[Elecraft] OT: More PS ?s...

[Don Brown]

Hi

Using a large zener diode as a blocking may not be a good idea. This diod=
e is going to have a larger forward voltage drop than a similar sized sho=
ttky diode. It may drop 1 or 2 volts at 20 amps. So it will drop more vol=
tage and dissipate more heat. They are not really designed to be used in =
the forward direction even though it may work. Most zeners are rated in w=
atts not amps so I suspect this a transorb not a zener. Transorbs are rat=
ed in pulse current at 10 milliseconds or so and will not work in your ap=
plication. At most they will handle 1-5 amps continuously. You need a sho=
ttkey diode rated at 25 or more amps and > 35 volts to block the reverse =
current and have a low forward drop

Don Brown
KD5NDB
  =20

----- Original Message -----
From: John Clifford
Sent: Thursday, October 17, 2002 3:21 AM
To: elecraft@mailman.qth.net
Subject: [Elecraft] OT: More PS ?s...

I solved my adjustable voltage problem... there wasn't a problem (only wi=
th
the operator!).

However... I've been doing a little more thinking about this to see if I =
am
trying to do this right.

I have been looking at the schematic
(http://www.alinco.com/pdf.files/Service/dm330ser.pdf, p.21) and don't se=
e
any reverse voltage protection.  At power off and after I let the voltage
dissipate to 0, I did a resistance check and got about 90 ohms... which
indicates to me that a battery across this guy when the power's off is go=
ing
to either blow my inline fuse or blow something in the power supply.  So,=
 I
know I don't want to hook up a battery without some sort of protection...
right?

I picked up several Motorola MR2535L diodes at the local Active Electroni=
cs.
Motorola IDs this part as being a member of their rectifier family, but u=
pon
looking at the data sheet it seems that this is a zener diode, rated
nominally at 20v/35A.  Minimum breakdown voltage is 24v, so I should be s=
afe
from reverse current with a 12v battery... right?  The only other questio=
n I
have is whether or not what I'm trying to do is a good application for th=
is
part.  I can't tell from the data sheet
(http://www.eicsemi.com/eicsemi/pdf/MR2535L.pdf) whether or not this diod=
e
is intended for this application.  Although it is rated at 20v/35A, will =
it
be good for steady-state usage of 20 amps or more, or is it designed to
handle these voltages  and currents for only very brief periods of time a=
s
it shunts reverse voltage?

What about hooking three or four of these up in parallel, in series with =
the
positive wire?  Would doing this give me considerable leeway as far as
voltage/current handling goes?  Would it also decrease my voltage drop (I
should be able to treat diodes like resistors when it comes to voltage dr=
op,
i.e., .6v / 4 would give me .15v drop 'cuz of 1/4th the resistance, right=
?)?

Is anyone else doing something like this (running a battery in parallel w=
ith
a switching power supply)?  How are you doing it without destroying your =
PS
when the power is turned off?  Are you using a relay instead and triggeri=
ng
it off of the AC line?  I was hoping for something more elegant (more sol=
id
state!) than that....

- jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1


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