18 Jun
2009
18 Jun
'09
2:22 p.m.
On Jun 18, 2009, at 2:19 PM, Tony Duell wrote:
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.
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... 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