Two parallel fuses or not to parallel fuses?

[geojoe1]

That is my question.
I have a DC power supply in my 'lab' that has a 70 amp automotive fuse in it. Well I keep blowing it. It's a total pain to change. I have two 1 and 1/4 by 1/4 inch panel mount fuse holders and some 35 amp fuses. Should I panel mount these to the outside of the case and put the fuses in parallel?

[jollyrgr]

I would HIGHLY say DO NOT DO THIS! Here is the situation. You have at least a 70 Amp draw if you keep blowing a 70 Amp fuse. Now there is little chance you will have both 35 Amp fuses blow at the same time if you parallel them. One might blow at 34.6 Amps, the other at 36.2. Once the one that slowly melts at 34.6 Amps fails the full 71 Amps or more is now seen across the remaining fuse. Violent blowing will take place.

This is just my logic on the expected results based on what happens to a fuse that gets overloaded in regular use. Hitting a 35 Amp fuse with 70 Amps just seems like asking for trouble.

Why not use 70 Amp AC cartridge fuses or better yet a circuit breaker? Here is one example:
http://www.mobiletraxx.com/catalog/i1563.html
(I know nothing about this vendor. The link is just an example of an available product. There are numerous brands and models of this device.)

They make resettable breakers like this and my assumption is you are using these for something electronic. What voltage, you did not state. But marine and truck parts stores should be able to assist you as a source.

You could also scavenge one out of a resettable service disconnect. These are used for heat pumps, central air conditioners, etc. The raw breaker should be available from your local home improvement center.

The curiosity is going to do to me what it did to the cat, but I must ask; what are you doing that you are tripping a 70 Amp breaker in the lab?

[Bob Scott]

That is my question.
I have a DC power supply in my 'lab' that has a 70 amp automotive fuse in it. Well I keep blowing it.

Why do you keep blowing the fuse? Is the supply regulated? Repeatedly overloading the supply may damage its innards.

It's a total pain to change. I have two 1 and 1/4 by 1/4 inch panel mount fuse holders and some 35 amp fuses. Should I panel mount these to the outside of the case and put the fuses in parallel?

That should work, but I'd tend to use a pair of faster blowing 30 Amp fuses. I disagree with jollyrgr on a few items. Since the fuses get hot before they blow, their resistances rise and the currents through identical fuses will tend to equalize. ie: If one fuse conducts more current, its resistance rises higher and will conduct less than the cooler fuse.

Check out Littelfuse.com for fuse specs. You'll see that an amount of current double the fuse rating will tend to blow the fuse in about 1 second, and faster at 10X rated current. Nothing "violent" happens when they blow.

Whatever you do, don't replace the internal fuse with a slow blow or a circuit breaker. Leave the internal fuse installed as rated by the PS manufacturer. The power supply probably contains semiconductors. Any substitute fuses may blow slower than the electronics in the supply is designed for.

Regards,
Bob

[Chris Smith]

IF you weren’t blowing fuses, I would say go for it.

IT does provide security, but why the hell is the fuse blowing if its ok?

Never try to cheat a known or unknown fault.

So no, not if you already have a problem.

Fix it and do the right thing, and only in a GOOD pinch, cheat with two alternatives.

[Robert Reed]

Geo
Two important questions-(1) What is the current rating of the supply at full load ?
(2) Does the fuse blow at startup, and if not , is there a pattern in when it blows ? (i.e.-Heavy load, no load or what)

[geojoe1]

I use this 12V supply to develope DC to DC converters. I am making a high power one now. I made this supply myself for use in my lab. It's just a transformer diodes and some caps. I scale the fuse to protect my prototype boards when I am testing for control loop stability. This time a 70 amp fuse is sufficient. I am having an unusually hard time dialing this control loop in and keep blowing the fuse on the supply when I remove the load from the converter. I actually wish I had used a digital control scheme like the one in the last issue of NV with the high voltage pic. Anyway I'm sure I'll get this loop damped. I just wish I didn't have to get into my supply and replace this huge 70 amp fuse everytime I test for stability.
I think this is what I'll do. I'll get some current shunts and measure the varying current in each fuse as I ramp up current. Then I'll see just how violently the second fuse blows with 70 + amps through it.

