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 Post subject: Variable current limiting fuse replacement device
PostPosted: Aug Fri 06, 2010 4:11 am 
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This idea is probably out in left field, but several times lately I've been working on newer stuff that may or may not be worth bothering to fix, and found blown power supply fuses, usually smaller size ones that are a pain to get.

I'd like to be able to test without going through too much in-depth troubleshooting, or too many $ worth of fuses, to determine if it's worth my time to bother fixing the items. Something I could use in place of the blown fuses that could be reset, and provide a variable level of overcurrent protection, to use in many situations, would be ideal, something that could be clipped to the fuse leads in temporary lieu of a real fuse, and set to a current value appropriate for the device.

I've never seen or heard of any such thing, but it seems like it should be possible. Any thoughts on this?


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PostPosted: Aug Fri 06, 2010 4:18 am 
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They do make resettable fuses that might be useable for what you are proposing, but you would need to keep several values on hand for the different current ranges. I don't think they make an adjustable version as of yet..

http://en.wikipedia.org/wiki/Resettable_fuse

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PostPosted: Aug Fri 06, 2010 6:10 am 
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Are you referring to primary line fuses? If so, I am currently using a Simpson "equipment controller" with my bench variac, that can be adjusted to trip at any value (with the right current transformer) via a front panel adjuster. It has a pot to adjust for time delay (like in slo-blo).

Charlie


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PostPosted: Aug Fri 06, 2010 1:45 pm 
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Some technicians I've seen over the years will actually jump a fuse out, or put a much bigger one in to see what happens when the equipment is brought up on a variac. This is really giving it the "smoke test," and is bad practice due to the chances of causing further damage to the equipment. There are a couple of simple things you can do, however, to get out of having to buy lots of odd-size and specialized fuses to test equipment with.

1. Take an old AC amperes panel meter and connect a couple of clip leads to it. Connect it across the blown fuse, then bring the equipment up slowly on a variac. You can see right away if the current is getting too high, and back off. Note that nearly all AC ammeters will work on DC, though perhaps with somewhat less accuracy.

2. Take a standard 3AG open fuse holder and connect a couple of clip leads to it, load it with a suitable fuse, then clip it across the blown one. If you use a variac to bring the equipment up, you can often see the filament in the fuse begin to sag and maybe glow before it pops. You may still lose a fuse or two this way, but you can often find Chinese 3AG fuses for a lot less than the other types, and you don't have to dismount and unsolder PC-board type fuses.

Note that these techniques will not always work out for equipment with switching power supplies, since the power supply might not "kick in" until a certain voltage is reached. They also might not catch certain problems like parts that do not break down until full voltage is reached, or power tubes with loose elements that work until they get hot, then flash over.

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 Post subject:
PostPosted: Aug Fri 06, 2010 1:59 pm 
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Since most anything with a power supply uses DC, why not buy a bench DC supply with current limiting? That way, you can trouble shoot and fix the unit, assuming there are no power supply problems, then replace the fuse when you're done.

Another approach, assuming you don't care about possibly frying the power supply, is to get some cheap Bi-metal circuit breakers. The auto reset type will continue to cycle the power, or you can get them with a manual push button reset.

http://www.e-t-a.com/uploads/prodb/D_104_105_106_e_240909.pdf

http://www.e-t-a.com/us_circuit_breakers.html

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Last edited by Mikeinkcmo on Aug Fri 06, 2010 2:54 pm, edited 2 times in total.

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 Post subject:
PostPosted: Aug Fri 06, 2010 2:02 pm 
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If someone wanted to experiment, I'd bet a GFI could be converted into a variable AC current trip switch. The bifilar sense winding would need to be changed to a single current sense for either the hot or neutral, and a variable resistor could be used to set the current trip point. The primary winding on the current transformer might need more or less turns--one would need to do a bit of empirical engineering.

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 Post subject:
PostPosted: Aug Fri 06, 2010 3:00 pm 
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A number of years (in the early 1990's) I recall seeing a project for a resettable electronic fuse in Radio Electronics (a magazine that I used to get. Perhaps your local library could get you a copy of the article.


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 Post subject:
PostPosted: Aug Fri 06, 2010 3:16 pm 
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Peter Bertini wrote:
If someone wanted to experiment, I'd bet a GFI could be converted into a variable AC current trip switch. The bifilar sense winding would need to be changed to a single current sense for either the hot or neutral, and a variable resistor could be used to set the current trip point. The primary winding on the current transformer might need more or less turns--one would need to do a bit of empirical engineering.


I like this idea. I saw a schematic for a GFI once with full details on it, which I'll have to dig up.

Most of what I fix where I have this problem involves SMPS's, and that's why I don't use the larger fuse/variac approach. Ideally, i'd like something metered where I could control and measure the amount of current through my variable fuse arrangement, so avoid "trial by smoke." An engineering degree would come in handy about now.


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 Post subject:
PostPosted: Aug Fri 06, 2010 3:18 pm 
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Alan S wrote:
A number of years (in the early 1990's) I recall seeing a project for a resettable electronic fuse in Radio Electronics (a magazine that I used to get. Perhaps your local library could get you a copy of the article.


