Switching PSU shutting off under every load

As it happens I will try fathoming a dedicated SMPS today that as it stands is in tick-tick mode, 1 ultrasonic cycle per second. Putting a dropped LED on each output shows life on all rails. Plan of campaign will be Run up on variac to find lowest point and will still run of sorts, tick-tick Put a x10 larger timing capacitor over the one on the 555 that feeds the UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated, LED of the optoisolator, over-riding the error feed. This will be a monostable of variable period from 1mS to a second or so with push switch, hopefully to elicit which area is playing up by gradually ramping up . All active, big Rs seem ok and caps ESR-check ok. I suspect false error signal. Anyone else any ideas/advice on a generic plan of action for these sorts of situations.?

you checked the esr and caps but I would desoldier the main 400v cap and test its capacity and esr out of the box. compare with a new one. also as you have a uc3842, I would do the same with the small (10uF or so) cap that is on the + and - pins of this uc. often this is the problem. regards,

--

Jean-Yves.

"supervinx"

** Many PC type switching PSUs need a load to be applied on ALL the outputs on order to work properly.

Why not apply load resistances ( not incandescent lamps either) to each DC rail and see if it holds up then ??

Thing is, when you load just the 5 volt rail, the other DC rails go UP in voltage - and that may trigger the out of range voltage detector circuit and stop the whole damn show.

.... Phil

Il Tue, 30 Mar 2010 21:14:04 +1100, Phil Allison ha scritto:

The PSU shuts off under normal operations while connected to his MB. I connected it to an external HD (load on both +5 and +12): I see the voltage raising and the voltage detector shutting the PSU down. I have no oscilloscope handy ... with a digital multimeter I can see the

+5 rising about 4.8, the +12 about 9 and then stop.

The main capacitors are 200V rated, but I see 215 across them :(

"N_Cook" wrote in news:hosb3o$r7f$ snipped-for-privacy@news.eternal-september.org:

is there any OVERvoltage shutdown circuit? some protection crowbar you've missed? some SMPS have crowbars on the 5v logic rail to protect the digital logic on the load.I've seen where a zener broke too soon and triggered the SCR too early and caused shutdown.

scope each rail to see how far it climbs. Maybe one rail is not climbing as much as it should.

use a scope with a current probe to see if you're hitting the current limit. Check the PWM IC's housekeeping cap.Maybe it's not holding the IC supply enough to keep the PWM IC running.(a common failure)

--
Jim Yanik
jyanik
at
localnet
dot com

supervinx wrote in news:fVksn.40988$ snipped-for-privacy@twister2.libero.it:

scope each rail to see if they rise proportionally.

A DMM will not respond fast enough.

By "main caps" do you mean the line supply filter caps,right after the rectifier? The raw DC supply for the switcher? If it's a doubler,you should have around 300VDC,if a single-ended rectifier,about 170VDC. Perhaps you don't have enough input DC. Or maybe you're seeing some AC along with the DC.

Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch changes it to single-ended for 220VAC operation.

--
Jim Yanik
jyanik
at
localnet
dot com

The PSU is 250Vac: the two capacitors are 680uF/200V (they seem to be series-connected) and they show, with no load, 215V across each capacitor. This value is suspect: I would expect 170V, as you stated, so the problem could lie in the primary side of the PSU.

The 680uF/200V is mandatory ? Can I try to substitute them with, for example, 1000uF (or more)/400V ? I have them handy ...

LOL

1000 would be ok for a short test but don't attach the SMPS to the MB. Load all the outgoing voltages with incandescent. Your problem is likely high ESR caps given the age of the SMPS.

--

digital

So far confirmed the UC3842 functions properly on a bench 13.5V supply with original 2nF and also27nF // to it. But gets overriden by a protect sub-cct on the hot oscillator side. Also that the 2 switching powerfets are conducting balanced. Problem seems to be a goodness of oscillation monitor on the hot side of the transformer that rectifies some of the drive and if not balanced or large enough starts the tick-tick mode. Rectified it must exceed 15V set by a zener or this mode cuts in , it would seem. Only reaching 4.9V. But in the process of checking this area a small electo now has an ESR of 200R wheras checking before any power on it must have been sub

2R or so sitting around idle. 1988 ps with a load of early SM and this cap of course in an awkward section to replace rather than test, a job for tomorrow.

Thanks for your help, the fault begins to be more clear ...

