Wikipedia:Reference desk/Archives/Science/2014 September 11

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September 11

Fuse voltage rating.

I have a power supply with a 5A/240v fuse that recently blew.

The power supply is available in 110v and 240v versions - mine is the 110v version, but I know that the only difference between the two versions is a jumper position, so I strongly suspect (without proof) that both versions have the same 5A/240v fuse.

The only fuses I have to hand are 125v rated - but at a bunch of different amperage ratings.

I think I can replace the 5A/240v fuse with a 5A/125v fuse - but some people are telling me that I need a 10A/125v (or so) replacement. My gut feel is that they are incorrect - but I'm at a loss to explain why.

My reading of Fuse_(electrical) is that the voltage number is the "rated voltage" which is the maximum voltage at which the fuse will reliably blow without arcing and conductive plasma keeping the circuit open. Since I'm running the unit at 110v, the 125v rated fuse should be OK.

So who is right...and (most importantly) why? I need the answer in simple terms (eh...simpler than that) so that the people involved in the original debate can be convinced one way or the other.

SteveBaker (talk) 01:07, 11 September 2014 (UTC)

""I need a 10A/125v" Utterly completely and dangerously wrong, you are trying to limit the current in the system, not the power. I'd use a 5A 125 V fuse of the same type - fuses come in different types and some will blow more quickly than others. Greglocock ([[User talk:Greglocock|talk]]) 01:28, 11 September 2014 (UTC)

That's what I think too...but a solid explanation is needed here. The Devil's advocate argument is that the circuit you're protecting has constant resistance - so it'll be handling different amounts of current with the two input voltages - so it'll need a different fuse in those two circumstances. I don't know how to counter that argument. SteveBaker (talk) 02:01, 11 September 2014 (UTC)

The fuse depends on the thickness of the wires inside the power supply, not the amount of power the supply is generating right now. Changing the voltage to the power supply does not change the thickness of the wires, so the Amp rating of the fuse does not change, it doesn't matter what you do to the voltage.

For voltage rating on the fuse on the other hand you just need a fuse rated for what you are using, not what the power supply is capable of.

I hope that's enough to convince them. Remind them the fuse is based on the physical size of the wires in the supply, not how the supply is being used. Ariel. (talk) 03:50, 11 September 2014 (UTC)

We need to know the specifications of the power supply to be sure that we are giving the correct answer. It is possible (but unlikely?) that your power supply actually draws a maximum of 1.2 kW regardless of the voltage of the source (within a stated range). In these circumstances, the required fuse would be 5A/240v when connected to 240v but 10A/120v when connected to 120v and this would explain why your fuse blew. I agree with the analysis given by Greglocock and Ariel for all normal power supplies, and you should not use a ten amp fuse unless the manufacturer recommends it. Dbfirs 06:18, 11 September 2014 (UTC)

Sadly, this is a made-in-China power supply - we have no schematics, no specification and no user-manual of any kind. The *only* guide as to what fuse it needs is the old. blown fuse I removed from it...which has some chinese squiggles, then a 5 and a 240...which we're merely assuming is the amperage and voltage rating. The fuse is the same size and shape as every other 5A/240v fuse you've ever seen - so we're presuming it's nothing especially exotic.

If it helps to know, this beast is generating just 25mA at around 40,000 20,000 volts(!) for our laser cutter. It's scary as all hell because it's allegedly capable of generating two inch long arcs that are more than able to kill you...and it has big capacitors inside so it can still zap you hours after you've unplugged it from the wall. This makes the simple act of changing the fuse way more exciting than you'd ordinarily expect! We don't know why the fuse burned out in the first place...but with the 5A/125v slow-blow that's in there now, there have been no further incidents and everything seems to be working OK. The ordinary shock risks or fire concerns that you might have from an overly-large fuse are really nothing compared to the potential lethality of the device when it's working just fine and the fire risk from the CO2 laser it's powering! SteveBaker (talk) 20:58, 11 September 2014 (UTC)

Even in that case the fuse would be 10A in both situations. A fuse is designed to protect against excessive current, not excessive power. (SteveBaker: Don't assume from this that we are recommending a 10A fuse.) Ariel. (talk) 20:41, 11 September 2014 (UTC)

Both the fuse and the wires behave as simple, fixed resistors (until the fuse blows). The heat load on a resistor is equal to the square of the current divided by the resistance, independent of voltage. The wires in the device will melt at some current greater than 5 A. You need a fuse that blows at 5 A to protect those wires. Voltage is irrelevant here.--Srleffler (talk) 16:36, 11 September 2014 (UTC)

Many thanks everyone! That clarifies things beautifully. I will now go and do battle with those who oppose me and crush them without mercy using the weapons you have provided!  :-)

SteveBaker (talk) 20:44, 11 September 2014 (UTC)

Err - 25mA at 40000 V is 1kW, so you _do_ need the 10A fuse - it's over 9A at 110V. The 5A fuse will probably blow again before long, but it doesn't indicate a fault in the device. Tevildo (talk) 22:45, 11 September 2014 (UTC)

You're sure it's not 25uA and/or 4000V? That's some serious EHT if your figures are right. Does it produce X-rays (which I hope are shielded) as well as the laser output? Tevildo (talk) 22:51, 11 September 2014 (UTC)

Oops! My bad...20,000 volts, not 40,000...which means that the 5A fuse is *really* marginal. No wonder it's blowing whenever there is a voltage spike. I'm starting to think that I should be going with at least a 6A fuse. SteveBaker (talk) 00:35, 12 September 2014 (UTC)

The slow-blow 5-amp fuse should cope with any normal voltage spikes, and at the same time protect against faults. Dbfirs 08:31, 13 September 2014 (UTC)

Suspension cells in tissue culture: congregation in middle of well

Is it usual for suspension cells to congregate in the middle of a well? Does it have something to do with water-tension and the meniscus? Figure of eight swirling works well enough for adherent cells but not my PGCs. I don't seem to have the problem when culturing in a flask. --129.215.47.59 (talk) 11:22, 11 September 2014 (UTC)

Yes, I see that all the time with my NSCs, only in small wells though (24/96 well or chamber slides). More medium may help. Fgf10 (talk) 11:58, 11 September 2014 (UTC)

If you stirred a liquid culture, and left it rotating, that would force particles either to the edges or center, depending on if they are lighter or heavier than the culture. StuRat (talk) 02:01, 12 September 2014 (UTC)