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Forum Index : Electronics : fuse not blowing
| Author | Message | ||||
| irishron40 Senior Member  Joined: 22/09/2014 Location: IrelandPosts: 251 |
I have this circuit build Its working fine but!!!! for the second time by accident I shortend the positive and negative , but the fuse didnt blow. Each time it blew a tip3055 and the replacement tip41c it also blew the 5 k potentiometer. the fuse is in the right place as shown in circuit any suggestions please ron |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Fuses are not fast enough to stop semiconductor devices from blowing up. What have you set the current limit to? The current limit part of the circuit should provide protection against accidentally shorting the output. Are you using a fast-blow fuse? |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
i have a 10amp red car fuse used, as that is the type of fuse holder i build in the casing. i havent used the current limiting as Its not connected yet . what size potentiometer should I use for the current limiting? I have a 10 k and a 50 k . but not sure if those vallues are ok. thanks dave ron |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
OK, car fuses are generally fast-blow. You should start with the recommended value. Try a fixed resistor if you can't get 1 Watt pots. I have built many power supplies in the last 40 years, but it is not readily apparent how the current is limited in this circuit. I suspect it is limited by the gain of the 2N3055 transistors and the allowable base current controlled by P2. Generally, one senses the current flowing to the output and use that as a feedback signal to control the series pass transistor. Google <images current limited power supply> and look at this current limit example |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
More complex, but a lot of good general power supply information. here |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
thats great thx dave ron |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
i didnt get that 47 ohm resistor yet but there must be a way of having a totaly protected system. this is what happened to my tip41 when i wrongly connected a 12 volt light bulb.it caused a big bang again fuse stayed intact.would there be other end transistors that would be imune to blowing up |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
A problem with incandescent bulbs, if that is what you are using, is that until they start glowing they look like a much lower resistance then you would calculate from the wattage. Also, using a different transistor (TIP41 versus 2N3055) could result in different limit currents ... in that original circuit. Yes, you can have a totally protected system with the correct implementation. Again, fuses are NOT going to stop this from happening. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
i wronly connected positive and neg to load. but what is the point having this fuse in this circuit if it doesnt protect it. I had a maplin power supply before (5a) and also NO limiter, and it never blew a transistor even when i had connected wire wrongly. could you explane how? thanks dave ron |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
To an incandescent light? I don't understand how it would make any difference how you connected that to your power supply. I cautiously suggest that maybe the circuit design is not "bulletproof". Also, as I suggested earlier you need a circuit that incorporates feedback. IE a properly designed current limiting circuit. One option is to buy a proper linear regulator IC hopefully that could drive external transistors for higher current capability. Try looking around the ti.com website and find their design tool (workbench?) There are other forums where you are more likely to get help for this topic, ie here Or, try to get recommendations for discrete circuit designs that are known to work. |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I had suggested you build a different circuit earlier in this thread simply because of this problem. you are better off building a circuit with current limiting, that way you will not blow up the circuit every time you make a mistake. It would seem you are hell bent on making a poor grade power supply that self destructs on the slightest human error. The design i suggested prior allows for the output to be shorted together and not blow the circuit due to current limiting within the circuit design. I think we all have done the simple design in the past, and like yourself killed the circuit many times before we realize it's better to do a circuit that is idiot proof to simple mistakes. You can keep flogging a problematic horse if you like, but what you are seeing will always be a problem. Pete. Sometimes it just works |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
ok Pete. i had a look back to the one you suggested . it seems simple enough to build and far less components that the one I build, love to see photo of your finnished product. regards. ron thx ron |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Hi. My power supply stopped working And I am lost. I have power going in untill R9 resistor.and maybe only a volt orso going to lm317. Resistor is changed and I checked all caps for a short.any help appreciated. |
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| Pete Locke Senior Member Joined: 26/06/2013 Location: New ZealandPosts: 181 |
Simple test. Lift one side of R9. If you have built the circuit per the original post, the LED should light up. Measure the resistance of R9. Better yet, with the regulator side of the resistor lifted (de soldered) from the circuit, what voltage do you get. Given the symptoms you describe, the regulator is dead. Also, just looking at the circuit, with a short circuit presented to the output, the LM317 is going to try straight away to raise the base/emitter voltage (current actually, but that's getting above what needs looking at here) to bring the output voltage back up. This will exceed the 2N3055 (TIP42) b/e limits VERY quickly and out comes the smoke. Also if you are using an automotive fuse, they are actually a VERY slow blow fuse. That fuse should be placed at the output of the main smoothing capacitor. A fast blow fuse in the output side of things should be used. Might help. Boot-strapping a regulator chip, which is what has in essence been done with the circuit, is a perfectly good thing. Very good concept used in all sorts of applications. But the regulator is 'blind' to the output of the power supply, being shielded from view by the output transistor. It can only see the base of the output transistor. No problem, as long as there is fast output protection provided. An automotive fuse will not provide that. The circuit as shown is an adaptation of a well proven design when used with proper output protection, and within its protection limitations. If you are using it as a bench supply to power experiments, I would suggest that using something as simple as a correct wattage light in series with the output will save you a lot of component replacements. Good luck. Cheers Pete'. |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
A very common question. 
