EMRFD Message Archive 7887
Message Date From Subject 7887 2012-10-21 07:38:28 peepholenz Fuses.. Maybe someone here can explain how to selct a fuse...
Here's the setup:Mains is 230v 50Hz.
Linear power supply for transceiver giving 14.0V DC at up to 20A.
Transformer primary dc resistance is ~4ohms.Secondary is bridge rectifier(large) to large(60000uf) capacitor,then linear reg giving 14.0v well regulated (drops a few tens of millivolt at 20A).
What type and size of mains fuse to "never" blow on turning on but definitely to blow if more than 25A load for more than say ten seconds.
Turn on surge can be as high as (230x1.414 peak volts)/4ohms,ie ~100A but steady current at 14v 20A assuming 80% efficiency is ~~1.5A.
Just got confused looking at fuse characteristics - maybe I'll have to consider series resistor in mains which is then shorted when relay is activated by the reg output....
7889 2012-10-21 09:46:13 Alex P Re: Fuses.. Have you checked into medium or slow blow fuses? With a 100 Amp in-rush surge the series resistor/relay is the way I would design your circuit with...it's a standard design method for high surge current equipment.
7895 2012-10-21 15:38:20 Andy Re: Fuses.. > Turn on surge can be as high as (230x1.414 peak volts)/4ohms,ie ~100AIt's been way too long since I've done this, but that is not how I
remember that calculation. The transformer is largely inductive, and
the point of maximum inrush current is not when the voltage is
Fuses are actually sloppy devices. They actually take a rather long
time to trip at or near (above) their rated current levels.
7904 2012-10-22 07:48:18 Hector Pascal Re: Fuses.. Fuse selection seems more an art than a science, but I'd be happy to learn
a better way, if anyone has a reference! Especially the major criteria
that determine the use of "fast" and "slow" fuses. I can't ever remember
seeing a fuse value calculation method that really predicts the best one
without some experiment.
On low current prototypes I have often characterized the transformer
primary switch-on current vs time, using a fast oscilloscope connected
across a small resistor in series with the active line. Then ignoring the
fastest transients, I select the smallest "normal" fuse one "size" greater
than the largest current seen at switch on, or at maximum load. Finally,
if this consistently blows, I uprate it in one "size" steps, until I find a
value that doesn't regularly blow from cold switch on, under the worst
environmental and voltage supply conditions designed for.
This adhoc method seems to work, but once a value is proposed, studying the
possible failure modes that will impact the fault current is probably a
good idea too!
[Non-text portions of this message have been removed]
7905 2012-10-22 18:02:07 Mike Re: Fuses.. I'll provide some comments below, in response to Hector's email:
> Fuse selection seems more an art than a science, but I'd be happy to learnSelection of fuses (and circuit breakers) is actually a well defined
> a better way, if anyone has a reference! Especially the major criteria
> that determine the use of "fast" and "slow" fuses. I can't ever remember
> seeing a fuse value calculation method that really predicts the best one
> without some experiment.
process when designing electrical power distribution systems (try a
google search on "overcurrent device coordination"). The goal (not
always attainable) is to provide a system where any overload or fault
("short circuit") condition will result in tripping (or fuse activation)
of only the device closest to the fault/overload, leaving all other
"upstream" overcurrent devices intact. This provides for mimimal
interruption of power to other loads.
To accomplish this, engineers generally work with time-current plots of
the tripping/interrupting characteristics, plotted on log-log paper.
Curves for different overcurrent devices (and for the served loads) can
be overlaid on each other to allow for selection (or adjustment in the
case of many types of power circuit breakers) of the individual portions
of the curves, so that selective coordination is attained (or at least,
approached). Typically, the curves for each overcurrent device are
actually a pair of curves (one with "minimum time to trip or interrupt",
another with "maximum time to trip or interrupt") so one has to deal
with a range of operating time for any given current. And the ampere
range for the curves typically run from a minimum equal to about the
ampere rating of the device, to a maximum that's approximately equal to
the maximum fault level on the system on which the device is to be
applied (which can be tens of thousands, or hundreds of thousands of
amps for an AC distribution system). The time range might span from
several thousand seconds to a few milliseconds. Hence the need for
> On low current prototypes I have often characterized the transformerThis is a very valid empirical method. Not only can the ampere value of
> primary switch-on current vs time, using a fast oscilloscope connected
> across a small resistor in series with the active line. Then ignoring the
> fastest transients, I select the smallest "normal" fuse one "size" greater
> than the largest current seen at switch on, or at maximum load. Finally,
> if this consistently blows, I uprate it in one "size" steps, until I find a
> value that doesn't regularly blow from cold switch on, under the worst
> environmental and voltage supply conditions designed for.
the fuses be revised to get something "close to" the mimimum size fuse
that will "hold in" for typical "power on" conditions, but the info can
also help you to decide on fast-acting vs time delay fuses. Of course,
you will need to pay for the fuses that you destroy during empirical
You can also work with the measured time-vs-current draw of the device
that you're trying to protect, and select a fuse that you think will
provide maximum protection without "blowing", by evaluation of the
measured "power on" current draw against the time-current characteristic
of the fuse(s) that you are considering. For example, the original
question raised by this listserver's member related to a 20A 12VDC power
supply. From his original call sign, I couldn't tell his country, but
he mentioned that it was a 240 (or was it 220?) VAC power source. My
experience (in the US) is primarily with 120 VAC power sources, for
which Bussman AGC (0.25 x 1.25 inch) fuses (with a 250 VAC rating) are
commonly used as AC protection for electronic "boxes". Time-current
characteristics for these fuses are available on the Cooper/Bussman
website if you sniff around a bit.
For the proposed 20A 12VDC power supply, coordination against upstream
devices is probably not an issue. So "keep it simple". I would suggest
trying a standard fast-blow fuse rated at about 200 to 300% of the
expected full-load current (on the AC mains). If that doesn't work, go
with a higher rating (up to perhaps 500 or 600%), and/or go with a
slow-blow fuse. For a DC power supply, the main reason (in my opinion)
for providing the fuse is to provide some protection against internal
faults ("short circuits") of the transformer, or the AC wiring (either
on the primary or secondary of the transformer). By contrast, overloads
of the power supply are (probably) handled with separate overcurrent
limiting (or detection) at the DC end of the supply.
> This adhoc method seems to work, but once a value is proposed, studying theAbsolutely! And learning new stuff is exactly what this hobby is all about.
> possible failure modes that will impact the fault current is probably a
> good idea too!
> David, VK6JT
> [Non-text portions of this message have been removed]
7906 2012-10-23 06:33:28 Harold Smith Re: Fuses.. There are NTC thermistors made especially as 'Inrush Current Limiters',
available in a wide variety of current ratings. I've bought them from the
usual sources, i.e. Mouser, Digi-key, etc. They are nothing more than a
resistor, the resistance of which goes down as it heats up.
7907 2012-10-23 16:13:49 n8oep_mike Re: Fuses.. Just remember that fuses are designed to protect the wiring and connected circuit elements...not the component(s)that are powered by them...
The 'fuse curves'; that you can download from various suppliers indicate; % overate and time to "open / fail".
A 50A fast blow fuse won't 'open' with 100A load in <0.5 second; but will 'open' in under X seconds under Y current.
Reduce the inrush current to minimize fuse fatigue...