Message 27413 in TECH.WW
From: G8MNY
Date: Fri, 07 Aug 26 07:11:00 Z
Newsgroups: TECH.WW
Subject: DRAE 24A 13.5V PSU
Message-ID: <1346_GB7CIP>
Path: N2MH4|VE3KPG|GB7CIP
R:260807/0720Z 34970@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260807/0719Z 28172@VE3KPG.#ECON.ON.CAN.NOAM BPQ6.0.25
R:260807/0711Z @:GB7CIP.#32.GBR.EURO #:1346 [Caterham Surrey GBR] $:1346_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have repaired 2 of this series so far, a 12A & a 24A model.
These are complex linear PSUs and use some unusual design features....
Short circuit current fold-back to 10A
Current limit @ 24A
Temperature limit trip
Over voltage limit @ 14V
Over voltage Crowbar @ 14.5V
Negative rail regulation
Earthed 2N3055 collectors
Current copiers extend current to 24A.
BLOCK SCHEMATIC (no diagram hence reverse engineered)
INPUT
The huge mains transformer's 2 secondaries each feed a bridge rectifier and
then on to each side of the capacitor bank. The +ve side is the + output in
this design, but it goes through several PCB links!
___ _________________________________
CONTROL CIRCUIT
Three opamps are used in the linear regulator in a cascade set up. IC1B
monitors the temperature and it's output together with a overvoltage sensing
transistor affect the current limiter opamp IC1C. This in turn offsets the
voltage setting pot on the voltage control opamp IC1D to affect the current
limiting.
The voltage reference zener D6 is initially powered via R17 and filter R15 and
cap C8 to delay power up, then from the output (D7 & R6) this gives the reduced
fold-back current on a shorted load, Voltage Opamp IC1C compares this reference
to the output sample from voltage setting pot P4. IC1D operates PNP TR1 to
drive the 2N3055 pass transistor TR2 to deliver up to 6A.
When 6A is reached in TR2, the voltage across R12 is compared to the value from
P2 by IC1C and it's output falls, -ve voltage is feed into P4 to reduce the
voltage and hence limit the current.
When too hot the voltage across D2 (by TR2) exceeds P1 setting and opamp IC1B
output goes high and this feeds IC1C to stop the output.
\ To Current Copiers /
COMPONENT LIST
No R | No R C TR D P IC TH L
17 1M | 1 3K3 4m7 25V BD131 5.6V 2K2 LM324N BTI51 ?
18 22R | 2 10K 4m7 25V 2N3055 1N4148 2K2 LM324N
19 0R05 2W | 3 6K8 4m7 25V TIP31B 1N4148 220R
20 | 4 2N3055 14V 2K2
21 22R | 5 10K TIP31B 1N4148
22 0R05 2W | 6 3K3 2m2 16V 2N3055 5.6V
23 22R | 7 10n TIP31B 1N4148
24 0R05 2W | 8 1K5 ? 2N3055 1N4001
25 22R | 9 27K 10n
26 3K3 | 10 22R 10W u1
27 27K | 11 22R 2m2 16V
28 3M3 | 12 0R05 2W 4m7 25V
| 13 10K 1n
| 14 15K
| 15 10K 4m7 25V
| 16 3K3 4m7 25V
CURRENT COPIERS
PROTECTION
Addition over voltage limiting circuit using TR9 shuts off the drive if output
rises above 14V. If there is no control due to a blown up pass transistor, a
SCR crowbar circuit will short the output as well (no fuse so destroying the
pass transistors?)
Warning due to the SCR crowbar, do not connect an unfused battery!
Several caps are across the output as well as a 7W load of R10, these soak up
any sudden changes in the loading.
STATUS LED Modification
IC1C
pin8
The -Ve going output of current Opamp IC1C that is also operated on over Volts
and Temperature, has just enough current to light the RED LED via the 1k & 470R
on doing so the GREEN LED goes out. The GREEN LED only lights if the +13.5V
output is good enough. All but the 1k are mounted to outside of the output
terminals with tight holes drilled for the LEDS, and +13.5 & 0V soldering to
the u1 cap leads that are across the terminals.
REPAIRING
Burnt out base emitter Rs means a 2N3055 is dud and has lost it's emitter
connection and it's driver will needed testing too.
This design is not that easy to work on you do need a flexible shaft 8mm
spinner!
Feet Transistors Feet Plan View
The 4 pass transistors are bolted on the underside with 2 sets of nuts, one to
hold transistors and internal heatsink to the case (no insulation kit needed in
this design!), and the 2nd set with washers holds the PCB in place. But the PCB
can only be removed after the 2 rectifiers have been moved out of the way.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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