Having no oscilloscope, I used spartan methods ;)

1) connected a 12V fan pair: the voltage dropped to 3.5V, but the PSU ran well 2) added another +12V fan pair, but connected to the +5V line: +12 line raised to about 9, +5 to 4.80 ... ran with no troubles 3) added another small load on the +5V (47 Ohm) and everything shut down ... The +5V line connected fans were motionless (too low voltage) and no motion was noticed adding the 47 Ohm load.

So I must assume there's no voltage peak: the protection scheme could be a "current sense" and gone wild ?

4) Retained the +12V line connected fans and the 47 Ohm load on the +5 Line (removed the fans on this line): starts ok 5) Tried to move the trimmer (which should adjust the +5V, very slowly) and power up, with the aforesaid loads. 6) Until 4.59 on the +5 line ... ok 4.60-4.66 starts, lasts few seconds, then shuts off >4.66 starts-stops immediately

That trimmer is connected to two transistor and some diodes (the small orange-glass-like ones, not the "usual" black ones.

So, I suspect the fault is in the protection circuit, generating a false positive.

I think of removing the two transistors and identify them. If some diode is a Zener, the only way I have to test is to desolder them and build a small auxiliary circuit (diode-resistor), to check the breakdown voltage.

One tool I don't have in my toolkit - a sniffometer. Nothing smelled while in use or close handling of this power supply. But on removing the electros , that distinctive fishy smell of leaked elecrolyte, with soldering iron temperature on the pins. Tiny space available to fit some capacitance so they used 2x squat format

22uF,35V in parallel. One leg pulled out of one cap on desoldering and the other a telltale drop of brown gloop immediately under the cap but not enough to be visible over the pcb. Both bright green nichicon 105 deg C. Found space on the hot side for replacement and powered up long enough at about 3KHz instead of 40KHz to give full V on all o/p ( unloaded) with very audible 3KHz coming off the transformer. As , presumably, the pair of switching power fets were operating at least partially linearly at an undesigned 3 KHz they heated up quite quickly. 15 V over that sensing zener . Removed the 27nF and now operating correctly with substantial test loading on all rails.

One mystery remains , the original tick-tick was about 1 a second, on adding the 27nF to the 555 timing capacitor the tick-tick rate increased to about

10 a second. I've not traced out the path from the "goodness of oscillation" circuit to the 555 but what sort of intermediary cct would lead to an increase in that tick-tick rate ?

"N_Cook" wrote in news:hov3ae$hvb$ snipped-for-privacy@news.eternal-september.org:

at TEK,I used to replace a lot of the green nichicon 100uF/25v electrolytics in the 1700 series waveform monitors and vectorscopes.ESR problems,probably aggravated by the long operating hours in hot racks at TV stations.

I think you answered your own question;you ADDED capacitance to the 555 timing capacitor.

what sort of SMPS uses a 555 for a controller?

--
Jim Yanik
jyanik
at
localnet
dot com

TV

Adding C usually decreases the f

UC3842 PWM controller , instead of simple RtCt pin , it uses the o/p of a

555 so that there can be diode steering for different timings

"N_Cook" wrote in news:hovkhd$due$ snipped-for-privacy@news.eternal- september.org:

Ah,nothing like taking a great simple PWM IC circuit and making it more complex....

Now I wonder about the circuit makes the ramp for the internals of the

3842,while not using the RtCt function.Seems like that could affect the current limit function.

--
Jim Yanik
jyanik
at
localnet
dot com

a

There is a lot more SM around the UC and 555 than any of the datasheets I've seen. There are at least 1 diode for changing the duty cycle of the 555 waveform. But increasing the single timing cap from 2 to 29nF upped the error mode frequency. Perhaps the duty cycle changed so much that it inverted and played havoc with timings or limits in the UC. I will use x5 rather than x10 next time . Incidently slowing things down so less likely to be destructive to drivers if the problem is shorted turns in the transformer.

I removed an SCR, 2N5061, with this behaviour: connecting an HD (or a load) doesn't shut off the PSU anymore, but voltage drops from 5 to 4.2, and it's not enough. I moved the trimmer with slight advantage (4.5) then the PSU shutdown again. I had to wait some minuites, discharge all caps before it started too work again. Under the SCR there's a fast switching diode 1N4148F, with a constant reverse voltage of 1.40. When everything goes bad, it's the first to receive 0 voltage. Across A-K of the SCR I read about 4.34 V, G and K are equipotential. May be this is the right zone to investigate ? A friend of mine owns a similar PSU (same PC), working, but he'll be back after easter. Having a similar model could help me tracing down the fault...