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How hot is too hot...

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Inducktion

Mon Nov 10 2014, 12:55AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Well, ordered this 120 watt chinese PSU for $12 on eBay in hopes of using it for a 100 watt LED fixture I'm trying to make for my plants...

I'm feeling a little skeptical and unsure if I should really trust it for as long as it's going to be on.

Doing a short 5 minute test or so, the outer PSU feels pretty warm (the case is the heatsink for the output diode and the transistor)

However, when I took the case off and felt around, holy shit

The things circled in black were HOT. I didn't circle the transformer itself but it was probably the hottest thing in the entire circuit board, and felt pretty dangerous.

I'm guessing I should probably just play it safe, and get an ATX PSU (which, of course I don't have...)

GrantX

Mon Nov 10 2014, 03:13AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

I have a friend who sells bitcoin miners. He bought a box of cheap knockoff xbox 360 power supplies for running his own miners and to package with each miner sold. They were supposed to be a 130W 12V supply, however they weighed about half as much as an actual xbox 360 supply and the box had no branding whatsoever, just stating "X 360 PSU" with a photo of the unit. Internally they looked about as bad as the one in your photo, messy glue and thermal paste, components leaning or bent, poor clearances, the whole thing looked rushed.

About 25 of the power supplies were set up to run the miners in my friends shed. One by one they ALL failed. About 5-6 failed the instant they were plugged in, the light flickered from green to red and the output dropped to about 2.2V. The rest didn't fail instantly, but they ran very hot, with the plastic case bordering on 70 degrees Celsius at 110-120W. Some lasted a couple of hours, some lasted a few days and one determined unit ran for a full month before cooking itself. When the dead units were opened each one had a severe burn spot (a hole was melted through the plastic case in 3 or 4 examples) where a SMT transistor was connected in series with the ground line. I didn't look at the circuit closely, but it looked like the transistor was supposed to cut off the output if the 5V sense rail was disconnected.

Moral of the story: If it looks suspicious, runs hot and cost less than $15, then its dodgy.

Inducktion

Mon Nov 10 2014, 03:45AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Well honestly, it runs alright, no problems with output voltage or anything, it's literally just a heat problem with the caps and transformer. It's also got some safety features (which kind of surprised me) like a fuse and decently isolated secondary/primary.

I think it'd be an alright PSU just not anywhere near the aforementioned 120 watt rating it supposedly has.

klugesmith

Mon Nov 10 2014, 05:51AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sounds like you got less power supply than what was advertised. Adding (more?) forced air cooling could make a big difference in its lifetime.

Back to the Subject of this thread, it's common for transformers to be designed to run hot. I once read an extreme point of view: "if a SMPS transformer doesn't get very hot, it is overdesigned & overexpensive".
Here is a citable reference:

"Historically most transformers have been designed to run at a temperature of 40 to 60Âº C above ambient and
therefore it was useful to minimize losses in the 60 to 100Âº C operating temperature range. Indeed this is the
case for 3F45 material as it is apparent that the losses are the lowest at all frequencies in the 80 to 100Âº C
range."

Aluminum-can electrolytic capacitors also self-heat, more so if underdesigned to save cost.
But they have finite lifetimes right out of the box, which become much shorter at temperatures that are business as usual for transformers. Check this out:

Sulaiman

Mon Nov 10 2014, 09:11AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Where I work our main business is repairing industrial electronics,
I guess somewhere between 1/3 and 1/2 of all failures are due to
electrolytic capacitors loosing electrolyte,
usually just resulting in high e.s.r.
sometimes causing pcb track corrosion.

We frequently find 85C rated electrolytics so replace them with 105C or 125C rated equivalents
often of higher voltage rating than the originals for longer life,
and we choose longer than usual (usually >= 2000 hours, up to 10,000 hours) life rated
because compared to our labour cost the little extra is negligible.

Many anufacturers pinch every penny they can,
which is why so many poor electrolytics are in service.

As above, ferrite is formulated for least loss at 80C to 100C
as that's how transformers are made cheaper/smaller/lighter.

The output rectifiers commonly dissipate more heat than the switching transistor(s)
as they operate at much higher current, so hot rectifier(s) is normal.

The snap-in capacitor in the foreground is probably 85C rated AND fairly high esr.

If this was my psu I'd replace the smaller electrolytics with good quality ones.
The snap-in will probably survive.

Poor/no airflow will DRAMATICALLY reduce lifetimes.

Shrad

Mon Nov 10 2014, 10:19AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

if you want a durable PSU use a server PSU

they are cheap, easy to work with, extremely efficient and designed to stay ON for years...

Conundrum

Tue Nov 11 2014, 09:27AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Bad caps?

Shrad

Tue Nov 11 2014, 03:42PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

fake caps?

2618014599 811b038cbc

hen918

Tue Nov 11 2014, 08:03PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Shrad wrote ...

fake caps?

2618014599 811b038cbc

WTF!

I've never seen anything like that before, it could be some sort of secret compartment!
I bet the weight would be different though

Shrad

Tue Nov 11 2014, 09:05PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

that's the reason I mentioned it... Inducktion said the PSU was lighter

hot capacitors (hot enough that one can't keep the finger on it) would tell me that the capacitors are full, though, as emptiness doesn't conduct heat that well...

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