Keystone CL-60...How Hot Should It Get?

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Mark A. Gulbrandsen

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Joined 2001
Then 2 in parallel seems the best way to go which is what I thought. 85 deg C. is better than 100! Also 2 give half the resistance when hot and the main fuse still holds fine. We'll see after repeated use about the fuse as my xformer and caps are larger than the factory units had in them. I suppose another way around the heat generator is to short it with a small relay 5 or 10 sec. after turn on. At least I can remove the giant variac from my bench till amp #2 is finished....hopefully this weekend.

Mark
 
Peter Daniel

Peter Daniel

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I wouldn't connect them in parallel, because it also reduces the resistance when cold and inrush limiting properties are not that good then. If you are afraid of heat the best way is to use relay which will short the thermistor after turn on.

I use to use CL-40 in my A75 amp and quite often I had a bridge failure. So now I'm using 2 CL-70 in my Aleph X and this should give me peace of mind about those soft recovery discreet diodes. The resistance when cold is 30ohm, when hot the voltage drop is negligable. They are so hot that you can't touch them, One other solution is to glue small heat sink using epoxy and this should lower temperature somewhat.
 
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fezz

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i was wondering what makes these current inrush limiters so special?

i am considering getting a inrush limiting thermistor for my zen amplifer which uses a 300va transformer on 230v, can i get away with a 2A thermister, but most importanly, why does everyone use this type of thermistor? and am i ok with a 'gerneric' one?
 
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fezz

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i would imagine that a higher resistance would not only reduce the effect of the turn on inrush, but because of the resistance it would also get hotter quicker so the effects of the current limiter are less evident?

anyway, it looks like another order to be placed with rapid eletronics (great company btw) because i missed the current limiters because they were in a different part of the catalouge (with filters and supression instead of resistors)
 
Nixie

Nixie

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CL-60 won't reach its lowest resistance unless ran near it's maximum 5 amps. For a 100 W Aleph-X, say, they will only get about 50% of that, and will not reach minimum resistance.
One trick is to put a limited amount of insulation to increase the temperature (must be able to withstand over 170*C), which will result in lower resistance for such a part in applications with insufficient current.
 
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Joe Berry

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Caution!

If you want to find the operating temperature of any device in series with a live AC line, DO NOT TOUCH as you may get shocked. Instead, use a laser/infrared thermometer. Get one if you haven't already - they're relatively cheap and have other useful applications.
 
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Mark A. Gulbrandsen

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Actually the stepped one is consistantly a soft start.... the thermistor is not consistant since it has to cool down before it can soft start again. Thats why a relay across the thermistor is also prefered as it allows the thermistor time to cool down. I prefer a power resistor and relay but the cost is higher. After you blow a few line fuses from not letting the thermistor time to cool you might feel the same way as I do. I do however like thermisistor in the chassis to power ground department!!

Mark
 
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RoboMan

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Nixie said:
Sure there is a need. The thermistor gives you a continuous soft start, instead of just a stepped one.
Click to expand...


Couldn't agree more.

Using NTC thermistor is much simpler than a power resistor and relay configuration. Limiting inrush current is necessary to protect all the components in the power supply (rectifiers, transformer, fuse, caps, power switch) and also your power socket (if you plug it in switch ON by accident) from extreme high surge current (when the super large caps are charging).

If you are using a power resistor instead of NTC thermistor, you have to allow a peak current equal to the resistor connected directly to the main (although the power limit of the resistor can be much lower). I used to make two 20W 5 ohm resistors in series while NTC thermistor was not widely available.

For Mark's case, 100 deg C is completely OK as it is still way below the 175 deg C limit.

Paralleling 2 CL-60 is necessary only when the steady current is over the 5A limit. Putting 2 in series will produce an effective resistance of 20 ohm (at 25 deg C) and will also double the steady resistance hence higher voltage drop across the thermistors.
 
Nixie

Nixie

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RoboMan said:
Paralleling 2 CL-60 is necessary only when the steady current is over the 5A limit.
Click to expand...
No, you should use 2 CL-60 if your maximum current is near 10 A. If you have a 6 A current, they will each be at only about half of their minimum resistance during operation. You should be using CL-40 instead. For optimum performance, always use a thermistor with maximum current rating just above your actual. A wide range is generally stocked by places like Digikey, so availability of an appropriate part is rarely an issue.
 
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