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using an smt resistor as a fuse

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Johnny B Good

Fri Jan 11, 2019 2:45 pm   

On Wed, 09 Jan 2019 18:27:30 -0600, Tim Williams wrote:

What are you fusing? To what end?

"High voltage" is particularly onerous because said fuse will continue
arcing the whole time the supply is draining down.

The transistor protects the fuse, not the other way around!!

It tends to be the other way round with SCRs though. Smile

Johnny B Good

Phil Hobbs

Fri Jan 11, 2019 4:45 pm   

On 1/10/19 1:01 PM, John Larkin wrote:
On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd_at_gmail.com

On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote:
I've placed 22-ohm 0805 resistors in series with the HV supply lines ...

If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms
would certainly fail, stopping the high power dissipation and removing the load
from the supply.

OK, I see I'm going to get roasted for this one.

OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf
because they're available, in a variety of specifications.

Three things the fuse has to do: carry the current, break the connection on overcurrent,
and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its
metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage
the PCB underneath it. Using a non-fuse-rated component risks the heat
damage (potentially, fire) that a fuse was intended to prevent.

It's that third part, repairing the damage and returning the gizmo to service, that
is the question: is it OK to throwaway the board on an overload (potentially, just
a stray wire) event, or would you want to repair/rebuild? And, is heat damage
limited to the resistor/fuse, or does it scorch the printed wiring?

I've never greatly admired surface-mount fuses, but you can also get fuseholders.
I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to
be only slightly annoying (tweezer-type irons help).

We've found surface-mount fuses (and surface-mount polyfuses) to be

Polyzens are very good for overvoltage. We current-limit elsewhere to
protect against short circuits. (RCs in the supplies of the output
amps, often.)


Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net

Bill Beaty

Wed Jan 16, 2019 12:45 am   

On Wednesday, January 9, 2019 at 10:48:05 PM UTC-8, Piotr Wyderski wrote:

If the transistor is not properly selected for fusing, one may also try
to augment it with the Soviet LED technique:


Transparent bolt! (The light from the junction is escaping.)

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