Consistent Blown Fuses

the reason why the fuse blown is that there is a short on your load or from the regulated dc..so focus on secondary not in supply..means you have to check from rectifier to regulator which is your MJE13005..or you can isolate from load ckt backward to regulator..as per your description,you have an ac input which is around 22v ac then rectified by bridge diode.if you have electrolitic (polarize) capacitor in input means you have a dc input.means that is dc-dc converter.but as you mention no transformer means your ckt is an ordinary regulator.

This is almost a perfect example that a picture is worth a thousand words. Had you included some cursory schematic instead of attempting to describe the circuit, things would be much clearer.

But looking at your symptoms, I suspect that you have a capacitor that is breaking down under high voltage. Your short finding voltmeter is not capable of generating a high enough voltage to make the capacitor turn briefly into a short circuit path.

With years of experience with switching power supplies, the most common problem is capacitors which lose ESR, meaning they don't filter as well. Since the cost of the capacitors is minimal, replace them. Also check the circuit for any smaller diodes, especially zener, (voltage sensing), which may have shorted. The lack of filtering can cause spikes in the output voltage far above rating, causing over-voltage protection, blowing the fuse.

I once had similar problem and having done the required calculation,i found out that the fuse rating used by the personnel who has been using the equipment was underated,so i bought the appropriate fuse rating,and since then the problem seized till date(3 yrs back).

you may connect 1A single pole breaker and try if its tripping,use 2A breaker,u may get yr problem solved,then u can use the appropriately rated fuse.

Patrick Whowha

Patrick,

On general principles I do not agree with replacing any circuit protection with anything but what was originally designed. Certainly, electrical engineers do make mistakes and may in error select an undersized fuse. But without careful calculation and measurements that proves the incorrect fuse sizing, changing the fuse size is wrong. The fuse opening may easily be preventing further problems like starting a fire from happening. Just because you cavalierly or meticulously succeeded once in changing a fuse size, is no excuse for bad advice.

I agree with the guy that said it is a capacitor breaking down. I have seen it many times in the power supplies for computer monitors.

One WARNING, and a few notes. Better ten blown 1A fuses than one blown board, or worse, one blown machine, or worst, one blown job. If there is any downtime for the machine, can you pull the good working unit out to compare? Physically read the value of the fuse. DO NOT change this value. You could also measure multiple resistances on both boards, using diode scale on your multi-meter. Check each component both ways.

The fuse, inductor, and bridge rectifier, indicate the circuit could work on AC or DC. However, if you know the correct supply is 220V DC, stay with that. 220 AC, is usually 220VACrms, root mean square, or average AC voltage. The problem is that the peak voltages in the sine wave will reach about 311VACpeak. This means the rectified and filtered output of the bridge rectifier could reach 311VDC. Since you don't have a schematic, or specs, I wouldn't try 220VAC.

Can you contact the manufacturer, or are they out of business?

The circuit you're describing, must have a small switching transformer, the positive output from the bridge rectifier goes to the primary of this transformer. The other primary lead should connect to the collector of the switching transistor. The emitter is usually returned to the negative side of the bridge rectifier, most often via a low value, i.e. .1ohm, resistor for current sense feedback. The IC you mention, uses between +5V and +15V (Vdd) at pin 16. The negative power to the IC, is pin 8, (Vss). It doesn't make sense that the emitter of the switching transistor would be the same as the positive power of the IC. If you determined this with an ohmmeter, recheck the resistance from IC pin 16, (+), and IC pin8, (-). The IC may be shorted, or other components, rectifier,etc. may be shorted. Caution, some rectifiers may be Zener diodes.

Another caution is this power supply has a hot side, and a cold side. This refers to the grounds. Any part of the circuit connected to the negative dc pin of the bridge rectifier is referred to as a hot ground. Be careful.

As I mentioned in my last post, the capacitors are the most likely cause of problems like these, and may lead to shorted parts as well. Occasionally, you can visually observe a bulging at the top of bad caps, where they have vented. Another method is the use of an ESR Meter. An ESR meter can check caps in circuit, with no power applied, and you are looking for ones that have opened, or decreased in value, not shorted. A short across the circuit the cap is in will give a good reading, so follow with an ohmmeter. The easiest way, after changing any shorted parts, is to simply replace all Electrolytic Capacitors. Be sure to use an equal, or higher rated voltage, and observe polarity. The total cost for a power supply like this is only a few dollars, much better than a machine's downtime, or worse, damage.

