Couldn't blow a fuse

[jimjones3630]

Hi all,

Some have placed fuses inline saving their mod when bad things happen.
http://www.candlepowerforums.com/vb/showthread.php?t=151747

Wanting to play with doing the same picked up a used Motorola DC power supply, R 1011B. 0-40v 0-40A.

What I like is not only the ability to blow every bulb I have with 40A including the 250w 24w Osram 64655 but it is switchable.

Selectable 0-5A, 0-20A, 0-40A ranges with limit adjusting control for each range plus 0-20V or 040V selectable range.

Hooked up Schultz 6 330d charger and verified on amp meter when it read 1A charge to the battery and so it was. I'm easily impressed.

Measured the startup in rush of amp to 64655 with a Fluke 87 meter, it was 42ish blowing the internal Buss fuse DMM 11A.

Plus it uses generic parts so should be easy to repair if and when.

Hooked up an 30A auto buss fuse connected to the high current output cranking up the supply to 40A and wouldn't blow. The 13g leads got hot and after a min. shut it down, easily able to repeat test with 5A and 10A fuses blowing.

Anyone have experience with a fuse not blowing? Faulty fuse?

I know would use a fuse size just slightly more the the bulbs rated amp, for the 100W probable a 15A would do never needing a 30A fuse.

Jim

 

[macforsale]

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[jimjones3630]

That is what I thought a defective fuse given I ran it for a min. or more at 40A. Think the reading on the meter is true given I blew Fluke 11A buss fuse measuring start up inrush amps (42ish) on a lamp.

Jim

 

[Nubo]

What voltage were you putting out? 30A auto fuse is rated at 12 volts (or maybe 14?). If you put it inline on a 6 volt circuit, it would take at least 60 amps to blow the fuse. I had the opposite situation recently with a 24V application -- I used auto fuses and had to divide the amp rating by 2 to get the value for 24 volt application. It's not amps that blow a fuse, it's *watts*. It's just that normally the voltage is fixed usually you need only think in terms of amps.

 

[RustyKnee]

What voltage were you putting out? 30A auto fuse is rated at 12 volts (or maybe 14?). If you put it inline on a 6 volt circuit, it would take at least 60 amps to blow the fuse. I had the opposite situation recently with a 24V application -- I used auto fuses and had to divide the amp rating by 2 to get the value for 24 volt application. It's not amps that blow a fuse, it's *watts*. It's just that normally the voltage is fixed usually you need only think in terms of amps.

I am not convinced voltage makes any odds. The fuse has teh same resiatnce no matter what ever the supply voltage is. the fuse gets hot due to the power disipated in it. assuming a fixed current for a number of supply voltages....the voltage drop across the fuse will be the same. V x I = Power will be the same, the fuse will get as hot for each supply V.

edit...this assumes somthing else is in the circuit atht drops the remaining voltage.....fust v drop + load v drop = supply v drop.

I think the Voltage they a rated for is more to do with dielectric strengths and safety.

Stu

 

[jimjones3630]

Fuse was inline to bench supply I turned up the V but no V went through because the fuse was shorted the circuit. Max resistance.

More Volts I turned up the more amps went up Volts remained 0 according to the meter. Since AXV=W there was no watts.

But the leads got so hot I turned it off after a min. or so. Watts is the power so why it get so hot?

Jim

 

[RustyKnee]

Fuse was inline to bench supply I turned up the V but no V went through because the fuse was shorted the circuit. Max resistance.

More Volts I turned up the more amps went up Volts remained 0 according to the meter. Since AXV=W there was no watts.

But the leads got so hot I turned it off after a min. or so. Watts is the power so why it get so hot?

Jim

unless you use super conducting leads there was a volt drop. it was just too small to register on the psu meter. cable res is measured in ohms per km. over short lenghs a multimeter won't see it...just looks like a short.

Stu

 

[jimjones3630]

Don't know that I follow since I wasn't measuring with a DMM. The meters on the bench supply which I had hooked up the fuse is what I looking at.

At the scale setting of 0-40A, 0-20V it is fairly sensitive on other measurements. Maybe there was some volts going thu but I don't think so.

With Pos. lead to one side of the fuse and Neg. lead to the other side of fuse.

Also tried wrapping a 20g wire around a 12" bar of iron connecting one lead from the bench supply to each end of the wire in an attempt to create a magnetic field. Couldn't get any volts to run thru but the amps increased heating up the leads.

Jim

 

[macforsale]

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[RustyKnee]

there volts being applied. its just that they aren't registere by the psu as the voltage is too small. try it with a dvm on the lowest voltage scale. in the real world...if there is current...there are volts Othey might be small though). unless the cables are super conductors with zero resistance.....which they aren't.
at 40A you only need 0.1v to get 4watts

 

[RustyKnee]

http://www.interfacebus.com/Reference_Cable_AWG_Sizes.html

you said 20g....that has about 10 ohms per 1000ft. so thats 0.01 ohms peer foot. so at 40 amps thats 0.4v droped over each foot of cable.

or 0.4v x 40a = 16watts per foot...that why the cable gets hot.

using the that guide link 20g is only rated for 1.46 amps...rated means continious. it also sayss it fuses at 58 amps.

Stu

 

[jimjones3630]

Thanks, what is so helpful at CPF is someone knows how it works. Will have to ready up.

That it fuses at 58 amps probable isn't a good thing.

My computor is down at home having little access to one at work I'll be back when possible.

Jim

http://www.interfacebus.com/Reference_Cable_AWG_Sizes.html

you said 20g....that has about 10 ohms per 1000ft. so thats 0.01 ohms peer foot. so at 40 amps thats 0.4v droped over each foot of cable.

or 0.4v x 40a = 16watts per foot...that why the cable gets hot.

using the that guide link 20g is only rated for 1.46 amps...rated means continious. it also sayss it fuses at 58 amps.

Stu

 

[LuxLuthor]

Here's another handy wire/current/resistance calculator.