I recently came across a really unique book written by Thomas Scarborough. It is an anthology of small designs of six electronic components or less. Sometimes simplicity is all that is needed for a particular solution to a challenge or just a new design. Many design architectures are needlessly complex and use more components (and more cost and board space) than is needed. Efficient design is good design practice.
Details about this book:
Copyright: Thomas O. Scarborough (Standard Copyright License)
Edition: Fifth Edition
Publisher: Tom Bomb Books
Published: March 24, 2014
Reverend Thomas Scarborough is a unique individual. He is a contributor both to philosophy journals and electronics magazines. His technological interests inform his philosophy, and his philosophy his technological interests.
I will be posting a series of these articles on my Power Management and Analog Design Center on EDN. Here is the first.
Editor's note: By all means, please do not build or test this circuit if you do not have an extensive knowledge of electronics and electricity — this circuit in NOT for beginners. There is danger of electrocution or fire unless you have seen the author's warning below as well.
Transformerless power supply
This is another “stock standard” design that could come in very handy. It provides about 12V 20 mA DC from a 230V AC or 115V AC mains supply. While 20 mA is not much to speak of, it is enough for most of the projects in this book.
This circuit, being transformerless , has the advantages of being both lightweight and cheap. However, it is not recommended for beginners. One is “playing with fire.” Not only is there a shock hazard which could be serious, even fatal — there is also the risk of sparks and fire, not to speak of the possibility of shutting down the whole house's electronics. Pay respectful attention to all the component ratings, let all wires be kept a safe distance from each other — say 10mm — and very carefully double-check the circuit before putting it into service.
The components shown here have higher ratings than are strictly needed — I prefer to play it safe. Capacitor C1 reduces the high voltage AC — this component must be rated 400V AC. Resistor R1 discharges C1 when the circuit is not connected to the mains, to avoid electric shock from C1. Bridge rectifier BR1 rectifies AC to DC. BR1 may be replaced by four 1N4007 rectifier diodes. R2 limits inrush current.
C2 reduces ripple in the DC supply, and Zener diode ZD1 regulates the DC output voltage. ZD1 need not be 12V — it may provide any DC output voltage that one would normally obtain from a battery. Take note of the Live (L — brown) and Neutral (N — blue) mains plug connections. In order to double power output, increase the value of C1 to 4.7μ (4μ7).
Keep the safety notes well in mind. A wise piece of advice while working with any mains circuits is to always keep one hand behind one's back — this can avoid more serious shock if one should seriously blunder. The whole circuit should of course be well insulated when in use. Take careful note of the orientation of the bridge rectifier diodes, C2, and ZD1. A 100mA fuse may be inserted in the Live (L) wire if desired, as an additional safety measure.
R1 . . . . . . . . 470k 1 Watt resistor
R2 . . . . . . . . 100R 2 Watt resistor
C1 . . . . . . . . 2.2μ (2μ2) 400V AC capacitor
C2 . . . . . . . . 470μ 25V electrolytic capacitor
BR1 . . . . . . . 1000V bridge rectifier (or four 1N4007 rectifier diodes)
ZD1 . . . . . . . 12V 2 Watt Zener diode (or voltage as preferred)
S1 . . . . . . . . . Mains on-off switch (optional)
100mA fuse and holder (optional)
Suitable plastic case
Suitably rated connecting wire
More information about Thomas Scarborough's books can be found here.
Please feel free to give us your feedback and comments on this circuit.
This article was originally published on EBN's sister publication EDN .