#385 widlar

Hacking the BoldportClub Widlar (Project #21) to get the classic μA723 to do something other than voltage regulation (LED blinky).

Build

Here’s a quick demo..

Notes

BoldportClub Widlar project is really all about two things:

Bod Widlar’s classic μA723 voltage regulator, in SMD packaging
a beautiful circuit board that breaks out the chip to allow building any of the standard circuits supported by the μA723 “voltage regulator toolkit”

The μA723 comprises a number of functional sub-systems:

internal voltage reference, 7.15V (typical)
voltage reference amplifier
error amplifier
current limiter
output control transistor
output zener diode

With a few external components, these allow the μA723 to be tailored for specific voltage regulation goals. I’ve experimented with some of the basic 723 circuits previously:

Although not recommended or particularly useful for modern designs, it is widely discussed and documented, so makes an excellent learning tool.

As it came out in the same year I was born (1967), I also find it a personally interesting benchmark from which to judge how much things have changed, … and how they haven’t;-)

The Kit

Another wonderfully packaged kit from Boldport..

widlar_kit widlar_pcb_front widlar_pcb_rear

Parts

Qty	Item
x1	μA723, TI UA723CDR
x1	NPN BJT TO-220, Fairchild BD239BTU
x5	470Ω resistor 250mW, Multicomp MCF 0.25W 470
x5	4.3KΩ resistor 500mW, RS PRO739-8809
x5	1.5KΩ resistor 500mW, RS PRO739-8767
x3	10KΩ resistor 250mW, Multicomp MCF 0.25W 10K
x1	10Ω resistor 600mW, Multicomp MCMF006FF100JA50
x1	3Ω resistor 600mW, Vishay MBB02070C3008FCT00
x1	10KΩ trimmer, Bourns 3306W-1-103
x2	100pF ceramic capacitor, Suntan TS15001H101KSBSA0R
x2	470pF ceramic capacitor, Suntan TS15001H471KSBPA0R
x1	PCB

Going Off Road..

This kit came packed with components for building a wide range of possible μA723 configurations.

But since I’ve already looked at the basic μA723 circuits, and I don’t really need another regulator, the attraction of course it to try and do something different (even though the 723 is generally though to “not have applications beyond the relatively narrow field of voltage regulation”).

But I spy an error amplifier, so that means it should be possible to get the 723 to behave like some form of oscillator.

Using the 723 voltage reference as a virtual ground, I configured an opamp oscillator circuit around the error amplifier, disabled the current limiter, and got a nice effect for “blinking” and LED at about 2Hz.

NB: Since I’ve left a DC offset on the oscillation, the LED blink is more a “glow”, as it never goes fully off.

I used a breadboard build to experiment and confirm component values:

723_blinking_breadboard