Re: Bulletproof your WG6100!

On Wed, 14 Jan 1998 15:13:33 -0800 (PST), aek (Al Kossow) wrote:

>"I have some short circuit
>protection schemes jotted down on paper, specifically a foldback current
>limit circuit, that could easily be added to the power transistors. I'll
>add it to my list of things to do when I am done with the fab order."
>
>hmm... did you have an urgent need for more of these original boards.
>wouldn't it make sense to add the current limiting to this BEFORE you
>make a bunch of them?

We knocked this back and forth the last time this came up.

The problem is the current limit design will require a power resistor on the
output of the pass transistor (to detect the current) and another transistor to
short the BE of the pass transistor as the current goes up.

Easy enough to do, but not doable as a drop in replacement for the current LV
regulator. You're going to have to cut and rearrange some things. Whereas now
all you need to do is hook into already existing points on the PCB board. No
hacks.

Also, the transistors used to current limit will have to be able to sustain the
current that can be delivered by the voltage regulators indefinitely. This can
be in excess of an 1.5 amps so they will have to be heatsinked beyond a little
tab heatsink. That adds a new level of pain in the mounting of the board.

You can also regulate the current going into the pass transistors and thereby
indirectly regulate to total current, but then you're depending a high tolerance
HFE value, which is seldom the case. The transistors are rate at minimum HFE.
Max can (it almost always is) much greater, which would render the current
regulation, of the base current, useless.

-Zonn

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Received on Wed Jan 14 15:38:13 1998