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Location: Plymouth UK
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Thermistor Question

07/30/2011 8:10 PM

I have a dead switch mode psu from a network switch. Closer examination shows that one of the components I need to replace is a Thermistor.

Knowing very little about Thermistors I read up some and have concluded that the item in question is NTC but I have yet to find a list/chart that would enable me to find the right value replacement.

The component is labeled 10SP 005

Anyone able to throw some light on how the numbering works?

In my box of spares I have other units fitted with 10SP 010's but before I go any further I would like to learn a little more.

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#1

Re: Thermistor question

07/30/2011 9:12 PM

The component is likely a NTC thermistor whose function is to limit the inrush current that can kill power components during power-on start up.

An NTC thermistor has a relatively high resistance at room temperature, so at turn-on, it limits the current charging the load's rail capacitors. As the thermistor heats up, its resistance drops, so the power supply becomes relatively low impedance.

The listing of NTC thermistor components above came from here:
http://www.allproducts.com/manufacture3/uei/08sp005.html

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#2
In reply to #1

Re: Thermistor question

07/30/2011 9:39 PM

What a fine fellow you are. That is exactly the chart I have been looking for.

Such components are, of course, available locally but without this or a manufacturers comparison chart I was struggling.

Thank you

I never thought to try Allproducts, I seem to forget to try outside of the UK

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#3
In reply to #2

Re: Thermistor question

07/30/2011 11:25 PM

Old chap, you should give the fine fella a GA.

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#4
In reply to #3

Re: Thermistor question

07/30/2011 11:30 PM

So should you, but I am the only one to do so thus far.

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#5
In reply to #4

Re: Thermistor question

07/31/2011 12:10 AM

Yeap !

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#6

Re: Thermistor Question

08/01/2011 6:44 AM

I will now give you the all time best advice for when repairing any SMPS, that you have not had training on, not got the manuals and not got the experience, knowledge and tools:-

Seriously, some can be exceedingly dangerous to long life expectancies.....

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#7
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Re: Thermistor Question

08/01/2011 6:14 PM

GA fixed

Thank you Andy. Good enough advice.

I will go ahead with this as my day job is as an electrician ...so lets just say that although I have little to do with electronics on a daily basis, I have the tools, the test equipment and, as a 50+ year old, I am aware of what electricity can do to the human body (and I have the burn scars to prove it) :)

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#8
In reply to #7

Re: Thermistor Question

08/01/2011 11:52 PM

OC, Andy beat me to it about the "life expectancy" warning...and your response indicates perhaps the nature of the warning; i.e., you know not to work on live equipment (even though you probably do it daily) but have you considered the effect on the life expectancy of the equipment connected to your "repaired" power supply? Andy's advice falls into the same category as advising against repairing a molded case circuit breaker, the Law of Unintended Consequences is most likely to rear its ugly head!

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#10
In reply to #8

Re: Thermistor Question

08/02/2011 4:12 AM

I figure if a 50+ doesn't understand the "ins" and "outs" by now, he will never learn them......let him have a go.

I worked in the computer business years ago and just for fun, tried to repair a few of them, either I did not find the new component for sale, or when I did, it did not fix the problem. Ergo, never managed to fix one.......gave up completely after 5 or 6......%They were just thrown away, not repaired when defective.

In my defense, the chips used were weird, some were unmarked. Several suppliers though I was joking when I found a number as they could not identify the chip or the manufacturer.......how much better the situation today is with the supply of parts I cannot say, my experiences lie at least 10 years ago.

I can say that in my experience, SMPSs are far more reliable than anything I ever used before provided the cooling is good.

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#11
In reply to #10

Re: Thermistor Question

08/02/2011 4:16 AM

I build my own PSs for CNC work, but I build a derivative of a SMPS, using some really great chips, they are called "Step down Linear regulators", all the clever stuff is in a cheap piece of silicon and few external components needed.

The basic design work was already done by Elektor Magazine 8 years ago for a 10 amp supply for voltages between 5 - 40 volts!!! 85% efficient.

If anyone needs further details for any reason, just ask me offline to send them to a non CR4 email address.....

Marking as off topic

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#9
In reply to #7

Re: Thermistor Question

08/02/2011 3:45 AM

You have all the requirements! Have fun!!

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#12
In reply to #9

Re: Thermistor Question

08/02/2011 12:48 PM

I have some of those in my box of parts too but I will happily acknowledge that the complexity of recent computer PSU's is beyond me. Besides I do not know the reason for their demise.

When the switch psu failed I saw what happened and why (a heatsink spring came away from its mount and found its way under the psu during transport) This is the reason I am willing to spend a little time and cash.

What I have just now is almost useless so as far as the equipment is concerned it is already broken....nothing to lose.

As a new member here I find the help and comment wonderful, and open too. I've only found this in a couple of other (computer forum) places.

Bravo all of you.

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