Recently I had opportunity to buy old Philips PE15xx series lab power supply in not bad condition for about $120, but I was too late and someone else bought it.

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The seller praised its very low noise, low ripple, low output impedance and good voltage and current regulation. I took a look at schematic in service manual to see how "good old linear power supply" is built. This is schematic found in service manual:

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It uses UA723CN and some old school BJT transistors. Datasheet says that UA723C with 5uF Cref capacitor has 86dB supply rejection. It looks pretty good for a part that cost about $1 (UA723CD).

From ua723 datasheet:

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I thought I might try to build a similar power supply for education purposes. I'm curious how it would perform compared with cheap chinese linear power supplies.

Of course price of PE1542 includes also high quality terminals, case, meters etc., but for educational purposes rebuilding just circuitry could be interesting.

My questions:

  1. Is this (Phillips PE15xx) power supply really very good design? Or just my seller glorified it?

  2. If it is - what can I use instead of ua723 and BDY (discontinued?) transistors to I build compareable low ripple low noise, good output CC/CV regulation power supply with modern parts?

  • 3
    \$\begingroup\$ BDY20 is the same as the still readily available 2N3055. \$\endgroup\$ Commented Apr 23, 2021 at 13:44
  • \$\begingroup\$ @Unimportant so... this is actually the the transistor used in many power supplies \$\endgroup\$
    – Kamil
    Commented Apr 23, 2021 at 14:07

1 Answer 1


Just about any contemporary NPN and PNP power transistors that are rated for the voltage and current will work as the output devices in this circuit.

It's a pretty good circuit if you have the manual to explain all of the adjustments. Rather than use the 723's internal current limit circuit that is based on the Vbe voltage drop of a transistor (and not very accurate), this schematic has R137, V102, and V103 as a much more accurate current limit circuit based on the 723's internal voltage reference and a true differential comparator.

The 723 was copied by many other companies. The LM723 from National Semiconductor (now a part of Texas Instruments) still is available. I'm sure European and Japanese companies still produce the part with different prefixes.


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