KPETERSEN@SIMTEL20.ARPA (07/25/85)
From: HaymakerLL <llh%drutx.uucp@BRL.ARPA>
The following is a letter to convert the switcher powersupply for use
in Packet Radio to allow the TAPR TNC to be used on 12VDC. The TNC is
a small 6809 based processor with a total current draw of around
800ma. This supply was used in packet radio use, but we have used it
in other applications, with good results. I might add that this
supply has been in use for better than six months with no problems,
and we have sent out about 200 requests for the schematic with no
negative feedback.
- 1 -
12 VDC POWER SUPPLY FOR TNC BOARDS
Packeteers in Colorado have adapted a surplus power
supply board for use with the VADCG and TAPR TNC's. The
supply is made by Iriichi Tsushin Kogyo Ltd., part no.
10053214-2. It is available from several sources, including
Radio Shack (#277-1016) and BNF Enterprises (#4NS0427). Our
modifications to the supply will convert the -5V output to
-12V (as required by the TNC), and will allow it to operate
from 10 to 15 volts DC (permitting construction of portable
packet stations and digipeaters operating from a 12 volt
battery).
Some changes to the TAPR boards will be required,
particularly on the older board layouts, to ensure
compatibility with the new supply. VADCG boards work without
modification, since they were designed for use with an
external power supply.
These modifications have been broken down into several
parts, so that you may perform only as much as necessary for
your particular application. For a copy of the schematic
and of these instructions, send a business size SASE with 1
once postage to: RMPRA -- Power Supply Experiments, 3775 E
115th Ave, Thornton CO 80233
**** DISCLAIMER ****
This information is a report on the methods and results
of the authors' experiments. The modifications we used were
not developed by the power supply manufacturer, the
Vancouver Amateur Digital Communications Group, or by Tucson
Amateur Packet Radio Corporation. Their approval of these
modifications is not implied by our use of their products.
We present our results for the information and assistance of
others interested in this work; it is still in EXPERIMENTAL
stages and those who attempt to duplicate our results assume
all risks of doing so.
PRECAUTIONS
**** For All Applications ****
1. Read this procedure through and be sure you understand
it, then do the job carefully. The switching power supply is
a more complicated and less forgiving device than the
three-terminal regulators it replaces.
2. Test the supply with dummy load resistors BEFORE
attaching it to the TNC. If there is a defect, you don't
want to find it by blowing out the expensive LSI chips on
the TNC board!
- 2 -
3. Use a fuse on the supply input lead. The switching supply
has NO other protection from excessive current. When testing
and connecting the switching supply, do not, ever, short the
output leads.
CONVERTING -5V OUTPUT TO -12V
**** For All Applications ****
Remove C14 (1000 UF, 10 V) and replace it with a
capacitor rated 470 UF, 16 V (Radio Shack 272-957). Be sure
to install C14 with the correct polarity (all the
electrolytics have "-" facing the same direction). The
reason for replacing C14 is that the 10 V rating will be
exceeded, and the capacitor would be likely to fail
prematurely. The replacement part must have a body diameter
of 1/2 inch (13 mm) or less, and lead spacing of .20 inch (5
mm), or it won't fit onto the PC board. Check the part
before you buy it, since a larger "equivalent" won't do the
job.
Remove R18 (30K, 2%) and replace it with a series
combination of 51K and 6.8K (the total value required is
58K). If you use 5% resistors you may have to change the
6.8K to get the exact resistance needed; you will determine
if this is necessary when you come to the TESTING section.
The output is rated at 50 MA when converted to -12V,
NOT the original 200 MA. This is plenty for the TNC's. The
derating is necessary to keep the peak current in TR4 and L4
the same as in the -5V circuit at 200 MA.
REDUCTION OF INPUT VOLTAGE (OPTIONAL)
**** Skip this section if you will use more than 15 V input ****
This modification changes two resistors, so that TR2
and TR4 will receive the same amount of base current at 12 V
input that the original values provided at 18 V input. This
is insurance against getting a board with low gain
transistors; the change is recommended even if the board
seems to work without it. When this change is made, the
recommended voltage from D1 "+" to D1 "-" is 10 to 16 VDC.
Replace R7 (240 ohm, 1 Watt) with 120 ohm, 1 Watt. Replace
R19 (910 ohm) with 470 ohm.
