Basic fuse question

[Cydrome Leader]

John Robertson <spam@flippers.com> wrote:

On 01/30/2015 5:41 AM, Phil Allison wrote:
John Robertson wrote:



** Yes - the word with that T as the initial letter.

It's the German word "Trage" meaning "dragged out" or leaden.

All the letter codes on common fuses are for German words.

F = " Flink " for quickly or rapid.

http://www.thefusewarehouse.com/pages/learn_fuse_markings.php



Isn't that chart just for the European fuses?


** Whooah - hold on a mo there.

Q. Who uses European standard fuses ?

A. Everyone does.

Common appliance fuses come in just two sizes: 20x5mm and 6.3x30mm or 3AG.

European and Asian makers produce both sizes and dominate the market because of lower cost. In 40 years of buying and using such fuses, I cannot remember seeing a pack labelled "Made in USA"



..... Phil




On this side of the pond we have Cooper/BUSS fuses which say "Made in
the USA" in my stock, just got some from Digi-Key.

remember the nice metal and plastic boxes the 5 packs came in from
Littlefuse?

 

[Phil Allison]

Cydrome Leader wrote:

..

remember the nice metal and plastic boxes the 5 packs came in from
Littlefuse?

** Errr - ain't that " LittelFuse ?

Also famous for the " Little Light" ?

( small incandescent lamp mounted on a goose neck with a BNC plug on the end. )


..... Phil

 

[Phil Allison]

Phil Allison wrote:

** Errr - ain't that " LittelFuse ?

Also famous for the " Little Light" ?

** FFS - I did it too !!

"Littel Light" !!!!!!!!!!



..... Phil

 

[Tim R]

On Thursday, February 5, 2015 at 11:35:42 AM UTC-5, Jeff Liebermann wrote:

Note: Some thermal circuit breakers will decrease in value if tripped
repeatedly. This is a safety feature as having the trip value
increase would eventually make the circuit breaker not trip at all.
Magnetic circuit breakers don't have this problem/feature.

I looked this up some time ago for the larger circuit breakers such as those in a main panel.

The manufacturers give specs for the number of cycles.

That number is very large for a circuit breaker being used as a switch to turn a circuit on and off. Many times lighting circuits, e.g., are operated with a breaker and don't have a switch. I don't remember offhand but I'm thinking in the range of 50,000 cycles.

Then you have the next case where the breaker "trips" on overcurrent. You have the toaster on when the refrigerator cycles, etc. You've exceed the 15 amp by enough to trip the breaker but it's not a dead short. The number of rated cycles goes down by a couple orders of magnitude but it's still high.

But now you have the dead short case, where you draw maximum fault current until the breaker trips. Breakers are only rated for 1 to 2 cycles of this kind of duty.

 

[Jeff Liebermann]

On Fri, 18 Oct 2013 12:21:09 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>** Fuses cost money and I don't like to see them blow at switch on.

I have a small collection of AC circuit breakers with clip leads
soldered onto the terminals. As I recall, 1A, 3A, 5A, and 10A. I
pick an appropriate value, clip onto the fuse terminals, and apply
power. If the circuit breaker doesn't pop, I install the real fuse. I
also use the variac trick, but only on my bench at home, as I don't
have one in the office.

Note: Some thermal circuit breakers will decrease in value if tripped
repeatedly. This is a safety feature as having the trip value
increase would eventually make the circuit breaker not trip at all.
Magnetic circuit breakers don't have this problem/feature.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Thu, 5 Feb 2015 09:08:16 -0800 (PST), Tim R <timothy42b@aol.com>
wrote:

On Thursday, February 5, 2015 at 11:35:42 AM UTC-5, Jeff Liebermann wrote:
Note: Some thermal circuit breakers will decrease in value if tripped
repeatedly. This is a safety feature as having the trip value
increase would eventually make the circuit breaker not trip at all.
Magnetic circuit breakers don't have this problem/feature.


I looked this up some time ago for the larger circuit breakers such as those in a main panel.

The manufacturers give specs for the number of cycles.

That number is very large for a circuit breaker being used as a switch to turn a circuit on and off. Many times lighting circuits, e.g., are operated with a breaker and don't have a switch. I don't remember offhand but I'm thinking in the range of 50,000 cycles.

Then you have the next case where the breaker "trips" on overcurrent. You have the toaster on when the refrigerator cycles, etc. You've exceed the 15 amp by enough to trip the breaker but it's not a dead short. The number of rated cycles goes down by a couple orders of magnitude but it's still high.

But now you have the dead short case, where you draw maximum fault current until the breaker trips. Breakers are only rated for 1 to 2 cycles of this kind of duty.

Sorta. There are many ways to use a circuit breaker, each with their
own unique way of causing problems. In my case, my collection of
assorted breakers came from a power supply that I designed, where I
made a big mistake. I used an AC rated breaker for the DC output. Bad
idea.

The contact material for AC and DC breakers are quite different. DC
breakers have a much larger spring to deal with the tendency for
arcing and welding the contacts when opening a high current DC circuit
under load. I did some crude testing and found that after tripping
about 10 times under 3x rated load, the contacts would weld. The same
breaker, with a 3x AC load seemed to work just fine. Moral: Don't
use AC only rated breakers with DC.

