Author Topic: Linear Power Supply used 7 days 24 hours  (Read 2637 times)

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Offline sdouble

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Linear Power Supply used 7 days 24 hours
« on: January 13, 2018, 09:14:12 am »
Hi folks,
I'm currently benchmarking a few low power linear power supply.
I'm actually after a 2-channel unit. It's supposed to provide me with clean rails (+9 and -5 V). Low power : only 4W on each rail.
My main concern (besides the noise level) is the fact that device should operate 24 /7 for a couple of years.
I wonder whether the electrolytic cap could sustain such a treatment.
Any experience on your side, guys ?
Thanks in advance,
J
 

Offline TimNJ

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Re: Linear Power Supply used 7 days 24 hours
« Reply #1 on: January 13, 2018, 09:29:50 am »
I think electrolytic capacitor life is always a concern. At 4W, however, it doesn't seem like the unit will make significant heat...which is an electrolytic cap's biggest enemy.

If you are really concerned, you can check out what manufacturer/series of caps they used when the unit arrives. Or check online to see if anyone has pictures.

You can use the rule of thumb that a capacitor's life doubles for each 10 degrees Celsius it is operated below it's rated temperature. That is, if you operate a cap rated for 1000hr @ 105*C cap at 85*C, then its expected life is 4000 hours. If they use garbage capacitors, then maybe take the manufacturer's specs with a grain of salt.

If you are really in love with a certain power supply from a design/usability standpoint but it uses crap caps, then you can always swap out the caps preemptively. (Though, I'd be wary of the quality of the other components too.)
 

Offline mariush

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Re: Linear Power Supply used 7 days 24 hours
« Reply #2 on: January 13, 2018, 09:42:09 am »
A polymer capacitor will have a much longer life, the formula used is

  Life = Original Lifetime x 10 ^ [ (Tcap_max - Tambient) / 20 ]  ...

so a 2000h @ 105c polymer running at 65c 24/7 in theory will last  2000h x 10^ [(105-65)/20]  = 2000 x 10 ^2  = 2000 x 100 = 200.000 hours or 83333 days or 8333 days or 22 years

Electrolytic capacitors ... yeah, you can approximate by doubling lifetime for every 10c under the max rating ...

If those regulators don't like low esr of polymer capacitors, maybe look into high temperature capacitors like the RX30 series from Rubycon which is rated for 2000..4000h @ -40c .. 130c : http://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/e_rx30.pdf


 

Online David Hess

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Re: Linear Power Supply used 7 days 24 hours
« Reply #3 on: January 13, 2018, 11:16:25 am »
My main concern (besides the noise level) is the fact that device should operate 24 /7 for a couple of years.

I wonder whether the electrolytic cap could sustain such a treatment.

The bulk input capacitor is sized to hold up the input voltage between power line cycles resulting in a massive ripple current derating so operating life is not a problem.  The output capacitor sees very little ripple current so its operating life is not a problem either.

You still have to pay attention to the operating life calculations but the application is not demanding.  Decades of operating life is feasible.
 

Offline retrolefty

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Re: Linear Power Supply used 7 days 24 hours
« Reply #4 on: January 13, 2018, 11:44:16 am »
 I worked with large (up to 60 amps or so) linear regulated power supplies (5v to 24v) in a large refinery
that used several hundred or more and most worked 24/7/365 for several decades and then I retired from there.   :-+

 Once or twice a year we might find a failed unit. Lamda brand supplies were the most used there but other brands were evident. I always attributed this reliability to the fact that they weren't powered up and down but left to run.
 

Offline james_s

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Re: Linear Power Supply used 7 days 24 hours
« Reply #5 on: January 13, 2018, 12:49:47 pm »
Linear power supplies are generally pretty gentle on caps, I have lots of 30+ year old stuff still running original electrolytics. Switching regulators wear them out a lot faster.
 

Online David Hess

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Re: Linear Power Supply used 7 days 24 hours
« Reply #6 on: January 13, 2018, 01:30:45 pm »
For the oldest linear power supplies with can capacitors, the failure I usually see is leakage of oxygen and water through the rubber seals of the input capacitors causing one of the aluminum terminal connections to corrode through.  Slightly more modern supplies with "computer grade" aluminum electrolytics eventually lose enough capacitance due to drying out that the input voltage is not held above the dropout voltage of the linear regulator between power line cycles.  In both cases, this happens at 30+ years.

Of course if you stick the capacitor where it is operating at high temperature, it will dry out faster.  I have worked on passively cooled enclosed instruments where this resulted in all of the small output capacitors having one half of their rated capacitance after 20 years but everything still worked.

On the other hand, the input and output capacitors on my switching power supply based cordless drill charger dried up completely after just a couple years causing it to malfunction.  The same happened with my AA/AAA NiCd/NiMH charger.

 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #7 on: January 13, 2018, 06:11:29 pm »
Thanks a lot guys for the info. that's pretty reassuring.
The rectifier bridge "sees" more ripple. Any special thing to look at in that direction ?
 

Online blueskull

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Re: Linear Power Supply used 7 days 24 hours
« Reply #8 on: January 13, 2018, 06:22:43 pm »
One way to do this cheaply is to use a master-slave ORing switch, for most of the time, master PSU takes the load and slave PSU is powered, but doing nothing. When master PSU fails, the ORing controller switches to slave PSU and issues a warning to tell the operator to do some maintenance.
This allows you to use some very cheap, non-critical PSUs in a critical environment.
The ORing structure can be very simple -- just 2 schottky diodes, and set the master PSU's output voltage slight higher than the slave PSU, so the slave PSU doesn't output before the master PSU fails.
 

