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Circuits designed by David Johnson, P.E.
Last Updated on: Thursday, April 07, 2016 02:04 PM

Master Category List - Dave's Circuits

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AC to AC Converters

0VAC to 280VAC Variable Isolated AC Voltage  -  I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/08/00]


20ma Current Loop Tester  -  A while back I needed to test a circuit at 240vac.  Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line.  So, to step up the voltage, I bought a small isolation transformer from Digikey . . . [Circuit by Dave Johnson P.E.,  05/29/11]

4-20ma Current Loop Tester  -  This circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps . . . [Circuit by Dave Johnson P.E.,  05/01/12]


AC Current Monitor  -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.001-ohm resistance.  It can easily handle 30 . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/10/06]


Caps Provide Voltage Boost to Series Regulator  -  This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply . . . [Circuit by David A. Johnson P.E.,  01/01/98]


Classic Plus & Minus DC Power Supply  -  This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter . . . [Circuit by David A. Johnson P.E.,  10/16/05]


Isolated AC Current Monitor  -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.001-ohm resistance.  It can easily handle 30 . . . [Circuit by David A. Johnson P.E.,  07/10/06]


Isolation Transformer Circuits  -  A while back I needed to test a circuit at 240vac.  Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line.  So, to step up the voltage, I bought a small isolation transformer from Digikey . . . [Circuit by David Johnson P.E.,  05/29/11]


Low Power 12,000 Volt Power Supply  -  If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line . . . [Hobby Circuit designed by David Johnson P.E.,  03/27/99]


Monitors AC Current  -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.001-ohm resistance.  It can easily handle 30 . . . [Hobby Circuit designed by David Johnson P.E.,  07/10/06]


Plus And Minus DC Power Supply  -  This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/16/05]


RS-232 Serial Interface Status Indicator Circuit  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volts.  Two dual color LEDs are used as indicator lights . . . [Hobby Circuit designed by David A. Johnson P.E.,  12/09/07]


Tester 4-20ma Current Loop  -  This circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps . . . [Hobby Circuit designed by David Johnson P.E.,  05/01/12]


Variable AC Power Supply  -  In electronics, it is often handy to have a fully isolated variable AC power supply.  With such a device you can safely test various AC powered circuits and even probe around in the circuit with an oscilloscope.  The hobby circuit below was designed to . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/08/00]


Variable Isolated AC Voltage Spans 0Vac to 280Vac  -  I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel . . . [Circuit by David Johnson P.E.,  07/09/00]


Voltage Boost to Series Regulator provided by Capacitor  -  This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/01/98]


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