Allen ADC-3000 inrush current limiter

[Powerin]

Hi all,

I apologise for this rather esoteric question. Our church ADC 3000 died a while back and emitted the usual burning electronics smell. With trepidation I opened it up and eventually tracked down the problem to a burnt out component in the AC power distribution box on the floor of the organ which has eight 117V outlets and is switched by the power switch on the console cheek. The component itself was wired in series with the power switch. I brought the organ back to life by simply putting a jumper wire in place of the component.

A bit of research revealed that this was a thermistor (about the size of a 10cent piece) used as an inrush current limiter. It was burnt and had no identifying label or marks on it. I emailed my "local" Allen dealer (800 miles away) who informed me he had changed one recently and that it was a varistor. From my basic electronic knowledge I knew that a varistor is used in parallel with a power supply as a voltage limiter....not in series as a current limiter like a thermistor (I think). So I emailed him back asking him to check this out to make sure and then ordered it.

The component that arrived was indeed a varistor which I can't use. So my question is does anyone know what specifications (such as how much current it needs to handle) I need to replace the original thermistor? The organ works fine without it but I would prefer the extra protection. If it makes a difference this is in Australia so the organ has a 240-117V stepdown transformer in front of this distribution box.

[indianajo]

In my opinion, MOS supressors are used in parallel with the AC power to limit the voltage when a parallel motor turns off, or lightning strikes down the line. These are also sometimes called varistors as in metal oxide varistor. The resistance decreases radically as the voltage applied goes over 1.4 time the AC voltage printed on them. See the sweber catalog for application notes.
Two other kinds of varistors are PTC thermistors and NTC varistors. NTC varistors have a high cold resistance, then the resistance decreases as they heat up. These are useful for limiting the surge current at turn on of a mains transformer driven device . I bought a PV-1.3k amp that dimmed the lights in my living room when I turned it on. I installed a GE CL-101 NTC thermistor to put in series with the power switch and transformer, to limit the on current surge. This one has a cold resistance of 0.5 ohms, max current of 16 amps, and hot resistance at current limit of .02 ohms. It is the biggest one GE sells. Datasheet is on datasheetcatalog.com under cl-101 or I found it under CL-60 for my mixer.
You will have to decide what current limit you want for your power strip, and pick an appropriate NTC varistor. I am unfamiliar with wiring practice and current limits in the Eastern Hemisphere.
Farnell sells the GE NTC resistors in the USA. Another brand is ameritherm sold by digikey in the USA. there are oriental made ones from somewhere which come without brand or part number on them in ATX power supplies. They are green, I salvage them for 2 amp service. They have the cold ohms printed on them. The datasheet is in another language I imagine.
You can also buy NTC varistors sold as thermistors, for very small load currents, for controls purposes. I attach these to heat sinks, pull the varistor up to DC supply, with a resistor, and wire to gates of fets. This can be used to turn on DC fans, or to set protection circuit latches for a stuck fan overheat event.
Another type of varistor is the PTC varistor. These were used in refrigerator compressors to shut off the second start winding after a certain time, instead of a bimetallic thermal timer with a contact, or a 555 timer controlling a relay requiring a separate DC power supply.
Best of luck.

[tucsondave]

It is indeed a varistor. It is there to prolong the life of the power switch. (From the ADC service manual)
The schematic shows Allen part number as 233-0027.
Allen's website shows their part number as 233-0021 and the generic part number as V150LA20A.

td

[Powerin]

Hmmm, thanks for that dave and joe. I wonder if what I have was done for AUS organs only. The 240-117V stepdown transformer already has voltage spike protection as far as I can tell. What ours had would definitely have been a thermistor as it was wired in series with the power supply (and I replaced it with a wire link). A varistor needs to be parallel to the power supply as it only conducts in an over-voltage situation. I can't really see why a varistor would protect the power switch as there wouldn't be any voltage spikes at switch on/off would there? On the other hand (according to what I have read) thermistors are often used to prevent the switch-on current surge in a switch mode power supply which might damage upstream components...and perhaps the switch?

So I don't know what to do. Obviously Allen did different things in the same organs. I guess I have to decide if the thermistor was just protecting the switch, as Allen says, or if it was to protect the power supplies. I can't see any point in changing the wiring to put a varistor in there. Actually it wasn't doing a good job of protecting the power switch as it has stuck on a few times over the years and I have had to dismantle it to clean the stuck contacts. Thanks for the info.

[indianajo]

NTC varistors and surge supressor varistors have entirely different functions. What you pictured that blew was a NTC varistor that goes in series with the power transformer to limit the turn on current surge. A voltage surge supressor goes after the fuse across the AC lines, and possibly also from an AC line to safety ground.
A 20 amp ntc varistor doesn't offer much contact protection. More modern ones do have greater cold resistance available for the same hot resistance I see from buying two ameritherm NTC varistors for this allen USPS-3 supply I repaired.
Actually, starting to blow the varistor may be a symptom that your allen has leaky power supply capacitors and is drawing more current at turn on than it is supposed to. One typical sign of age of old electronic devices. That is apparently what blew the varistor in the USPS-3 which was built in 1987.

[jbird604]

I replaced one of those in an ADC5000 with a wire jumper about 30 years ago and the wire is still there. The organ is still playing and no other problems have occurred. As long as you have some good surge protector on the AC line ahead of the organ, I'd say just leave the wire in there.

[tucsondave]

I too have used a wire jumper so I agree with John, just leave the wire in there.

Looking again at the Allen parts list shows the possible reason for the confusion. They refer to both the surge protector (parallel) and inrush current limiter (series) as varistors.
Also the part numbers have changed over the years.

td

[Powerin]

Thanks guys. It's a 30 year old organ so a wire link it is.