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Minimum relative humidity for power supplies

I have noticed that some power supplies have a minimum relative humidity specification while others do not.

For example the MeanWell IRM-15-12 has a specified temperature and humidity range of -30 to +70 degC and 20% to 95% RH.

Why would there be any need for a lower limit to the relative humidity when some other manufacturers only specify the upper limit?

My best guesses are potential drying of electrolytic capacitors and increased electrostatic discharge risk, but neither of these seem really convincing.

Has anyone come across this before and discovered the real answer? (If there is one!)

John

Reply to
jrwalliker
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** I'll give it a go.

The PSU has a single low voltage ( 12VDC )output and is rated as being Class II standard - implying it is safe for humans to handle the output or something running from it.

The in/out withstand voltage is given as 3KVAC plus leakage is < 0.25mA @ 240VAC. So there is likely a Y cap of about 3nF from output to AC supply for EMI suppression.

In cold and very low humidity conditions, static electricity may build up on some item or person that exceeds the DC rating of the Y cap. 4kV is a voltage that is often quoted.

.... Phil

Reply to
Phil Allison

Someone, thoughtlessly, put an environmentsl test-chamber vertex into the spec sheet, as a 'limit', when it's really just the only measurement accuracy the test chamber can achieve, at -30 degrees C.

How would anyone know better than +/- 20% what the 'relative humidity' of air is, at such a low temperature? It's about 500 parts per million for 100 percent. humidity at that temperature.

Reply to
whit3rd

What happens is that some supplies are low voltage all the way through (relatively low), and others have HV subsections in them.

Moisture presents shorting issues, and that even in LV supplies where feedback and control signals could be corrupted by moisture present between circuit board conductors.

Some quality departments simply want to declare what their test parameters and therefore their "compliance window" is at.

Commercial contracts can be specified as to be conformal to certain national and international specs. Government contracts can be very specific as to what compliance windows must be adhered to. It can also then end up as a sales "feature".

Nope. Static is a problem in a lab with bare chips and circuit sub assemblies floating around unprotected and that from low humidity, not high.

Reply to
DecadentLinuxUserNumeroUno

Wow, DLU1 returns from sabbatical. Is it really you? Now all we need is Guy Macon, and we can party like it's 2008. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

That is an interesting idea. I had a look inside the PSU. It has a 1nF ceramic capacitor between input and output. The capacitor is a Murata KX250 102M rated as X1 Y1. It is specified as being suitable for double/reinforced insulation. The data sheet indicates that it is tested at 4kVrms for 60s and with multiple 8kV impulses.

This seems to be a better specification than that used in many products, so I am still puzzled about the reason for the lower humidity limit when similar products from other companies have no lower limit.

Maybe whit3rd is right and it is simply a misunderstanding. I will ask them.

John

Reply to
jrwalliker

I was hoping that Rich (G?), the pot smoker, would chime in on the pot thread. :^)

George H.

Reply to
George Herold

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