Fuses protect wiring, more then anything else. They are there to blow and prevent a short-circuit from cooking the wiring into a lovely smoking, melted mess. Also good for electrical-fire prevention! 
In cars, this is especially true, as you can get several hundred amps out of a car battery, so the fuses are there mainly to prevent a fault from melting the wiring loom, which is car-strip-down time. It was the same in houses of old, when you had fuse carriers before circuit-breakers came into the mix. A fuse - even a fast-blo one - will happily carry two to three times it's rated current for a short time. While this may only be a few seconds, in that time, a 10A fuse could pass 20 or 30 amps for a short period before it blows. In that time, much damage can be done a a semiconductor device, as you have found out. 
Lets say you have a 10A fuse, but a semi which, due to a fault, is sucking 50A. Within a fraction of a second, the semi will blow itself to bits, but the fuse will still be intact, as it has not had the large load on it long enough to melt the wire. As other members have been saying, you need a current-limited PSU. They are not that hard to come by depending on your load requirements. Pretty much a "Must-have" thing for any PSU capable of more then a couple of amps. As my brother told me when I was younger and wanted him to bring home an old 10A tranny from work(he's a sparkie): "10-amps is a lot of current. Make big explosion." 
You can get some quite affordable PSU modules etc from eBay - how much current do you need to pass? EDIT: These PSU modules are quite affordable, good for 7-10A, are buck/boost(so can step up a 12v input to 32v max), and fully overload protected. They are a nice little module, and work very well. A video of a guy using one of these to build a current-limited PSU. Smoke makes things work. When the smoke gets out, it stops! |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks for that info Grogster, I ordered one. Perhaps it might get me some output from that too slow turning VAWT that's ornamenting my place. I did enjoy that 'jinglish' of the sellers, quite entertaining but not much good for better business on e-bay. Klaus |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
The 'Great Scott' videos are great. He builds lots of electronic stuff, and you can follow along and build one yourself. It was that video that got me interested in those PSU modules, and they seem extremely good, being buck/boost(so a 12v input voltage can be adjusted between 0.8v and 30v or so), and full current limiting which the bit you really need the most.
Please let us know how you get on. Smoke makes things work. When the smoke gets out, it stops! |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
before the R9 resistor i havefull input voltage, after the r9 only 0,8 volt. i removed the r9 resistor and it tested ok, but replaced it anyway with a new one with same vallue. I also eliminated the the 2N3055 transistor from the board , but just dont get it why I suddenly only get 0.8volt going to lm317. I also removed this transistor and tested perfect .again to be 100% sure I replaced this lm317 also with a new one. it was working before with this setup. cheers ron |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
problem solved/ the r9 had blown and i replaced it, but for some silly reason I made the mistake to replace the 47 ohm one with a 47 k. the reason it blown is that i was drawing to much current and its only a 1.4 watt one insted of a 0.5 watt one i have stuff ordered to make the one power supply pete had suggested a while back. thx all. ron |
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