17 Jan
2010
17 Jan
'10
11:42 a.m.
On 17 Jan 2010 at 18:40, Tony Duell wrote:
Oh, right... The OP says he has the schematics, do they show any such
resistor.
3) There is resistor in parallel with the
regulator.
MITS did this routinely and it wouldn't surprise me if they did this
on the 680. In particular, I remember that the +5 supply for the 4K DRAM boards had this dirty little trick. Anything, I
suppose, to
shave a few dimes off the manufacturing cost.
I came across it in the HP59500 HPIB interface for the multiprogrammer.
This is basically a unit which send/receives ASCII strings over HPIB and
has 16-out, 13-in TTL level interface to the multiprogrammer.
It's a large board of TTL (something like 70 chips) and a little PSU
board giving the necessary 5V (that's the only supply it needs). The PSU
board has 2 TO3 componnents on it, one is a regulator (I forget it it's a
fixed of adjustable one) and the other is an SCR [1] for the crowbar
circuit. Anyway, there is a reistor shunted across the regulator to carry
most of the load current. If you try to test that supply without a dummy
load oe with too small a load, the crowbar will fire and the resistor
gets hot and bothered.
[1] We (UK) often call that component a 'thyristor' by analogy with
'thyratron' (a gas-filled triode or more complex valve, line the 2D21
(EN91), etc). Is that name recognised elsewhere?
Everything's fine with a full load, but if the load should drop for
any reason, any IC connected to the regulated (?) line were toast.
Indeed.
-tony