is to protect the circuits in case something shorts out or opens?