Hardware Hobbyists vs. Emulator Jockeys
mcguire at neurotica.com
Thu Jun 18 16:22:03 CDT 2009
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
> circuit. That's not a smoothing choke, it's the inductor of a
> 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
> 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
> regulator, the designers didn't realise it could be used as a
> 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.
Port Charlotte, FL
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