BYPASS INPUT RECTIFIER (OPTIONAL)
**** NOT for switching supplies which will have AC input ****
The input rectifier bridge has a voltage drop of about
1.5 volts. This needlessly reduces the supply efficiency
when only DC input is used, and also increases the required
DC input voltage. Connect a jumper from the "+" terminal of
- 3 -
D1 to the terminal of D1 that connects to L2. Likewise,
connect a jumper from the "-" terminal of D1 to the fourth
terminal of D1, which connects to L1. Be sure the jumper
wires cannot short to anything else. Check the continuity
from the input leads (with SW1 on, and no power applied):
red wire to D1 "+", and white wire to D1 "-". This
modification retains the input RFI filter and on/off switch,
and also uses D1 as a reversed-input clamp diode. Be sure to
use a fuse in the input lead; 1 1/2 amp normal blow should
be suitable. The supply draws about 1/2 amp producing 5.1 W
output with 13.5 volt input.
**** THIS IS THE END OF THE POWER SUPPLY MODIFICATION ****
Users of TAPR TNC's should continue with the following
sections, which deals with some pitfalls peculiar to that
device. VADCG and other users should simply disconnect their
old supplies and hook up the switching supply; then rejoin
this procedure at the TESTING section.
TAPR BOARD CLOCK CIRCUIT
**** ALL TAPR USERS be sure your board is up to date ****
The clock oscillator on older TAPR TNC's would
sometimes work at a parasitic frequency (not the one stamped
on the crystal). The different rise-time of switching supply
(compared to the original linear regulator) can make the
problem appear in TNC's that were previously "clean". If the
TAPR logon message is gibberish after installing the
switching supply, your clock is malfunctioning!
The most recent TAPR kits have modifications in the
oscillator to prevent the parasitic mode. If you have a 7404
at U1, you have the new board. If not, replace U1 (7414)
with a 7404. Connect a 4.7 K resistor from pin 5 of U1 to
pin 3 of U3 (for a neater installation, tie the U3 end to a
free pad in the wire-wrap area and run a wire to U3 -- the
resistor leads are much less likely to short out other pins
that way). Resistors R12 and R13 should be 1.0 K; some early
models have 1.5K.
The latest layout has also deleted CY (20 PF from pin
13 of U1 to ground), and added a 470 ohm resistor between
C12 and the reset switches. We did not make these
modifications, but you may wish to do so if you want an
exact duplicate of the latest TAPR boards' circuit.
TESTING
**** For all applications ****
- 4 -
For initial testing, use dummy load resistors instead
of your TNC board (in case something is wrong!).
RECOMMENDED TEST LOADS
OUTPUT VADCG TAPR MINIMUM LOAD
-------------------------------------------------------
+5V 6.8, 5W 6.2, 5W 51, 1/2W
+12V 62, 3W 180, 1W 470, 1/2W
-12V 470, 1/2W 470, 1/2W 1K, 1/4W
If you don't have the recommended load resistors,
substitute higher resistance values (up to "minimum load").
Please note that the switching supply's outputs should NOT
be tested without any load at all; some current must flow in
L3 and L4 so that the semiconductors operate in their normal
modes. POOR PERFORMANCE UNDER NO LOAD CONDITIONS IS NORMAL
IN SWITCHING REGULATORS!
Connnect the input wires to a 12 volt source (or 18V,
if the reduced input step wasn't performed). Watch the
polarity if you have modified for DC input: RED is "+".
Don't forget the fuse! Turn on SW1, and measure the +5V
output. Adjust RV1 to set it to 5.0 volts. Now check +12
volts; it should be between 11.5 and 12.5 volts (there is no
+12 adjustment). Check the -12 volt output; it should be
between -11.5 and -12.5 volts. If it is necessary increase
the -12V, increase the 6.8K (part of R18); likewise to lower
the output decrease the 6.8K. The output changes by .25 V
for every 1K change in resistance.
If you have an oscilloscope, you can check the output
ripple. It is typically 10 to 20 millivolts peak to peak, at
a frequency of 100KHZ. NOTE: poor grounding of the scope
will cause noise pickup from the switching transistors, and
give a false reading. When the probe tip is touched to the
probe's ground lead clip, anything but a flat trace
indicates you are picking up noise, not just output ripple.
HOOK IT UP!
**** FOR ALL APPLICATIONS ****
**** TAPR BOARD USERS: SEE DETAILED PROCEDURE IN NEXT SECTION ****
The output of the switching supply should be connected
to the board with wire no smaller than #24 AWG, using leads
of the shortest practical length. If you choose to install
connectors in the leads, be sure they are polarized so you
CAN'T POSSIBLY hook it up wrong. If you don't want to use
connectors, you can remove the pins at the switching supply
output and solder the wires directly to the PC board. VADCG
TNC's use the regular power supply connections. DOUBLE CHECK
- 5 -
YOUR WIRING BEFORE YOU TURN ON THE POWER.
