• DRAE 24A 13.5V PSU

    From G8MNY@21:5/101 to TECH on Tue Jul 2 05:32:47 2019
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    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 & 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) ┌──────┐┌─────────┐┌─────────┐
    │Mains ├┤ Dual ├┤Capacitor├────────────────────┬─────┬─────>+13.5V
    │Trans-├┤ Bridge ├┤ Bank │ ┌──────────────┐ │ │
    │former││Rectifier││ ├─┐ │Temp & I limit├─)──┐ │ └──────┘└─────────┘└─────────┘ │ ├──────────────┤ │ ┌┴──┴───┐
    │ │ 6A linear ├─┘ │ Over │
    ├─┤ reg PSU ├─┐ │Voltage│
    -ve│ └──────┬───────┘ │ │Crowbar│
    │ ┌──────┴───────┐ │ └───┬───┘
    ├─┤Current Copier├─┤ │
    (-10V)│ ├──────────────┤ │ │
    ├─┤Current Copier├─┤ │
    │ ├──────────────┤ │ │
    └─┤Current Copier├─┴─────┴─────> 0V
    └──────────────┘
    INPUT
    The huge mains transformer's 2 secondaries each feed a bridge rectifier & 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!
    ___ _________________________________
    L─o-o──o\─┬──┐||( Rectifier + │ │C2│ │C4│ │C15│ │ +13.5V
    6.3A │ )||(__Bridge 1───)─┐ │ === │ === │ === │
    neon )|| ___ __│ │ === │ === │ === │ ===
    │ )||( Rectifier + │ C1│ │C3│ │ │C12│ │C16 N──────o\─┴──┘||(__Bridge 2_____│___│__│__│__│__│___│__│____-Ve (-10V)
    E──┐ 2x 17V 20A

    CONTROL CIRCUIT
    Three opamps are used in the linear regulator in a cascade set up. IC1B monitors the temperature & 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 & filter R15 & 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 & it's output falls, -ve voltage is feed into P4 to reduce the voltage & hence limit the current.

    When too hot the voltage across D2 (by TR2) exceeds P1 setting & opamp IC1B output goes high & this feeds IC1C to stop the output.

    ──────────┬──────────────────────────────┬───┬───────────┬──>+13.5V
    5│\IC1B │ C8│+ │ _│_
    ┌──┤+\ 7 R1 IC1D === R13 D6 /_\'
    │ │ >───)───R9──┐ /│12 │ │ │
    │6┌┤-/ │ │ From R26 14 /+├───┴───)───R15─┬───┤
    │ ││/ D3 ├─┐ ├─<overV ┌──< │ VOLTS │ │
    │ ├─R2─┤<├┤ R3 │ │ \-├──┬──>POT R17 │
    │ │ ┌───)─┴─┐ │ IC1C │C9 \│13│ P4 │ _│_
    │ │ T │ I │10│\ ├─┤├─────┘ │ Start│ \_/D7
    └─)──>POT │ POT<──)──┤+\ 8 │ D5 │ up │ │
    │ P1 │ P2 │ │ >──────)─────R5──┤<├─┤ -ve │
    │ │ _│_ │ ├──┤-/ _│_TR1 │ │
    │ │ '/_\ │ │ 9│/ PNP/ \e 250mA │ R6
    │ │ D1│ │ R8 ┌────┤ └─┬───┐ R14 │
    _│_D2│ │ │ │ R11 _│_ L1 R25 │ │
    \_/ │ │ │ │ │ e/ \TR2 │ │ │ │ ──┴───┴───┴──┬┴R12┴┬────┴┬─┘ ├───┴┬──┴─────┴───────────┴┬─>0V
    -ve │ │ └─┤>├─┤6A │ ┴
    │ │ D8 ┴ │
    \ 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

