NTC 5D-9 Thermistor?

Thanks for the spec sheet, but I'm not sure I can find everything I was looking for here on that sheet or another I also found via google search. The sheet clearly answers one question: the "5" in the 5D-9 part identifier is the resistance in ohms at 25 degrees Centigrade (I assume this is the startup resistance and that it decreases as the temperature increases due to current passing through it). If I read the table at the end, it appears that the steady state "hot" resistance drops to 0.18 ohms, but at what current, steady state temperature, and how quickly?

More importantly, if I wanted to beef up this part to prevent this from happening again, what generic parameters should I be looking for in a thermistor at the local RS store? The 5 ohms and .18 ohm are a good place to start, but I did not see any type of current limit or power rating (assuming the .18 ohms) for the device.

Additional answers/advice would be appreciated.

Bob

If it is a 600W non-PFC power supply we might assume at 80% efficiency the input power to be somewhere around 750W.

750/120 = 6.25A (sound familiar) - full power operation.

Per Ohm's Law: 5 ohm starting resistance = 31W if starting inrush is

100% of full power load (not likely). .18 ohm ending resistance = 1.125W at full power.

Note that the inrush current limit for the 5D-9 is only 3A, so either you don't have a 600W supply or (most probably) the max starting surge is calculated at 50% of the full power load or perhaps as you stated the device is underrated. You decide.

Travis,

Thanks again. Yes, I can do the math and understand ohm's law... and your

75% efficiency number seems about right (I assumed 80%) and consistent with the fuse sizing, but the piece I missed in the spec sheet is the maximum 3A in rush current. Where was this specified?

I think you've hit upon why the part exploded and burned up. It appears it was indeed undersized for the application. So from their listing, which part has the higher in rush current limit? Again, I did not see this specified in the part list table. Thanks in advance.

Bob S

Column labeled 'at 25ºC (A)' - 3A

25ºC is the starting temperature and thus the 3A must be the inrush current, or at least this is the way I read it.

I would replace it with a NTC 5D-18 which has a 7A current rating.

Oops - my error - you want the same terminal resistance so use a NTC

8D-18 (8A).

Or a Thermometrics CL40 should work too.

*527CL40*&N=0&crc=true

Thanks again for taking the time to help educate me. From your explanation of the table (which makes perfect sense), I think the 5D-15 (6A in rush) seems to double the original current limit, but retains the 5 ohm startup resistance and 0.18 ohm steady state resistance. The 5D-18 would probably also do the trick ... it just has a slightly lower resistance, but I do not think this is an issue at all since ideally it would be zero anyway.

Now to find this beast.... The local retail RS had nothing. They also did not have anything listed in their catalog either. in fact,. the only thermistor they carry was a 10K ohm initial value. I'll have to try some of the local electrical supply houses. Thanks also for the alternative CL40. I'll have to see what I can find.

Bob

Unless you have a distributor nearby that will sell one to you you'll probably spend more time and effort trying to find it than it is worth - I'd simply order it online.

Try Allied:

or Mouser:

Yes, exactly. We did have one local electronics house here, Grainger, but they did not stock this item. I took your advice and ordered the CL40 from Mouser. My son is home from school for a couple weeks so hopefully we can get this in before he leaves to return to school.

Thank you again for the help here in understanding the specifications.

Bob

No problem - and I hope you'll get his PC back up and running.

I just saw that CompUSA had an 500W Antec supply for $20 after rebates, sale good through tomorrow. You might want to get one of these for a spare or in case the thermistor fails again due to some undetermined problem. Order it online and you won't even have to drive to a store. Cheap insurance (but not as much fun as fixing a broken supply!).

OTOH if you need a good source for a replacement PS in the future I'd recommend these guys.

Oops, just noticed that this is a CompUSA branded supply, not an Antec supply! So buyer beware on that one (although I've installed a few 350W CompUSA supplies and they worked fine and are still in service after a few years).

Travis,

I'll go for the replacement part thanks since I am 99+% certain that this will fix the supply given my previous experiment jumping around the blown part coupled with the fact the protection fuse was not damaged. Hopefully by putting in a replacement part that has twice the previous' current limit this should not happen again.

I am a firm believer in minimizing waste and really hate to see anything at all go to a landfill that can be repaired. And besides the ecological aspect, it is so much more rewarding to fix something that is considered dead by most people. By the way, the part (1 piece) from Mouser was $1.39, and although I'm sure that the shipping and handling will increase that figure, it still should be much less than a replacement supply.

Bob

I agree, but unfortunately more and more often now I am finding that it simply is too hard (time consuming and frustrating) to repair some of today's electronics.

One of my Enermax high-end switching PC power supplies crapped out last week and while I quickly found the problem (a shorted Mosfet switching transistor and a couple of associated passive components) it was simply impractical to repair the supply without hours and hours of labor. The Mosfet was riveted to a heat sink that was in turn buried under several layers of other components. Removing all the layers would have taken a lot of time and extensive desoldering. While I would have preferred to save the supply I decided instead to replace it at a cost of $90. But now I've got some nice quiet 8mm and 9mm fans that I can use elsewhere.

OTOH I recently repaired a 10 year old 27" Sony TV that had a failed power supply IC - it was easy to get to and while I was at it I improved the heat sink so it won't likely fail again. So you win some and you lose some.

I hope you win the PS battle and that it only costs you $1.39 plus S&H. Post back and let us know if you succeed.

CompUSA PSUs are made by several different companies, some great, like Win-tact, some awful, like Powmax/Leadman, and their supplier told me that the brand can change from week to week.

Raidmax is now supposedly OK and is made by Topower, the same company that makes Aspire, OCZ, and Tagen. Topower's quality varies, but I believe that the Raidmaxes are made from the better models. Still, I'd probably rather buy a Fortron-Source, which can be as little as $40-50 for a 400-450W model.

I have a very well-built OEM PC PSU that has two thermistors, one for each AC line. One thermistor is fairly large, the other only half its diameter. What's the purpose of using two different sizes like this?

I use Raidmax for the low end and Seasonic for high end applications. Enermax used to be my preferred high end PS but I've had a few problems recently with their supplies produced in 2002/03 so I'm waiting to see if they have addressed whatever supplier or design issue caused that.

Good question. Are you sure that they both are thermistors and that one isn't a fuse link or RF filter?

Here's a photo:

On the circuit board they're labelled NTC1 and NTC2, indicating that they're thermistors. The RF filter is behind them in the photo.

Here's a photo:

I'm pretty sure that the two black things are thermistors because they're labelled NTC1 and NTC2 The RF filter is in back of them in the photo.

I'm pretty sure you are right. It is likely they are to provide two different NTC inrush curves - I suppose we'd have to ask the designer what s/he had in mind. Sounds like overkill to moi, or maybe they got a 'two for one' deal on the thermistors.