18 Jun
2009
18 Jun
'09
11:19 a.m.
On Jun 17, 2009, at 6:26 PM, Eric Smith wrote:
I'm looking at the schematic for the H744, and I see an LM723 with
a big fat 2N5302 (Ic=30A) wrapped around it. Now, I'm aware that the
LM723 can be used in a switching regulator topology, but at first
glance that's not what it looks like to me.
I'm darn sure it is a switcher. Look at that darn great inductor in the
circuit. That's not a smoothing choke, it's the inductor of a switching
regulator.
Sitck a 'scope on one. You'll see switching waveforms...
Another clue... The input is 20-30V AC> So the voltage at the output of
the bridge recifier could be over 40V. The output is 5V, so 35V is
dropped in the regulator. The maximum output curret, IIRC is 25A. If it
was a linear, then 875W could be going out as heat -- from _every_ H744
in the system. Does the heatsink really look big enough for that? I know
it's a room-heater, but not that much of one...
Incidentally, the 723, as I am sure you know, is a 'regulator building
kit' consisting of a reference voltage source and an op-amp in one
package (OK, that's a slight simplification...) It can be wired as either
a linear regulators (op-amp as the error amplifier, possibly driving an
external pass transistor) or a switcher.
I've heard a rumour that early 723 data sheets only show it as a linear
regulator, the designers didn't realise it could be used as a switcher.
After it started getting used in the latter configuration, example
circuits were added to the data sheet. Does anyone have an early 723 data
sheet which doesn't show a swtiching regulator circuit?
-tony
While I
agree with you here, I have to admit that, if my
PDP-11/70 had switching power supplies, I'd probably run it a lot
more often. I'd *never* make it an irreversible modification,
though.
Unless you've already replaced them with something else, your
PDP-11/70 *DOES* have switching power supplies!