Thermistor spec changes?

[Pure_Brew]

Hi,
I'm using a pair of CL-60's and I measured them just over 10 ohms prior to installing them before the transformer, like 2 months ago.
I checked them with my multimeter immediately after power down today, and found that they measured around 2 ohms.
I turned off the amp and after like 5-10 minutes, they slowly reached up to 5 ohms but never returned to the original measurement.
It took like 6 hours for them to get up to about 5.7 ohms. I tested an 8ohm resistor I had lying around to see if something else was off, but my multimeter measured it just fine.

Is it normal for these devices to go out of spec?

The next thing I tried was leaving the multimeter connected across the thermistor, turned the amp on and turn up some music for a minute or 2. I got a voltage reading (slightly varying) of approx 2v AC. (us voltage 120v)


Any thoughts or input on this would be appreciated -thx

[bob91343]

In order to evaluate a thermistor you must measure the temperature as well as the resistance, and make sure the resistance measurement isn't affecting the temperature.

I have no idea what CL-60s are. When you mention a model number, ALWAYS also mention the manufacturer. Don't assume we know what you are talking about.

[bobodioulasso]

GE SENSING / THERMOMETRICS|CL-60|Thermistor | Newark.com

How much current are you running trhrough it?
How big is your transformer? Has it a dual primary winding?

[Pure_Brew]

The CL-60's are Digikey part#KC006L-ND (similar to the GE example above)

They are current inrush limiters. I am using them on the dual primary windings going into a PS Audio 200c.

Known ratings: 200w x 2 @8ohms/1000w x2@1.5ohms
Transformer: Hi-Pot tested @ 2.5KV, EI type. 2pair of primary windings fed from 120v US, 3 pair of secondary: +57/-57, 0/0, +57/57. The large capacitors are 10000uf x2 per channel.

I wanted to lower the arcing at turn on for the Relay = Magnecraft W199AX-9(40A).
The original design suggested the BIG blue flash was normal, and to leave the amp on 24/7. But the amp is old (1986) and I'm hoping to get more life out of it (plus I don't want to keep it on 24/7)

The thermistors indeed lower the arcing to next to nothing, and the amp works fine. But I wonder if these thermistors will ever go back to a cold 10ohms and I don't think they will at this point.

Or maybe...I'm measuring them incorrectly? The 5.7 ohm reading is measuring from the leads while they are installed.

[bobodioulasso]

You mean 57v ac x 2? (57v ac give about 80v dc)
What is that shorted secondary winding 0/0? There must be a mistake here.
...
Anyway,
I think cl60 is quite small for this big transformer. The peak must be huge at powering on.
This thermistor is generally used with smaller transfos.
I would try 2x cl60 in series with each primary winding in order to reduce the peak. Or use a soft start. The relay being already there.
I use cl 60 with 250va transformers, it works fine though i never measured it after duty .
A condenser across the contacts of the relay could help to reduce the spark too.

[Pure_Brew]

Originally Posted by bobodioulasso

You mean 57v ac x 2? (57v ac give about 80v dc)
What is that shorted secondary winding 0/0? There must be a mistake here.
...

Apologies, my vocabulary in electronics is limited.The 2 pair of windings prior to rectification would be +57/-57 volts AC. The center pair is paralleled and goes to a chassis grounded bus-bar. Indeed this would provide 80vdc after rectification (75vdc is needed for each channel). I think I'm saying this correctly?

Originally Posted by bobodioulasso

I think cl60 is quite small for this big transformer. The peak must be huge at powering on.
This thermistor is generally used with smaller transfos.
I would try 2x cl60 in series with each primary winding in order to reduce the peak. Or use a soft start. The relay being already there.
I use cl 60 with 250va transformers, it works fine though i never measured it after duty .
A condenser across the contacts of the relay could help to reduce the spark too.

Well that makes some sense, but according to that GE link on suggested usage, based on capacitance, I would need like 2 per cap = 8 x CL-60's. Then again, my arrangement looks nothing like the model. The arrangement I'm using is like the Nelson Pass burning amp, prior to the transformer, not after rectification.

Calculating the exact values seems like a daunting task. However, while I'm not quite sure what the maximum in-rush is, the maximum steady state current of 5 amps each seems reasonable. They still seem to minimize the turn-on flash and are hardly warm to touch even when the amp has been on for a week+. What's the worse that can happen, one breaks? Maybe I should take one out and measure it outside the amp.

Interesting thought on the condenser, I'll have to look into that.

[bobodioulasso]

Thermistors on primaries are enough.
The inrush of an EI transfo is not as huge as the inrush of a torroidal one.Though yours is really big. (it must weigh a ton)
When the amp is at idle or playing not loud, the current drawn is little, that is why the thermistors do not really heat.

Yes, unsolder one to measure it.
A condenser of about 0.01uf /250v across the contacts should reduce the sparkle.

[bobodioulasso]

Thermistors in primary windings makes the transfo surge smaller, avoiding to blow the main fuse.
Thermistors in secondary windings avoids overloading the rectifier when caps are charging. It is sometimes used in class A amps where capacitor values are high. Which is not your case.
When i say 2 cl 60 in series, it is to make the resistor larger, not for current purpose.

[Pure_Brew]

I took out the thermistor and re-measured it. As it turns out the device is fine, I didn't account for paralleled resistance, which is why I got the low reading.
When I measured resistance across one, I was actually picking up the parralelled resistance of both.

I think they heat up just fine at Idle, dropping resistance almost instantaneously.

And yes, the transformer is pretty huge for this type, weighs almost 30 pounds.

[bobodioulasso]

So, everything is fine.
It's quite a rule to unsolder one end when mesuring resistors.
Nice amp!
Though, the power rating on low loads is optimistic.
2x1000w@ 1.5 ohms is not possible using three pairs of 12A / 150w devices.