Last thought, ask the boss if there is a company you can send the board to for rebuilding. If you don't have the correct power supplies, loads, and test set-up, it can be a trial and error situation, leading to damage of an expensive machine. This supply needs bench repair, and a company that rebuilds them will guarantee specs after the repair. The best tech does not always fix everything, but rather, makes the best decisions.

Find out if anybody replaced the fuse before you got the board to repair it.

They MAY have fitted the wrong type of fuse and you are unwittingly just replacing like-for-like. Is the fuse on the 'good card' the same type and rating?

If you are sure there are no faults further down the line and think it is safe to up-rate the fuse, then do so. Alternatively, you could power up the dc side of the card with a bench supply and see what level of current draw you get for starters. Adopt usual safe power-up procedures to avoid destroying your board !!

Good luck.

Currently I am tracing the complete schematic diagram of the card before I proceed with the troubleshooting. When I am done I will let the folks know about the result. Thanking you folks for the help and advices given to this matter. I am greatly touched by the earnest efforts put up our enthusiatic friends. Thank you again.

I have found the defective component that causes the 1A fuse to blow. It is a capacitor, labelled 680 H that was found short. The power after the full bridge rectifier, positive side goes into the winding of a transformer and the other end of the winding goes to this defective capacitor and via a diode returns to the negative side of the bridge rectifier. The trouble I am facing now is to decipher the type and value of the capacitor to buy. The only marking on the capacitor is this label: 680 H and it is quite vague. First I know the capacitor is a non polarized type and the working voltage should be at least 300 V. The capacitor looks like the film plastic type. Could the 680 H marking denote the value of 680 nF or 680 uF? Anyone can help? I do not know how to post the traced diagram on this forum. Sorry for my ignorance. I would appreciate it if someone can give me the guide and I post the diagrams here. Thank you folks.

Most times the third digit is a multiplier, if I understand your description of this capacitor. I would read the value as 68pF. Here's a handy help chart,

http://www.justradios.com/uFnFpF.html

I normally do the math in my head, a 684= .68uF, 683= .068uF, 682= .0068uF, 681= .00068uF, 680= .000068uF=68pF.

I selected a part I would use in this application, from Digikey. This link is just the capacitor section, since it is a large catalog, but you may want to download the entire catalog. The part I found is rated at 1KV. Part # P9616-ND (Qty,10, $3.16USD) They have a search feature, and showed an ESR meter for $75.00 It wouldn't have solved this problem, but may help in the future.

http://pdfcatalog.digikey.com/US2010/sectf.pdf

Your circuit description implies that this "capacitor" maybe acting as a surge snubber. A schematic would certainly clarify this. The H instead of a u, p or m bothers me though, for H is the abbreviation for Henry, the unit of inductance. Then again, 680 Henries would be huge in value and would not likely be confused for a capacitor. It might help if we knew the color of the part and if it's discolored. The shape and size of the part can help to narrow things down. Does it have axial or radial leads?

Do you have any component designation on the circuit board like C42 to imply that this is a capacitor? I ask because if it isn't a capacitor, replacing this with a capacitor might just compound your troubleshooting and not help you.

But if this is a snubber capacitor, a 68 or 680 pF capacitor would be a feasible value.

It is a capacitor and its marking on the PCB board is C7. Yes, the capacitor is part of the diode snubber. The capacitor is in parallel with a series of zener diodes. The zener diodes are also in parallel with the MOSFET transistor that switches the primary winding of the transformer. A resistor is also in parallel to all these component. It is a snubber circuit similar to other snubber circuit whose C7 value is 680 pF 600V.

Thanks to both Redfred and Techee1 for the reassuring comments. I can safely buy a non polarized 680 pF 600V polystyrene or plastic film capacitor as the replacement for the C7 capacitor. I guess the H could mean the high working voltage of the capacitor.