I didn't have much trouble using such a breaker as an on/off switch on
the AC side of the power supply. By much, I had to install a PTC
thermistor inrush current limiter on the AC line because the rather
large inrush current was sometimes tripping the breaker immediately
after it was turned on.

The breakers in my pile are the "push to reset" flavor and are not
used as an on/off switch.

A 5 amp breaker will conduct 5 amps forever and not trip. The
time-current curves are far from linear or simple:
<https://www.google.com/search?q=circuit+breaker+trip+curves&tbm=isch>
For example, a "typical" breaker will require about 3 minutes to open
at 2x the rated current, and about 0.1 sec at 10x the rated current.

After my initial screwup, I spent some time looking at such
time-current curves trying to select a proper circuit breaker. It
would have been much easier if I were allowed to use a combination
thermal-magnetic breaker, but those were far too expensive.

One could also use a PTC inrush current limiter as a temporary fuse.
Too much current and it gets hot and goes "open". The problem is
selecting the right combination of on-resistance and trip current.
<http://en.wikipedia.org/wiki/Resettable_fuse>
I've had to go through the design process a few times trying to find a
suitable replacement for such a device, that had burned itself to a
cinder and no parts list was available. There's no problem at either
extreme of low current and over-current. However, the dissipation
reached a peak in between these two extremes. At some current,
usually just below the trip point, it gets VERY hot but does not open.
Be careful with these.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Thu, 05 Feb 2015 09:49:52 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

One could also use a PTC inrush current limiter as a temporary fuse.
Too much current and it gets hot and goes "open". The problem is
selecting the right combination of on-resistance and trip current.
http://en.wikipedia.org/wiki/Resettable_fuse

Argh, that's wrong. Clarifying my muddle:

An inrush current limiter is an NTC (negative temperature coefficient)
device. When cold, it has a high resistance. As current heats up the
device, the resistance goes down.
<http://en.wikipedia.org/wiki/Inrush_current_limiter>

An electronic fuse is a PTC (positive temperature coefficient) device.
When cold, the resistance is low. When too much current goes through
the device and it becomes hot, the resistance goes up.
<http://en.wikipedia.org/wiki/Resettable_fuse>

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

On Thursday, October 17, 2013 at 7:55:39 PM UTC-4, Dan wrote:

I need to replace a PSU fuse in an LCD monitor. The open fuse is
ceramic, 5 x 20 mm, and marked "T3.15AH250V", with no obvious spacing
between the various markings. I understand the speed rating (T) and the
voltage (250V), but I'm unclear on the amperage. Is it 3.15 amps, .15
amps, or 15 amps? Did some searching, but all I can find are pages
trying to sell fuses, not any giving how to read the ratings.

TIA

Dan

Jeff
You mentioned that you have a number of different breakers fitted with clip leads for testing before you install an actual fuse. I hate to repeatedly blow fuses too, but doesn't that increase the time it takes for the circuit to open, thus subjecting the equipment to possible further harm? Lenny

 

[Jeff Liebermann]

On Sat, 7 Feb 2015 18:01:09 -0800 (PST), captainvideo462009@gmail.com
wrote:

Jeff
You mentioned that you have a number of different breakers fitted
with clip leads for testing before you install an actual fuse.

Yep.

I hate to repeatedly blow fuses too, but doesn't that increase
the time it takes for the circuit to open, thus subjecting
the equipment to possible further harm? Lenny

Chuckle. That would be like comparing the old screw in electric panel
fuse with the more modern circuit breakers. Actually, the magentic
circuit breaker is faster than the thermal fuse up to about 2x rated
current.

However, it doesn't matter. If the power supply is shorted, blowing
the circuit breaker a bit late isn't going to make much difference as
the power supply is already fried. Whatever is blown or shorted will
probably protect the rest of the circuitry. However, if everything is
working correctly, the circuit breaker won't trip so it make no
difference how fast or slow it opens.

Where it might get a bit sticky is a fairly low resistance short
across the power supply output. That won't draw enough current for a
instanteous breaker trip and might cook a few devices or traces
between the power supply and the low resistance short. It's a
definite risk, which I'll gladly take, because I haven't seem much of
that. Most commonly, it has happened when I stupidly install
electrolytic caps in backwards, which hopefully will not become
chronic or epidemic.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

On Thursday, October 17, 2013 at 7:55:39 PM UTC-4, Dan wrote:

I need to replace a PSU fuse in an LCD monitor. The open fuse is
ceramic, 5 x 20 mm, and marked "T3.15AH250V", with no obvious spacing
between the various markings. I understand the speed rating (T) and the
voltage (250V), but I'm unclear on the amperage. Is it 3.15 amps, .15
amps, or 15 amps? Did some searching, but all I can find are pages
trying to sell fuses, not any giving how to read the ratings.

TIA

Dan

I've probably had a few thousand TV's come in here over the years. (not any more). NAP sets seemed to use circuit breakers a lot. Some other brands I recall that had open B+ fuses smoked and then blew the fuse when a new one was installed for testing. Often wondered why it didn't smoke the first time before the first fuse opened. Lenny