Hardware Hobbyists vs. Emulator Jockeys

[Dave McGuire]

On Jun 18, 2009, at 2:19 PM, Tony Duell 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...

   Yes, Eric educated me.  I'm somewhat embarrassed to admit that I  
was mistaken here.  I have repaired scads of these but have never had  
to dig so deeply as to understand the whole circuit.  When I see an  
LM723 with a big fat series pass transistor wrapped around it, I  
think "linear regulator".  I've replaced a few electrolytic  
capacitors and a few of the transistors, usually the same ones.

> 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.

   Yup.  I've built a few power supplies with LM723s with external  
pass transistors, but always linear, not switchers.  I know from the  
datasheets that it is possible but I've never done it.

> 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?

   I have some pretty old NS databooks; I will check.

            -Dave

-- 
Dave McGuire
Port Charlotte, FL