Improving the SW HV PCB, how about this ?

Level42

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I've always seen the extra resistor (R20 and R21) mounted over the 7824 and 7924 as a poor hack to get more current from the power supply.

Apart from the fact that they get freaking hot (and thus waste energy), it also greatly reduces the functions of the 7824 and 7924.

Now there's a much better solution (I think), using a transistor instead of a resistor. This kind of set-up is mentioned on several datasheets too but here's a schematic I found:

000033.gif


And here's the negative version:

000134.gif



This set-up can deliver 5 Amps (compared to 1 amp for a unmodified 78xx set-up)....

Any idea's if we could incorporate this (safely) into the HV board ? Any reason not to do it ?

The good thing is that the transistors needed can be mounted on the cooling body that's already present on the HV board (which doesn't get very hot).

I've uploaded a scan of the HV PCB schematic that came with my manual, this actually includes the resistors R20 and R21 which I haven't seen on any of the schematics that are available on-line.

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It doesn't matter if you run the regulators over current, shunt the regulators with a resistor to get extra current, or use a transistor biasing setup to get more current (and I'd really be worried about startup transients with that setup, since there's no regulation on the base of the transistors -- even if those schematics were drawn properly, which they aren't)...

...you're still burning I*(Vout-Vin) power... you're just moving the heat around to different components...

I like to put my series resistors on the deflection board so the heat's on opposite sides of the cab.
 
OK so that's not an option then. My "plan B" is getting a couple of resistors that can be mounted on the cooling body already on the HV board.

Like these:

dropping_resistor_lores.jpg


These are designed to handle much more than the 5W ceramic resistors. The case will already do a better job on dissipating the heat and mounting it to the metal body will even improve that....

Also, this moves the resistors from the board itself which is very crowded in the area where they originally mounted them.

I like to put my series resistors on the deflection board so the heat's on opposite sides of the cab.
Not on the cockpits ! :)
 
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