the
weekender
Inrush current protection
for the
SB-220
linear
Do you have adequate surge protection
for your
SB-220? If you own this fine piece of gear or similar
equipment without the benefit of built-in surge pro-
tection, this article should be placed at the top of
your project list. For about $10 in parts and six hours
of bench work, you can breathe easy when you push
the power switch.
I
call it the $10 insurance policy.
The subject of surge protection has been addressed
by many in the past few years. In my opinion, one of
the better articles was written by K.
M.
Gleszer,
WIKAY, entitled "Upgrading Your SB-220 Linear
Amplifier," which appeared in
QST,
February, 1979.
Specific solutions were offered for operation with
117-Vac for filament inrush current, diode-transient
and voltage-equalization protection, plus other
items. But conspicuous by its absence was a scheme
for diode inrush current protection. This protection is
easily obtained with the simple circuit described
here.
One other area where I'd suggest
a.change is the
time-delay relay. The time-delay function is auto-
TO
SELECTOR
SWITCH
OMIT THIS SCREW
FRONT
ON REMOUNT
TO CHASSIS
TO PLATE
HOLE AL TRANSFORMER
fig.
1.
SB-220 rectifier board.
By
F.
T.
Marcellino, WSBYM,
13806
Parkland Drive, Rockville, Maryland
20853
NO.
14
(1.6mml
BUS WIRES
PC'i5cm1 LONG
5
6
TO MOUNTING
SCREW
II
FROM
L
(FIG. 31
fig.
2.
Relay connections for the surge-protection
circuit.
matic with a standard relay coil and a current-limiting
resistor. Therefore the high cost, plus purchase time
and final alteration, of a time-delay relay can be
avoided.
The mods I've installed are not unfamiliar, as
they've appeared in several 1970-series of the
Radio
Amateur's Handbook.
However, I've described the
procedures in a detailed order using short, some-
times elementary, phrases for clarification. I'm a
stickler for the smallest detail, so you needn't bother
with assumptions.
With the mods installed, the following benefits will
be added to your SB-220:
1.
Rectifier transient surge protection.
2.
Rectifier reverse voltage equalization.
3.
Rectifier inrush current protection.
4.
Inrush current protection for the 3-5002 filaments.
This procedure is divided into two parts: rectifier
protection and surge protection. You can elect to
cancel one, but because the amplifier must be un-
caged for installation of either,
it
seems wise to in-
clude both.
The fourteen original diodes in the SB-220 were
not replaced with higher
PIV
units. This action is not
necessary unless you break some during disassem-
bly. These diodes are rated for
1
ampere average for-
ward current at a
PIV
of
600
volts. The ratings are
adequate for this application, and, combined with
the modification, they will have a long life.
The nominal delay was selected as
5
seconds. This
time can be altered by varying the total limiting resis-
tance. A resistance of 200 ohms caused a long delay,
and the resistors dissipated much power. At the op-
66
december
1980
To Next Page
Loading...
