Power Supply Inrush Current

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
Hi All,

So I designed an amp requiring a maximum current of 2.5 Amps. There for I have purchased a toroid transformer rated at 25VA at 7V AC at 3.57 Amps. So I thought the rating of the toroid was reasonable but I noticed that the inrush current charging the rectifier filter capacitors was 5 Amps even though the circuit is only outputting 2.5 Amps at most. The inrush current is only 0.001 second, should I be worried about this?

Thanks,
Kevin
 

MrChips

Joined Oct 2, 2009
26,477
I would say the transformer will be fine but I would watch the rating of the rectifier diodes.
What is the size of the capacitor and the part # of the diodes? Is it a bridge rectifier or full-wave center-tap configuration or center-tap bridge bipolar supply?
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
Its a bridge rectifier, no center tap. I will have 2 capacitors in parallel rated for 20V 10 000uF which I believe is more then enough for what I need. I forget which diodes but there rated for 5 amps which that has me worried now I'll look at the rating since I assume the diodes have an inrush current rating. Should have mentioned that the 5 amp 1ms peaks are constant and not only at the beginning when the circuit gets turned on
 

MaxHeadRoom

Joined Jul 18, 2013
25,466
The inrush current is only 0.001 second, should I be worried about this?
Kevin
I don't see a transformer self destructing in .001 seconds, even if you cycled it rapidly the inrush would be much reduced due to the capacitor retained charge.
Another indication is heat produced.
Max.
 
Last edited:

MrChips

Joined Oct 2, 2009
26,477
2 x 10,000 uF = 20,000uF is a heavy AC load. Do you really need that much filtering?
I would bump up the bridge rating or try it first with only one capacitor. Measure the DC and ripple voltage under full load and report back.
 

ian field

Joined Oct 27, 2012
6,536
Hi All,

So I designed an amp requiring a maximum current of 2.5 Amps. There for I have purchased a toroid transformer rated at 25VA at 7V AC at 3.57 Amps. So I thought the rating of the toroid was reasonable but I noticed that the inrush current charging the rectifier filter capacitors was 5 Amps even though the circuit is only outputting 2.5 Amps at most. The inrush current is only 0.001 second, should I be worried about this?

Thanks,
Kevin
The charging surge for the reservoir electrolytics can in extreme cases damage the rectifier.

A lot of SMPSU circuits with the bridge rectifier connected directly to the mains, use a NTC thermistor to limit inrush, at room temperature the thermistor has high resistance, but as soon as you switch on; the current through it heats it up and lowers its resistance.

I can't recall ever having seen a NTC in front of a transformer input - its far more usual to find a series current limiting resistor that's shunted out by a time delayed relay.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
My only concern is that I will damage the toroid transformer because of the high peaks of current I get when it charges the filter caps.
 

MrChips

Joined Oct 2, 2009
26,477
You are not going to damage the transformer. You are more likely to fry the bridge rectifier.
Make sure you put an appropriately sized fuse on the input to the primary windings of the transformer.
 

MaxHeadRoom

Joined Jul 18, 2013
25,466
My only concern is that I will damage the toroid transformer because of the high peaks of current I get when it charges the filter caps.
The inrush current is only 0.001 second, should I be worried about this?
Read the previous posts!.
What do suppose is going to happen in .001 sec. ?
It is heat caused by prolonged excessive current that usually destroys a transformer.
Max.
 

ian field

Joined Oct 27, 2012
6,536
Read the previous posts!.
What do suppose is going to happen in .001 sec. ?
.
Max.
In *REALLY* extreme cases, the sudden rise in magnetic field causes the windings to contract.

Most transformers are vacuum impregnated and cured in an oven to prevent any "vibration" in use, but as the OP mentioned; its a toroid with high peak currents - so its always going to be just that little bit nearer to pushing the envelope.

What I'd suggest is; survey any similar designs that can be found online and study the measures other people have used.
 

Kermit2

Joined Feb 5, 2010
4,162
a modest sized choke and 1500 uf would filter about as well as your single large capacitance arrangment and would reduce the size of your surges.
you should research such alternative filter methods.
 

ian field

Joined Oct 27, 2012
6,536
a modest sized choke and 1500 uf would filter about as well as your single large capacitance arrangment and would reduce the size of your surges.
you should research such alternative filter methods.
Choke-input filters went out with the valves - there's probably a very good reason for that.
 

Kermit2

Joined Feb 5, 2010
4,162
cost was increased so the bean counters really frowned on iron core copper wound parts vs. newer(cheaper) large value electrolytics. Performance was not as big an issue as you might like to think... but suit yourself. i merely suggested the OP do some research.
 

MaxHeadRoom

Joined Jul 18, 2013
25,466
...Also I think to some extent with the advent of smaller-portable-solid state devices, weight and size becomes an issue, however the R/L π filter is/was an efficient one.


Max.
 
Last edited:

ian field

Joined Oct 27, 2012
6,536
cost was increased so the bean counters really frowned on iron core copper wound parts vs. newer(cheaper) large value electrolytics. Performance was not as big an issue as you might like to think... but suit yourself. i merely suggested the OP do some research.
With that bit of research - the OP would find out what those very good reasons were.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
Thanks for expressing the concerns but I have found that in my case if I put small valued resistance such as 0.33 or 0.66 ohms works fine and limits the current.

Thanks
 

ian field

Joined Oct 27, 2012
6,536
care to share? You seem very sure that old is inferior. Is there a reason other than the cost/weight/size considerations we have already listed?
AFAICR; there was a fad that followed soon after the advent of solid state amplifiers, for getting the best possible transient response possible by the brute force & ignorance method of making the PSU as "stiff" as possible.

The belief that any resistance *AT ALL* between the secondary and rectifier ruined performance, persisted and inevitably led to some amp builders letting the magic smoke out of said rectifiers and/or transformers.

The method I described previously; a current limiting resistor in the primary circuit, that is shorted out by a relay after a short time delay became the most popular solution to inrush current, as once the relay contacts closed there was no unwanted resistance in the supply feed.

I can't recall ever having seen a NTC thermistor used in the primary circuit of an amplifier - after all, they can't go down to 0 Ohms as they need some I2R to keep them warm.

That little bit of resistance would go against the "holy scriptures"!
 
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