Offline IanMacdonald

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Re: Linear Power Supply used 7 days 24 hours
« Reply #9 on: January 13, 2018, 09:31:08 pm »
I'd have said that a greater problem in linear PSUs is under-rated rectifiers. Usually because power factor is overlooked. If you use good-brand caps it's unlikely they will give trouble.
 
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Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #10 on: January 13, 2018, 10:52:12 pm »
here is what the seller says :
"This power supply uses a Nichicon capacitor, MUR1560 rectifier diode, not a rectifier bridge. "
 

Online blueskull

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Re: Linear Power Supply used 7 days 24 hours
« Reply #11 on: January 13, 2018, 10:53:44 pm »
here is what the seller says :
"This power supply uses a Nichicon capacitor, MUR1560 rectifier diode, not a rectifier bridge. "

MUR1560 for a 4W PSU is waaaaaay overkill even under any reasonable power factor an surge conditions.
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #12 on: January 13, 2018, 10:54:24 pm »
here is the rectifier diode
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #13 on: January 13, 2018, 10:55:37 pm »
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #14 on: January 13, 2018, 10:57:21 pm »
here is what the seller says :
"This power supply uses a Nichicon capacitor, MUR1560 rectifier diode, not a rectifier bridge. "

MUR1560 for a 4W PSU is waaaaaay overkill even under any reasonable power factor an surge conditions.
certainly. It will run continuously (7/24) at 4 to 5 W but it's a 65 W power supply
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #15 on: January 13, 2018, 11:01:00 pm »
another pic of the beast :
 

Online blueskull

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Re: Linear Power Supply used 7 days 24 hours
« Reply #16 on: January 13, 2018, 11:10:53 pm »
I can see it's a solid unit and should be fine.

Here are some pros and cons:
Pros: very solid build with good components and looks neat, usually if something looks neat, its designer had spent time on it, and that's never a bad thing. It's not going to kill you by a faulty transformer -- primary and secondary windings are physically separated, and mains voltage doesn't run on PCB, crimped connector has insulation sleeves and fuse is placed inside input connector module.
Cons: output cable is too thin, doesn't look like something rated for 65W. The 5V rail, assume that's rated for 30W, that's 6A. I don't think the thin cable (AWG20?) will be happy passing 6A continuously. But for your 5W application that's more than enough.
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #17 on: January 13, 2018, 11:23:00 pm »
my only issue with that unit is that the seller can't remove its logo on the front plate. I can't re-brand it. That's clearly a problem...
BTW, thanks blueskull, i'm happy to learn that it won't kill me ! ^-^
 

Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #18 on: January 14, 2018, 11:38:00 pm »
does anyone know that transformer brand ?
Among the possible issues are transformers going bad.
I don't know those and wonder whether  those chinese transformers are reliable ?
 

Online blueskull

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Re: Linear Power Supply used 7 days 24 hours
« Reply #19 on: January 14, 2018, 11:45:02 pm »
does anyone know that transformer brand ?
Among the possible issues are transformers going bad.
I don't know those and wonder whether  those chinese transformers are reliable ?

Don'y know, but it should be very well built. Those are not crap, those are HiFi linear power supplies, and those things usually have a decent profit margin that they don't have to race to the bottom.
Anyway it's a heavily derated unit, and I won't worry about it if I were you.
 

Offline james_s

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Re: Linear Power Supply used 7 days 24 hours
« Reply #20 on: January 17, 2018, 07:40:09 am »
Of course if you stick the capacitor where it is operating at high temperature, it will dry out faster.  I have worked on passively cooled enclosed instruments where this resulted in all of the small output capacitors having one half of their rated capacitance after 20 years but everything still worked.

One of the products a former employer made back before I worked there had a power supply with electrolytics right up next to a large heatsink that wrapped around and folded over part of the board. As one might expect capacitor failure was common after a few years of 24/7 service. I rarely find bad bulk filter caps but on those I encountered several that rattled like a maraca when shaken. Usually the failures would show up after a power outage, boxes would be running ok but after losing power they wouldn't start up again.
« Last Edit: January 17, 2018, 12:10:22 pm by james_s »
 

Offline floobydust

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Re: Linear Power Supply used 7 days 24 hours
« Reply #21 on: January 17, 2018, 09:55:38 am »
another pic of the beast :

R-core transformers, ugh!  :palm:
 

Offline james_s

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Re: Linear Power Supply used 7 days 24 hours
« Reply #22 on: January 17, 2018, 12:11:13 pm »
What's wrong with the transformers? I've used some cores like that, they're a lot easier to wind by hand than a toroid, seemed to work fine for what I've used them for.
 

Offline mikeselectricstuff

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Re: Linear Power Supply used 7 days 24 hours
« Reply #23 on: January 17, 2018, 12:15:19 pm »
For electrolytics, it's ALL about temperature. Life doubles for every 10 deg.C reduction.
They can last decades if kept cool and well within their ripple current rating ( to reduce internal heating)
« Last Edit: January 17, 2018, 12:17:19 pm by mikeselectricstuff »
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Offline sdouble

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Re: Linear Power Supply used 7 days 24 hours
« Reply #24 on: March 31, 2018, 05:26:45 pm »
dear all,
I just received the PSU.
It looks good, pretty heavy as expected. most of the weight is in the transformers.
The big output caps are Nichicon and rated 85 degrees C 35 V.
The low voltage wires are 16 awg, which is a bit low to ensure a constant voltage with varying loads
the noise level is satisfactory.
I will run it now for some weeks at 1.5 Amp on both rails.
Don't hesitate to comment on the design. This is still a prototype for me. If you see a mod which could improve the  reliability of the device, please mention it.
I intend to buy a bunch of those within 2 months if the burn in phase provide satisfactory results.













« Last Edit: March 31, 2018, 05:39:18 pm by sdouble »
 


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