Cool, I'll look into that.


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 Post subject:
PostPosted: Aug Fri 06, 2010 3:19 pm 
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tinwhisker wrote:
Are you referring to primary line fuses? If so, I am currently using a Simpson "equipment controller" with my bench variac, that can be adjusted to trip at any value (with the right current transformer) via a front panel adjuster. It has a pot to adjust for time delay (like in slo-blo).

Charlie


Mostly fuses internal to power supplies, but not line fuses. Do you have a picture of the Simpson creature? I've never heard of that.


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 Post subject:
PostPosted: Aug Sat 07, 2010 12:13 am 
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Location: Rochester NY USA
There are thermal fuses which look like disc capacitors which will go to a high-resistance state with overcurrent. 1A is available at 120V - larger values for lower voltage. Here's one: http://www.newark.com/tyco-electronics- ... dp/02M9705


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 Post subject:
PostPosted: Aug Sat 07, 2010 12:50 am 
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"Mostly fuses internal to power supplies, but not line fuses. Do you have a picture of the Simpson creature? I've never heard of that." [/quote]

I am "babysitting" my mother at the moment, so I am not at home to look up the model number.

I suspect that these units are obsolete, replaced by digital units. I found mine "new" at the local surplus store, at a fraction of the original cost. These analog units look like conventional ammeters/voltmeters, have one (or two) knob(s) to set upper (and/or lower limits). When the meter hand coincides with the adjustable, (usually red), pointer, internal electronics trips an internal relay, that changes state n.o and n.c. contacts. These can be used to release a relay with high current capable contacts in series with the "device under test".

Naturally a digital controller could do the same thing, at great cost. This may sound complicated but is simple to implement.

Charlie


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 Post subject:
PostPosted: Aug Sat 07, 2010 4:13 am 
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This idea is probably out in left field, but several times lately I've been working on newer stuff that may or may not be worth bothering to fix, and found blown power supply fuses, usually smaller size ones that are a pain to get.
Neodymium Magnets


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 Post subject:
PostPosted: Aug Sat 07, 2010 4:57 am 
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Location: Dayton, Ohio
F.W.I.W. my variac ammeter is a Simpson set point controller model 3344 A. The way I have it set up is to act as a "variable fuse" with adjustable time delay, if desired.

Charlie


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 Post subject:
PostPosted: Aug Sat 07, 2010 2:45 pm 
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muchuanlin wrote:
This idea is probably out in left field, but several times lately I've been working on newer stuff that may or may not be worth bothering to fix, and found blown power supply fuses, usually smaller size ones that are a pain to get.
Neodymium Magnets


Ummm, Mr. Moderator, please?


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 Post subject:
PostPosted: Aug Sat 07, 2010 2:49 pm 
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Location: St. Paul, Minnesota
Thanks Charlie, that sounds like it might work inside a device under test too, I'm going to look into it, and see if I can get my hands on a manual or schematic.

I was unable to get my hands on that Radio Electronics article, but I like the devices Tom posted. That would get me started in the right direction. I need to look into those more, and get some spec sheets.


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 Post subject:
PostPosted: Aug Mon 09, 2010 1:07 am 
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Here's a URL to a digital AC/DC ammeter with programmable alarm relays that might do the job for you.

http://cgi.ebay.com/AC-Programmable-Ammeter-Current-Meter-Relays-150A-Coil-/350383686289?pt=LH_DefaultDomain_0

It also might be possible to construct your own device using a 1.5 amp 6.3 vct transformer with 1/2 of the primary connected in series with the AC load, the primary output rectified/filtered and connected to a pot that is adjusted so that 1 amp of load produces 1 volt at output of the pot. This could drive a LM3914 dot/bar led driver which would light leds in .125 amp steps from .125 amps to 1.250 amps. The led output corresponding to the trip current desidred would reset a latching relay. The latching relay would be set with a pushbutton switch.


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 Post subject:
PostPosted: Aug Mon 09, 2010 3:19 pm 
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This is the perfect time to use a "Dim-bulb tester", use a large light bulb in series with your item. In case of a short, the bulb will light normally, current is limited to the bulb's rating, and no smoke, as almost no power will be dissipated in the shorted item. Bulbs have a very low resistance when not lit, so they won't drop the voltage much with a good item being tested.
Don


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 Post subject:
PostPosted: Aug Tue 10, 2010 2:19 am 
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Take some stranded hookup wire and remove a few inches of insulation. Remove a single strand and wrap that onto the old fuse, using that to hold the strand in place in the fuse holder. Smaller gauge wire uses tinier strands, so by that method it's pretty easy to find something small enough to substitute. Now, you have an endless supply!

-Ed


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 Post subject:
PostPosted: Aug Mon 16, 2010 11:21 am 
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Peter Bertini wrote:
They do make resettable fuses that might be useable for what you are proposing, but you would need to keep several values on hand for the different current ranges. I don't think they make an adjustable version as of yet..

http://en.wikipedia.org/wiki/Resettable_fuse

Such a very amazing link!
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