DEC H744 +5 supply

Brent Hilpert bhilpert at
Fri Sep 21 18:17:13 CDT 2018

On 2018-Sep-21, at 3:20 PM, Noel Chiappa via cctalk wrote:

>> From: Mattis Lind
>> The H744 is a buck converter. You can read about buck converters here:
> Wow, that was incredibly hard to read; no clear and simple explanation of the
> basic concept of how it works, before getting into the details!
> If I understand it correctly, it stores part of the incoming energy of a block
> of current in the field around the inductor; later, switches change state to
> create a loop that includes the inductor, and it uses the stored energy to
> cause electrons to flow around the new (temporary, because of the switch)
> circuit. Is that about right?

Yes, that's a basic part of the operation.

> The H744 manual doesn't really talk much about that aspect of the circuit's
> operation (at least in terms of 'we use this trick to get all the energy out
> of the incoming current flow'); it just describes the stuff around the coil as
> "an LC filter". It says "This type circuit is basically only an averaging
> device", which I wouldn't say is really on point - that would describe my
> (incorrect) prior description as well as the correct one.
> And just to make it even more confusing, it says "most of the input voltage is
> absorbed across the emitter-collector of Q5", but I looked, and Q5 is tiny,
> and I eventually figured out that that only applies to the +15V needed to run
> the voltage regulator.

Q5 is functioning as a common-base stage in the driver chain (base tied to fixed +15V).

It is not part of the +15V supply to the 723, that is provided by R2, zener D2, C2.

Q5 'voltage isolates' the 723 from the higher voltages around Q4-3-2 while nonetheless providing a control signal path from the 723 to the drivers.

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