On Sun, 1 Nov 2020 22:49:59 -0500, ABLE1
wrote:
On 11/1/2020 5:38 PM, Jeff Liebermann wrote:
Are you prepared to pay for the best? (The worst you already have in
the form of an SMT fuse). For a little more $$$, the manufacturer
might supply a PPTC resettable fuse:
https://www.littelfuse.com/products/polyswitch-resettable-ptcs.aspx
Nothing to replace when it blows. Just give it time to cool down.
So Jeff, let me ask you.
If I would replace or jumper the B = RXEF010 resetable fuse the board
would work, meet code, and almost never have to concern myself with
it in the future??
No. I would replace it with as close to the original SMD fuse as
possible. In an emergency, I might replace the tiny SMD fuse with a
much larger fuse with wire leads. If doing development work on a
prototype, I would probably install a rather expensive socket and
rectangular fuse. However, for consumer use, it will always be the
original fuse.
The reason is that I don't know anything about the device that you
own. Most important, I don't know what it will do if a low AC voltage
is applied to the device. That's what will happen with a PTC
resettable fuse. The circuit does not go to completely open when the
PTC fuse is tripped. It goes to a fairly high resistance, and then
only as long as the fuse remains hot. To keep it hot, there has to be
a high current going through the fuse. That means there's power
applied to the rest of the PC board while the fuse is tripped. If you
look at the specs, that's the "hold current". Done correctly, that
will keep the fuse warm, and the power MOSTLY removed from the rest of
the circuit. Done wrong, it is possible that the fuse will cause the
device to cycle on and off repeatedly as the PTC fuse cycles through
hot and cold cycles. There are places and devices where a resettable
fuse is appropriate and safe. However, since I have no idea what
you're working with, I can't predict what will happen if you cram in a
PPTC resettable fuse and certainly don't want to take the risk
suggesting you do something that might be unsafe.
Also, is this really a problem that is worth solving? I doubt it. Put
in the correct fuse and you won't have to worry about the device
burning your house down.
Incidentally, I once repaired a small 13.6V 30A switching power
supply, which had a PPTC resettable fuse in the AC line. The problem
was obvious as there was a hole burned into the PCB where the PTC
resettable fuse had been. I could find no part number or specs on the
fuse making replacement difficult. I soon discovered why they used a
resettable fuse. It wasn't for convenience or size. It was because
the inrush current on this power supply was so high, that even a
slow-blo fuse would blow up if switched on with a 15A load. The
designer decided to use the PPTC fuse as an inrush current limiter,
which is a really bad idea. At some point along the load curve and
near maximum current, the operating point landed on the transition
line between low and high resistance. Since there was no hysteresis
in the transition line, the maximum power dissipation point for the
PPTC fuse was also somewhere along the curve. As bad luck would have
it, the customer's was operating almost on the same point of the
curve, causing the PTC fuse to become very hot. After a few days of
this kind of abuse, it burned a hole in the PCB and carbonized the PTC
fuse.
I made a few measurements and calculations and found a higher current
replacement PTC fuse. That brought the PTC fuse transition line to
somewhat higher than the maximum current. That fixed one problem and
created a new problem. The PTC would never trip because the
over-current protection in the power supply would turn down the power
supply before the PTC fuse was able to do the same. However, the
over-current protection was set to begin at about 10A above the
maximum output current and had no hysteresis. So, this mess is going
to require some redesign. I changed a few parts to bring the
over-current threshold to the maximum rated current and enabled
hysteresis by installing a resistor that the designer forgot. I
replaced the PTC fuse with a Slo-Blo conventional fuse, and inserted a
PTC thermistor inrush current limiter intended only to prevent blowing
the fuse under normal operation. Problem solved and it's been working
just fine for about 3 years.
Are you prepared to do something similar with your "circuit board"?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558