    │ │ 3│\IC1A │
    │ ├───┤+\ 1 │
    │ │ │ >───┐TR3│ Three more 2N3055
    │ │ ┌─┤-/ _│_ │ each with a driver
    │ │ │4│/ e/ \ │ & opamp are used
    │ │ ├──R18─┤ └─┤ to get the current
    │ │ │ _│_ │ up to 24A.
    │ │ │ e/ \TR4 │
    ├─R19─)─┴────┘ ├───┤
    │ │ 12│\IC2D ┴ │
    │ ├───┤+\ 14 │
    │ │ 13│ >───┐TR5│
    │ │ ┌─┤-/ _│_ │
    │ │ │ │/ e/ \ │
    │ │ ├──R22─┤ └─┤
    │ │ │ _│_ │
    │ │ │ e/ \TR6 │
    ├─R22─)─┴────┘ ├───┤
    │ │ 10│\IC2C ┴ │
    │ └───┤+\ 8 │
    │ │ >───┐TR7│
    │ ┌─┤-/ _│_ │
    │ │9│/ e/ \ │
    │ ├──R23─┤ └─┤
    │ │ _│_ │
    │ │ e/ \TR8 │
    └─R24───┴────┘ ├───┘

    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!

    ─────────────────────────┬───────┬────┬──┬───┬──┬──┬─>+13.5V
    _│_ │ │ │ │ │ │
    D4/_\' │ │ │C6 │ │ │
    ┌──────┬────┤ FB│ │ === │ │ ===
    e\│ === │ |│| │ │ === │ │u1 on
    Over- TR9 ├─┬──┤C13 │ TH1_│_ │ │C11│ │ │terminals
    volts PNP/│ │ │ │ \_/ │ │ │ R10 │
    to IC1 <─R26─┘ R28 │ TRIP<──┬─/│ === │ │ │ │
    pin9 │ │ P3 │ │ C10│ │ │ │ │
    >-ve──┘ R27 │ C7=== │ │ │ │ │ │
    ┴────┴────┴──┴────┴──┴───┴──┴──┴┬─>0V

    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

    ──┬───────────────┬───>+13.5V
    470 │ ┌────────────────┐
    ├────┐ === u1 │ ┌─┐ │
    RED _│_ _│_ GREEN │ │ └─┘ │
    LED ≡\_/ \_/≡ LED ├───>0V Red │o │
    FAIL │ _│_ OK ┴ │ (∙)+ │
    │ /_\' Green│o │
    wire│ │ 9V │ (∙)- │
    │ ┴ │ │
    Current+ 1k └┬┬────────────┬┬┘
    Temp amp │
    │>────┴───R5─
    IC1C
    pin8

    The -Ve going output of current Opamp IC1C that is also operated on over Volts & 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, & +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 & has lost it's emitter connection & it's driver will needed testing too.
    This design is not that easy to work on you do need a flexible shaft 8mm spinner!
    ┌──────────────────────────────┐
    ▄├─┐ fuse ≡≡≡≡≡≡≡≡ ≡≡≡≡≡≡≡≡│
    Side View ▀├─┘ │ ▐ p4 p2 p1 ▐ ││
    │ ││mains │ ___ │ p3 o+ ││
    │SWITCH │heat- Rect ││lead │ -'' ''- │ th1 o ▐▌│
    ▄│ │sinks ▐▌│ │ / _ \ │ o o - ││ ▀│----------- │ ▒Cs▒ ││ ││ / \ ││ o o ││
    ▄│ MAINS |│ PCB ▒ ▒ ││ ││ \_/ ││ - o o ▐▌│ ▀│TRANSFORMER|│===============││ │ \ / │ ▐ ++ o ▐ ││
    └┬─┬───────────┬─┬──────┬─┬┬─┬┘ │ -..___..- ≡≡≡≡≡≡≡≡ ≡≡≡≡≡≡≡≡│
    └─┘ ~ ~ └─┘ └──────────────────────────────┘
    Feet Transistors Feet Plan View

    The 4 pass transistors are bolted on the underside with 2 sets of nuts, one to hold transistors & internal heatsink to the case (no insulation kit needed in this design!), & 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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    * Origin: HAMRADIO telnet lu9dce.dynu.com (21:5/101)