TAPR INTERNAL POWER SUPPLY HOOKUP
**** TAPR BOARD USERS only ****
Feeding power to the output of integrated three-
terminal regulators can cause them to fail [see "Ciarcia's
Circuit Cellar" in BYTE, Jan. 1985]. To prevent this from
happening, the regulators will be removed from the TAPR
board, and the transformer connected to the switching
supply. With this modification in place, the TAPR board can
run on either 115 VAC or 12 VDC. YOU SHOULD HAVE FOLLOWED
ALL OPTIONS UP TO THIS POINT.
Remove the +5 , +12, and -12 regulators (U24, U22, and
U23) from the TAPR board. U24 may have been mounted on your
cabinet back-panel. Remove D9 and D12 from the TAPR board.
Connect an insulated jumper wire (#24 AWG or larger) from
the ANODE side of D9 (not banded) to the output of U22 (pin
3). Connect another insulated jumper wire from the cathode
of D12 (banded) to the output of U22 (pin 3). These jumpers
bypass the 12 V rectifiers and regulators, so the +/- 12
supply can be connected at J4.
Disconnect the three wires from terminals 3, 4, and 5
of the power transformer. Now examine power connector J4.
If you had the +5 regulator mounted outboard there will be a
wire at pin 7 (closest to C1). If you don't have a wire
there, prepare one (using one of the extra pins supplied
with the TAPR kit), and install it at pin 7. Connect the
wire from pin 7 to the switching supply +5 output. The wire
at pin 6 used to go to the transformer. Hook this one to the
+12 output of the switching supply. The wire at pin 5 also
went to the transformer; hook it to the -12 output of the
switching supply. Finally, hook the wire from pin 4 to the
output ground of the switching supply.
If you had the +5 regulator mounted outboard, remove
pin 3 from J4 by pressing down the locking barb with a small
screwdriver and pulling out the wire. Then salvage the pin
by removing the old wire. Connect the RED (positive) input
lead of the switching supply and the cathode (banded end) of
a 1N4001 diode to the pin (or the last spare pin, if the
regulator was mounted on-board). Put the pin back into pin 3
position of J4. Slide a piece of spaghetti tubing or heat-
shrink tube over a piece of wire, and solder one end of the
wire to the anode end of the diode. Slide the tubing over
the diode for insulation. The diode isolates the DC supply
from the transformer, so the transformer won't put current
out of the DC input leads. Connect the other end of the wire
to one side of a fuseholder (for the +12V fuse). Connect the
- 6 -
other end of the fuseholder to a wire (which will be the
positive DC input lead). The white lead from the switching
supply is the negative DC input lead; splice on additional
wire if necessary. The input leads should be run to the +12
supply in a workmanlike and secure way (chassis connector,
strain relief, etc.: take your pick).
Check with a VOM to be sure the wiring is correct (from
switching supply outputs to the power traces in the wire
wrap area). Check for shorts from +12 input to ground
(resistance is normal...a SHORT is not). Check continuity
for +12 input and ground, from input leads to the switching
regulator card.
**** NOTE ****
The TAPR transformer is less than optimum for this
application, but it will work at normal line voltage. For
low voltage ("brownout") conditions the transformer may be
replaced by a 12.6VAC, 1A transformer, or a lower primary
tap selected on the TAPR transformer. If you do use higher
transformer voltage, remove C1 (2200UF, 16V). It will be
operating too close to its maximum rating, and the switching
supply has its own fllters. If a battery is connected to the
12V input for backup, the transformer SHOULD BE CHANGED, or
else the battery will be loaded constantly (the transformer
/ rectifier output has to exceed the battery voltage to
prevent this).
MOUNTING
**** For all applications ****
The switching power supply may be mounted with #6
hardware, using a set at the holes near each corner. When
mounting in a TNC cabinet, keep it as far away as possible
from the critical analog circuitry of the modem.
The mounting hardware also serves to ground the the
power supply board. In particular, the mounting hole closest
to IC3 may be grounded through the mounting hardware to
minimize noise on the +12V.
The mounting hole near SW1 MUST HAVE INSULATING
HARDWARE to avoid shorting the power input leads to ground.
Use either a plastic washer on BOTH top and bottom, or use
non-conductive mounting hardware (ie, nylon screw and
standoff). Don't rely on the green soldermask film; it is
thin and not intended to be used for insulation.
READY TO GO!
- 7 -
Turn on the TNC (by powering up the switching supply).
The logon message (or autobaud routine message) should
appear. Measure the +5V on the TNC board, and make a small
adjustment to RV1 if the voltage is off. If you are using a
TAPR board with provisions for both AC and DC power, be sure
it works both on 115VAC and 12VDC.
If you have any further results, we welcome them. They
may be sent to us by mail, via the RMPRA bulletin Board
(N0BRZ-1), or via N0CCZ on DRNET.
Good luck and 73,
--Don Brown, N0BRZ
--Tim Groat, KR0U
--Linc Haymaker, K0ZCO
(Rocky Mountain Packet Radio Assoc.)