"Doug Smith W9WI" wrote in message
...
Lucky wrote:
I have a Icom R75 and Lowe HF-150 I want to run off a Astron PSU model
SL-11a. It's 13.84V and 7Amps. Now, I have some questions please about
how to protect my radios from any mishaps from the PSU or otherwise.
Now the specs for the R75 says it's current drain at 13.80v + or - 15% is
:
Standy 0.9 Amps
Max. Audio 1.1 Amps
I bought a 2 conductor DC accessory cable that has a fuse holder on it. I
will use that to connect the PSU to the radio.
It it means anything it is 22 AWG RS part # 270-025
1] Now, what size fuse should I put in the fuse holder to make sure the
radio is totally protected from the PSU?
About 2 amps. (as a rule of thumb I would fuse a circuit at approximately
twice the maximum expected current under normal conditions) Most likely
failures will cause the circuit to draw a LOT more than the rated 1.1
amps, more than enough to blow a 2 or 3-amp fuse.
2] Am I protecting the radio from too much volts or amps or both? I
figured Amps.
The fuse protects against excessive current -- too many amps.
To be honest you're not as much protecting the radio as you're protecting
the power supply & your house. If the radio starts drawing much more than
1.1 amps, it already has a serious problem. However, without the fuse it
might cause the power supply to overheat, or (more likely) cause the
wiring to overheat, or whatever part in the radio is defective might start
smoking or even burning.
That said, you're also protecting the radio against reverse polarity.
(getting the power supply connected backwards) The radio probably
contains a polarity protection diode. If you connect the power supply
backwards, this diode limits the voltage that can appear across the radio
to less than 1 volt - but it does so by causing the radio to draw very
high currents. (at least 10 amps, probably more) The idea is that the
fuse will blow, disconnecting the voltage, before the protection diode can
get hot enough to burn out.
According to the Astron website, the SL-11A power supply has builtin
protection against excessive voltage. Actually, it's pretty similar to
the polarity protection in the radio -- if the output voltage from the
supply gets too high, a diode within the supply draws very high currents,
causing a fuse inside the supply to burn out before the attached equipment
(your radios) can be damaged.
3] I have some 250V 1.6 Amp fast acting fuses. The are 5 x 20mm GMA type
fuses. Radio Shack part number 270-1051. Are these OK? Is the main thing
the Amp rating and not the volt rating?
They're OK. They *may* be more likely to burn out (if the radio for some
reason draws a bit more current then specified) but the only consequence
is you'll have to buy more fuses. It won't damage the radios.
The voltage rating must be high enough to ensure the power doesn't arc
across the fuse if the fuse blows. A 250-volt fuse is more than adequate
for your 13.8 volt circuit. (it is possible to get 32-volt fuses that
certainly would NOT be adequate in a 250-volt circuit!)
4] The Icom Radio comes with
3 Amp FGB fuses internally and for the DC cable. Now if the radio draws
say no more then 1.1 Amps, why are they using a 3 amp fuse? Why not 1.5A
or even 2A? Why 3A?
5] So is 1.6A too low, too high, or just good? Should I go even lower to
be sure?
Probably a bit low but if it were my equipment, I'd try it.
I wouldn't go any lower, you'd probably have nuisance failures.
6] Now, the fuse in the Astron PSU itself.
Is that to protect from volt surges or amps surges?
Both. (see my answer to #2) (some power supplies don't have overvoltage
protection, in which case the PSU fuse only protects against current -
amps - surges.)
7] Do you think I'm going to extreme measures to protect my receivers
since the Astron PSU is supposed to be that good? I want total peace of
mind knowing I did all I could to protect them.
I don't think fusing the power cords to the radios is paranoia.
8] My Lowe HF-150 will use 13.80v and the same PSU also. According to
the manual it can use between 10-15V and it draws approx 300mA but I
think it's more in the 500mA range to work properly.
9] Do you think I need a different fuse size in the holder to protect the
150 or will what I use to protect the R75 be sufficient for the 150 also?
I would expect any likely failure in the HF-150 to draw at least 5 amps,
so any fuse in the 1.6-2-3 amp area should be OK.
10] What is the difference between "GMA" and "FGB" type fuses? Is it
better to use the longer ones, the mini ones or it doesn't matter?
Without actually seeing the pictures don't use me as the ultimate
authority, but I think the only difference is physical size and shape.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com
Wow good info explained simply Doug. Thank you that was nice of you. It
clears up lots of things for me. I thought I was protecting the radio more
then anything else but it seems I'm doing lots more that I didn't know
about.
I'd just like to know a couple more things please if you don't mind.
I will be using that 22AWG DC cable plus a 6 ft extention adaption cable for
the adaptaplug that RS sells. Obviously both cables are different gauges and
carrying capacities.
1] Will I compromise any of the excellent qualities of the Astron PSU or
effect how the radio receives it's voltage purely by using these differing
cables?
2] You say the higher voltage rating for the fuses keeps the power from
arcing. How is it determined what voltage rating you should use? By the
voltage that is going to flow thru the wires or by what the PSU can put out?
Thanks for being a radio buddy and helping me out. Your website looks like
it's full of other good information too.
Lucky