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