Toroid inrush current testing

[zdr]

Originally posted by ratza



Oh, yes you would. Do this experiment: on a single toroid PS, keep one speaker connected and on the other channel mount an 8 ohm resistor instead. Now put some music or a signal on this channel and hit the volume. Do you hear anything in the loudspeaker? Yup, I thought so. With dual toroids, this will never happen.

I guess I would hear the disconnected speaker not playing, resulting in seriously degraded stereo image :>

Seriously, I don't think I will be ever using this kind of "mono" setup, so I don't see the point of this test? I still own Vincent SV-236 which is playing pretty damn good with it's single 500VA toroid.

[ratza]

Originally posted by zdr


I guess I would hear the disconnected speaker not playing, resulting in seriously degraded stereo image :>

Actually it will sing quite louder than you would expect. Try it.

[TheMG]

I think what he was saying is, connect an 8 ohm resistor (of sufficient wattage!) to one channel, and drive ONLY that channel (don't put a signal into the channel with the speaker on it).

The result: you might hear some sound coming from the speaker attached to the channel not being driven.

[BWRX]

Originally posted by peranders

My amp with 600 VA /230 VAC toroid generates 77 A, worst case.

Measure the primary winding, should be 3-4 ohms. Take (230*1.4)/3 = maximum theoretical current

Over 300 VA you will need a softstarter, to spare the mains switch and to let you have a fuse which protects better.

The inrush current will also depend on how the secondaries are wired, how much capacitance is used, and whether or not the capacitors have any charge on them when power is connected. Large caps with no stored charge will appear as a short across the secondary on start up, resulting in a larger inrush current.

I have a smaller 120VA 2x15VAC toroid transformer with 20,000uF after a bridge on each secondary and the lights in the room dim a bit when power is connected! In comparison, a 120V 18-0-18VAC EI core transformer with 3900uF on each rail doesn't dim anything when power is connected. Technically, that isn't a fair comparison but if the EI core transformer had 20,000uF on each rail I bet it would dim the lights at least a little.

[richie00boy]

This dummy channel driving test is a good idea, but I wonder if the bleed comes not because of a transformer issue per se but a grounding issue?

[TheMG]

I'd say it's a result of the power supply voltage fluctuations caused by the channel that is being driven. Of course, on a well built amp you have to drive the channel with the dummy load pretty hard to hear sound from the other channel/speaker.

A good real life example of when something like this becomes quite noticeable, I'm sure you may have noticed it before, but on some intercom systems (in a school for example) when the intercom is being used to announce to only one part of the building, it is possible to faintly hear the sound from the speakers in the other part of the building.

[zdr]

Originally posted by TheMG
I think what he was saying is, connect an 8 ohm resistor (of sufficient wattage!) to one channel, and drive ONLY that channel (don't put a signal into the channel with the speaker on it).

The result: you might hear some sound coming from the speaker attached to the channel not being driven.

That is a valid point, are you referring to some design flaw in Overture chipamps and their PSes, allowing for signals to traverse over PS between channels? Problem is, I don't have a resistor 8 ohm with that power rating, and I wouldn't want to put my curtains on fire.

My point is: if amp and ps is constructed well, it probably won't matter if you run single or double toroid.

Interesting suggestion for test though, has anyone tried it yet?

I am about to finish single toroid chipamp and already have dual toroid one, so comparison is to come soon!

[BWRX]

Originally posted by TheMG
I think what he was saying is, connect an 8 ohm resistor (of sufficient wattage!) to one channel, and drive ONLY that channel (don't put a signal into the channel with the speaker on it).

The result: you might hear some sound coming from the speaker attached to the channel not being driven.

There is a name for this specification: channel separation.

[zdr]

My 500VA toroid arrived few days ago, so I decided to give it a go with elektor softstart circuit. I connected it with a remote wireless power switch so that I can detonate it from another room. Softstart has a fast blow 3.15A 20mm fuse, which was working fine with 2x225VA toroids.

First few attempts went fine, and since I did not hear a detonation I entered the room encouraged, to do I few more. Suddenly, I thought I saw a glimpse of light from resistor pack area, just before relay shorted them. I switched it off and after a while, back on again. BOOM. Fuse blew up. So much for the reliability of Elektor softstart.

I realized what happened: one of the resistors died after the pack absorbed energy from inrush current pulse. Relay then shorted them, everything appeared to be fine - until the next cycle.

Relay switched resistor pack back on when PS was powered down. This time however, series resistors were just an open circuit, so next switch-on of the pack did not do anything. When relay shorted them, inrush current hit the toroid at it's best, blowing the fuse (and the 16A mains circuit breaker).

I replaced the resistor pack with two NTCs of 10ohm in series. Seems to work much better, although there are still many drawbacks for this design too. I am aware that NTCs need time to cool down, so fast PS switching is likely to blow the fuse too. They are however less likely to fail as they can be left in the circuit even if the relay fails to short them.

I think I will go for pic controlled dual relays allowing for instant reset of the softstart circuit.

[peranders]

One of the main features of my softstart was a very fast reset. If you use a MCU you will also have the possibility to malfunction of the MCU. A main idea for me was to keep it as simple possible and by that also reliable.

If you have a very varying load you can experience trouble with NTC resistors.