[haklesup]

Since your supply is very simple, you don't need a fast blow but you might want that to protect the load you are testing. Two 35A fuses will do in a pinch but you may find the trip point to be somewhat unpredictable as you change out blown fuses (and they will blow in pairs). You'll need heavy solder joints, big wires and good fuse contacts to prevent current hogging due to a resistive connection (which would tend to reduce the trip point) I don't see any scenario where the trip point would be higher than 70A though (time delay notwithstanding).

If you are blowing a lot of fuses and do this kind of testing frequently, you should get a DC circuit breaker. The cost of such is similar to a hand full of blown fuses. Fuses are cheap only if you never blow them.

One example of many:
http://www.bluesea.com/product.asp?Prod ... 58&l2=6607

IMO, measuring the input impedance of the load with an ohmmeter before powering it up might save you a few fuses. If it's less than 170mOhms, you are begging for a trip to the fuse store. It's not foolproof but it can be a useful test (unpowered impedance can be different than when powered for example).

So long as you are not ignoring the voltage rating, the most violence you will get is a flash of light inside the fuse. By paralleling two sacrificial 35A fuses and testing the trip point with a known load and ammeter, you can verify the assumption that it will trip near or below 70A. Keep in mind the delay curve. I would expect the second fuse to burn very quickly after the first fuse blows, might even find a combination of fuses that blow almost together if you are lucky.

[Robert Reed]

Geo
Why are you concerned about the situation in where the second fuse blows? Its no diffrerent than a 35 amp fuse in any other circuit when a dead short is impressed upon it. It just blows the fuse like its supposed to do. The only 'violence 'associated with fuse blowing is the voltage across it at the instant it ruptures and this is related to spacing between the end terminals. Most low voltage fuses are rated at 32 volts or more, shouldn't be any problem in your case. Have you considered fusing the power supplys primary (120VAC). This would allow for a smaller more conveniant fuse/circuit breaker. This device would have to have a small amount of time delay to allow charging what are probably huge caps, upon startup. Also if you go with a circuit breaker, be sure to check the specs carefully. They usually have a variable trip point vs. current. For example- a 20 amp breaker will trip instantly under dead short conditions. But at 21 amps, it may take several minutes, and at 30 amps several seconds.

[Chris Smith]

The best you can expect is a slo blo fuse, and after that you are going above the stats and peak causing trauma, and a dead fuse.

IF this is the case, deal with it, not the fuse.

[geojoe1]

You know on Myth Busters when the can't get something to fail catastrophicallly they go over board and for an explosion or violent fialure of some type. Well no matter how hard I tried I could not get the fuses to blow violently. I even tried to blow them with higher voltages. Just a flash.

So anyway. When I carefully ramped up the current in the two fuses in parralel I couldn't detect a difference in current in either one. My meter resolution was about 50mA when measuring > 20 amps. (It's actually a very nice current meter) Anyway, there was, as expected, a time delay between the two fuses blowing. But apperantly both fuses were close to blowing already and so when the first fuse blew the second blew imperceptably fast. I only caught one good instance on the scope, the second fuse blew 80 m S after the first. So since I still have about 150 35 A fuses left I think this will be a good solution for now. Now I have a new question about automotive alternators. Check my new post.
Thanks for all of the help. If I end up burning all of my fuses I will certainly go and get on of these circuit breakers.

[haklesup]

When the first fuse blows, the current in the second instantly doubles pushing it from the 110% zone to the 220% zone and it blows very fast. It dosen't have a chance at that point. (110% since you're slowly ramping the current). I'd be willing to guess less of the fuse (ribbon inside) is left on the second as compared to the first.

You have to go far enough over the voltage rating to get arcing across the fused gap before you would see anything exciting. A little over and maybe the rating is off by some amount. I can see this being a problem in the wedge style car fuses as the remaining gap is small but I suspect you are using the large cylindrical kind (sorry don't know the code right now)