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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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SMPS wall wart failure.
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#3
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SMPS wall wart failure.
On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber"
wrote: The zener diode is paralleled by the filter cap, not the power diode. If the zener is there strictly for protection, the power supply should work normally without it. Have you tried using it without the zener? If it's in parallel with the output, it's likely some voltage slightly over 5V, such as 5.6v or 6.8v. If the zener is expected to short upon overvoltage, I would guess(tm) that a low power zener would be more appropriate than one that can handle the power. I've had my share of 5v 2A power supplies blow up. I don't recall if they were made by Jentec, but when I replaced the usual bulging capacitor(s), most would not recover. I never bothered to troubleshoot further and just recycled them. Thanks for the hint about the zener. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#4
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SMPS wall wart failure.
Also, please see my corrections to my original post. The zener diode is paralleled by the filter cap, not the power diode. Although strictly speaking it is correct no matter which way round you say it, that still comes across sort of wrong, as though the zener has some primary function, and that of the filter cap is secondary. It would be better to say that the filter cap is paralleled by the zener, implying that it's the zener that has the secondary function. Probably just me splitting hairs though ... :-) Arfa Thanks for your reply. -- David Farber Los Osos, CA |
#5
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SMPS wall wart failure.
"Arfa Daily" wrote in message
... Also, please see my corrections to my original post. The zener diode is paralleled by the filter cap, not the power diode. Although strictly speaking it is correct no matter which way round you say it, that still comes across sort of wrong, as though the zener has some primary function, and that of the filter cap is secondary. It would be better to say that the filter cap is paralleled by the zener, implying that it's the zener that has the secondary function. Probably just me splitting hairs though ... :-) Arfa Thanks for your reply. -- David Farber Los Osos, CA 1. Cap develops ESR. 2. High frequency AC waveform appears on the desired DC output. 3. Circuit interprets as low DC - attempts to compensate. 4. Actual DC runs away. In the Panasonic example, the 14 volt line would go to 35 volts. 4. Zener shorts. 5. Ideally this throws off the "Q" of the circuit so much that it simply stops oscillating. Power supply stops but does not blow up. "Ideally". The old Panasonic switchers still usually fried the primary circuit switching transistors in this scenario. Remove the shorted zener, replace the cap, observe the voltage at that point, replace with an appropriate zener. For example, Panasonic put an 18 volt zener across the 14 volt line. If your rated output is 5 volts, maybe a 6 volt zener. Mark Z. |
#6
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SMPS wall wart failure.
Arfa Daily wrote:
Also, please see my corrections to my original post. The zener diode is paralleled by the filter cap, not the power diode. Although strictly speaking it is correct no matter which way round you say it, that still comes across sort of wrong, as though the zener has some primary function, and that of the filter cap is secondary. It would be better to say that the filter cap is paralleled by the zener, implying that it's the zener that has the secondary function. Probably just me splitting hairs though ... :-) Arfa Thanks for your reply. -- David Farber Los Osos, CA Hi Arfa, I agree with you regarding the phrasing. Your way does sound better and it is splitting hairs. (-: I was wondering what would happen if the choke were placed before the capacitor and zener diode. Wouldn't the choke filter out the spikes and then make the zener diode unnecessary or at least less likely to short? Thanks for your reply. -- David Farber Los Osos, CA |
#7
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SMPS wall wart failure.
David Farber wrote:
I was wondering what would happen if the choke were placed before the capacitor and zener diode. ** A choke would present a high impedance in series with each current pulse from the switching tranny - causing a big drop in the voltage appearing on the electro cap. Wouldn't the choke filter out the spikes and then make the zener diode unnecessary or at least less likely to short? ** The zener (probably 6.2 V) conducted heavily and failed short when the electro went high ESR and caused the peak output voltage to go high. An electro that has developed high ESR cannot smooth the current pulses being delivered by the switching tranny and diode, so the output wave has continuous high peaks with a low *average* value. The control loop responds to the low average and tries to correct it by making each current pulse stronger, which only makes things worse. In short, the output electros in a SMPS are critical to it operation and in many cases there in nothing to prevent the output voltage going high when they wear out. I have seen serious damage done to 5V logic when this happens. .... Phil |
#8
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SMPS wall wart failure.
Phil Allison wrote:
David Farber wrote: I was wondering what would happen if the choke were placed before the capacitor and zener diode. ** A choke would present a high impedance in series with each current pulse from the switching tranny - causing a big drop in the voltage appearing on the electro cap. Wouldn't the choke filter out the spikes and then make the zener diode unnecessary or at least less likely to short? ** The zener (probably 6.2 V) conducted heavily and failed short when the electro went high ESR and caused the peak output voltage to go high. An electro that has developed high ESR cannot smooth the current pulses being delivered by the switching tranny and diode, so the output wave has continuous high peaks with a low *average* value. The control loop responds to the low average and tries to correct it by making each current pulse stronger, which only makes things worse. In short, the output electros in a SMPS are critical to it operation and in many cases there in nothing to prevent the output voltage going high when they wear out. I have seen serious damage done to 5V logic when this happens. ... Phil Hi Phil, I was forgetting about the pulsing action. I guess I was thinking about a standard AC transformer operating in the 60 Hz range. I imagine it would hold true that whatever the transformer pulse/sinusoidal frequency is, the choke belongs at the end of the line. Thanks for your reply. -- David Farber Los Osos, CA |
#9
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SMPS wall wart failure.
