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inrush current limiting on the SECONDARY of a transformer
inrush current limiting on the SECONDARY of a transformer
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Old 20th October 2020, 09:20 PM   #21
evanc is offline evanc  United States
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inrush current limiting on the SECONDARY of a transformer
Thermistors should not be run in parallel
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Old 21st October 2020, 04:32 PM   #22
Holmer is offline Holmer  Germany
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I actually had to open the amp I built in 1988 to have a look at the soft start, I get the feeling my brain developed a soft start of its own. I only remembered it use some trick circuit but forgot the details.

It puts a 3uF motor start capacitor and a 47R 5W WW resistor in series with the primary for as long as it takes to charge the cap bank to 80%, then they get bridged with a relay. The relay is powered from the main caps via some Z-diodes, so if something is wrong the start cap will never get bridged and power stays limited. The start cap cannot burn out (unlike resistors), the 47R is just to prevent contact welding when the cap gets shorted.

I do not even use speaker relays, the rails come up so slowly (20s) you hear no thump at all (please don’t say DC fault now).
This method only works for low bias, or the rails will not come up far enough to engage the relay.
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Old 22nd October 2020, 03:14 AM   #23
TonyTecson is offline TonyTecson  Philippines
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soft starting the primary is all you will ever need.....

1. soft starting was invented to avoid your lamps dimming at turn on, but since incandescent lamps are almost extinct, and led's do not dim as much as incandescents, no need to worry too much...

2. soft starting was invented to avoid switch contacts from melting or getting welded oa turn on or turn of, the obvious solution is to use switches of sufficient interruption ampere ratings..

3. soft starting also lets you use smaller ampere rated fuses than otherwise would melt without it...

so for 520kufd filtering i will go for soft starting because of the above reasons and for nothing else...
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Old 22nd October 2020, 03:37 AM   #24
TonyTecson is offline TonyTecson  Philippines
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Quote:
Originally Posted by Holmer View Post
I actually had to open the amp I built in 1988 to have a look at the soft start, I get the feeling my brain developed a soft start of its own. I only remembered it use some trick circuit but forgot the details.

It puts a 3uF motor start capacitor and a 47R 5W WW resistor in series with the primary for as long as it takes to charge the cap bank to 80%, then they get bridged with a relay. The relay is powered from the main caps via some Z-diodes, so if something is wrong the start cap will never get bridged and power stays limited. The start cap cannot burn out (unlike resistors), the 47R is just to prevent contact welding when the cap gets shorted.

I do not even use speaker relays, the rails come up so slowly (20s) you hear no thump at all (please donít say DC fault now).
This method only works for low bias, or the rails will not come up far enough to engage the relay.
brilliant, will try this also, a capacitor is a good way to limit current without the heat incurred in a resistor....
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Old 1st November 2020, 03:30 PM   #25
GUNFU is offline GUNFU  Russian Federation
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Quote:
Originally Posted by cakyol View Post
All power supplies for high power audio amps have a current limiter resistor on the primary of their transformers for the obvious reasons.

However, why is this not a practice on the SECONDARY ?
I have never seen this done on any amplifier.
Here is the reason for my question:

I am building an amp (more like a welding machine LOL) with a 2.5kva toroidal which will have secondaries at 2 x 70 volt AC and 520,000 uF of filter capacitance. With such a beast, on the secondary, such high capacitance will present an instantaneous short circuit, as soon as the power is switched on.

Is a secondary current limiter not needed on account that if a lot of current is drawn at the secondary, this will also translate to a proportional current increase on the primary and hence collapse the secondary voltage so that it literally self limits ? Even though that sounds reasonable, I have my doubts.

Or would it be always safer to insert a 5 ohm thermistor (limits current to 70/5 = ~14A at switch on) in series to each secondary, just in case ?
As more power is required & current drawn, thermistor's resistance would reduce and hence effectively become more transparent in the secondary, but obviously at the expense of generating some heat.

As you can imagine, such capacitors are extremely expensive and have to be very heavily protected.
Would a secondary current limiter not immensely improve the long term reliability and the life of the rectifier and the capacitors ?
Thoughts please ?
Thanks
The current is limited precisely in the primary winding in order to reduce the inrush current of the transformer caused by the hysteresis loop in the magnetic circuit. No one knows in what position of the hysresis loop the transformer was turned off before. But I would install along with a thermistor a circuit breaker with a D characteristic.
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Old 2nd November 2020, 01:23 AM   #26
TonyTecson is offline TonyTecson  Philippines
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you can of course use soft starting at the secondary if so desired...
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Old 2nd November 2020, 06:39 AM   #27
GUNFU is offline GUNFU  Russian Federation
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In Audio Research amplifiers, inrush currents in each capacitor circuit are typically limited by RL filters. They are probably trying to extend the life of the capacitors, thus reducing the ripple of the charging currents and distributing the charging current more evenly through each capacitor.
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Old 2nd November 2020, 07:43 AM   #28
JMFahey is online now JMFahey  Argentina
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Quote:
Originally Posted by cakyol View Post
It will be mostly indoor rock music, at about 70 - 80% of its max power.
The amp is a 500 Watts/channel RMS design.
Then it will be clipping all the time.

Unless you *only* play sinewaves that is.
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Old 2nd November 2020, 09:03 AM   #29
bansuri is offline bansuri  Italy
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The magnetizing current is creating saturation in the core because it might double the value when switched on. Then it is constant even with open secondary.

A saturated transformer creates big spikes in the primary, only ohmic resistance of the winding limits it at up to 10x normal. So a primary resistor can limit it.



The magnetizing current is 90į out of phase, the maximum occurs when the mains is 0V and the energy has to come from the (limited) flux in the core. it is dependent on the core data, number of windings and mains voltage usually 70-80% of Isat.


The secondary current is pulsing in phase when the mains is high and can provide higher current without saturation.
But diodes and Caps might be stressed, that is why also in the secondary NTCs are used sometimes.


Here the limiting factor is the heat created by ohmic resistance of the winding.

Last edited by bansuri; 2nd November 2020 at 09:11 AM.
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Old 2nd November 2020, 09:18 AM   #30
H713 is offline H713  United States
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Usually audio amplifiers don't have a big enough transformer for the inrush current caused by the iron itself to be a real issue. Even with the transformers in some of the biggest PA amps, it wouldn't cause an issue. A bit of a thud maybe, but nothing to harmful.

With really big transformers I've measured that inrush current to be over 3 kA, but that was a bit of an unusual (and enormous) transformer.

The problem audio amps have is that we have a toroidal transformer with thick secondaries (so it can take big peak currents) and up to 60 mF of capacitance.

Last edited by H713; 2nd November 2020 at 09:21 AM.
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