Hardware Hobbyists vs. Emulator Jockeys

[Tony Duell]

> 
> On Jun 17, 2009, at 6:26 PM, Eric Smith wrote:
> >>   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!
> 
>    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