Jeff Liebermann wrote:
On Sun, 7 Jun 2015 12:04:01 -0700, "David Farber" wrote: The zener diode is paralleled by the filter cap, not the power diode. If the zener is there strictly for protection, the power supply should work normally without it. Have you tried using it without the zener? If it's in parallel with the output, it's likely some voltage slightly over 5V, such as 5.6v or 6.8v. If the zener is expected to short upon overvoltage, I would guess(tm) that a low power zener would be more appropriate than one that can handle the power. I've had my share of 5v 2A power supplies blow up. I don't recall if they were made by Jentec, but when I replaced the usual bulging capacitor(s), most would not recover. I never bothered to troubleshoot further and just recycled them. Thanks for the hint about the zener. Hi Jeff, As you suggested, I installed a 5.6v zener and it works fine now. The output voltage is a solid 5.17v. Thanks for your reply. -- David Farber Los Osos, CA |
#10
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SMPS wall wart failure.
Mark Zacharias wrote:
"Arfa Daily" wrote in message ... Also, please see my corrections to my original post. The zener diode is paralleled by the filter cap, not the power diode. Although strictly speaking it is correct no matter which way round you say it, that still comes across sort of wrong, as though the zener has some primary function, and that of the filter cap is secondary. It would be better to say that the filter cap is paralleled by the zener, implying that it's the zener that has the secondary function. Probably just me splitting hairs though ... :-) Arfa Thanks for your reply. -- David Farber Los Osos, CA 1. Cap develops ESR. 2. High frequency AC waveform appears on the desired DC output. 3. Circuit interprets as low DC - attempts to compensate. 4. Actual DC runs away. In the Panasonic example, the 14 volt line would go to 35 volts. 4. Zener shorts. 5. Ideally this throws off the "Q" of the circuit so much that it simply stops oscillating. Power supply stops but does not blow up. "Ideally". The old Panasonic switchers still usually fried the primary circuit switching transistors in this scenario. Remove the shorted zener, replace the cap, observe the voltage at that point, replace with an appropriate zener. For example, Panasonic put an 18 volt zener across the 14 volt line. If your rated output is 5 volts, maybe a 6 volt zener. Mark Z. Hi Mark, As soon as I found the short, those marvelous Panasonic VCR power supplies came to mind. I installed a 5.6 volt zener and it works fine now. Thanks for your reply. -- David Farber Los Osos, CA |
#11
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SMPS wall wart failure.
"Phil Allison" wrote in message ... David Farber wrote: I was wondering what would happen if the choke were placed before the capacitor and zener diode. ** A choke would present a high impedance in series with each current pulse from the switching tranny - causing a big drop in the voltage appearing on the electro cap. Wouldn't the choke filter out the spikes and then make the zener diode unnecessary or at least less likely to short? ** The zener (probably 6.2 V) conducted heavily and failed short when the electro went high ESR and caused the peak output voltage to go high. An electro that has developed high ESR cannot smooth the current pulses being delivered by the switching tranny and diode, so the output wave has continuous high peaks with a low *average* value. The control loop responds to the low average and tries to correct it by making each current pulse stronger, which only makes things worse. In short, the output electros in a SMPS are critical to it operation and in many cases there in nothing to prevent the output voltage going high when they wear out. I have seen serious damage done to 5V logic when this happens. ... Phil David - see Phil's answer regarding the choke being first. And +1 on his observations about serious damage occurring when a cap goes high ESR and the control loop 'lets go' in its efforts to correct for what it sees as the low output voltage. Notable among these cases are cheap Chinese DVD players and set-top boxes, but I have also seen it happen on equipment from what you would normally consider to be 'reputable' manufacturers ... Arfa |
#12
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SMPS wall wart failure.
Arfa Daily wrote:
In short, the output electros in a SMPS are critical to it's operation and in many cases there is nothing to prevent the output voltage going high when they wear out. I have seen serious damage done to 5V logic when this happens. David - see Phil's answer regarding the choke being first. And +1 on his observations about serious damage occurring when a cap goes high ESR and the control loop 'lets go' in its efforts to correct for what it sees as the low output voltage. Notable among these cases are cheap Chinese DVD players and set-top boxes, but I have also seen it happen on equipment from what you would normally consider to be 'reputable' manufacturers ... ** The examples I had in mind were Yamaha Multi FX processors like the SPX90 and a few relatives. The SMPS was on its own PCB and there was enough heat to dry out the electros on the DC rails - particularly the 5V one. That rail could rise to nearly double voltage and take out a whole bunch of 74LS ICs and others that were strictly 5V. The same PCB had the dreaded "yellow glue" dobbed all over the place eating the leads off small electros, zeners and resistors in the control loop. After a full clean up and fitting all new electros, I added a SCR crow bar to the 5V output to protect the main board in future. ..... Phil |
#13
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SMPS wall wart failure.
"Phil Allison" wrote in message ... Arfa Daily wrote: In short, the output electros in a SMPS are critical to it's operation and in many cases there is nothing to prevent the output voltage going high when they wear out. I have seen serious damage done to 5V logic when this happens. David - see Phil's answer regarding the choke being first. And +1 on his observations about serious damage occurring when a cap goes high ESR and the control loop 'lets go' in its efforts to correct for what it sees as the low output voltage. Notable among these cases are cheap Chinese DVD players and set-top boxes, but I have also seen it happen on equipment from what you would normally consider to be 'reputable' manufacturers ... ** The examples I had in mind were Yamaha Multi FX processors like the SPX90 and a few relatives. The SMPS was on its own PCB and there was enough heat to dry out the electros on the DC rails - particularly the 5V one. That rail could rise to nearly double voltage and take out a whole bunch of 74LS ICs and others that were strictly 5V. The same PCB had the dreaded "yellow glue" dobbed all over the place eating the leads off small electros, zeners and resistors in the control loop. After a full clean up and fitting all new electros, I added a SCR crow bar to the 5V output to protect the main board in future. .... Phil Seems like a good move ... Arfa |
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