Workbench: wet square wave output of capacitors
Frank Hertel follows up on a question about his LED dimmer project
Posted: July 14, 2021
Rob Atkinson, K5UJ, is an engineer from Saint Charles, Ill. And one of the many readers who enjoy the DIY projects that we feature from time to time in Workbench.
He writes to say that he admires Frank Hertel’s ingenuity in home-making the LED dimming circuit we told you about in the May 26 issue. However, as a ham, Rob is concerned about the RFIs caused by variable square waves and that there was no mention of RFI mitigation measures in Frank’s submission.
Rob points out that any type of square wave based control circuit can be a terrible spectrum polluter, especially if the wires leading to the controlled device are long and unshielded. His concern is that these ideas often end up on the Internet, where they are reused by unsuspecting short- and medium-wave enthusiasts.
Frank Hertel replied, “Since radio amateurs sometimes operate with weak signal reception” – which Frank does when he measures distant AM and FM signals – “I can understand Mr. Atkinson’s concerns.
Frank says Rob is right. See Fig. 1, the diagram of the dimmer. The 555 Timer IC produces a fairly square wave output, so filtering is important. However, the 6.8K resistor that is in series with the output of the 555 to the gate of the MOSFET MPF102 is bypassed to ground with a 1 MFD capacitor (unpolarized) and does a fairly good job of transforming the square wave. into a semi-sine wave (asymmetric shaping).
Finally, an additional damping filter capacitor, the 100 MFD capacitor of the D718 emitter to ground that connects to the LED fixture, filters a bit more.
Frank realizes that he could have made a more sophisticated output filter array, but found that all of the emissions from the dimming circuit were consistent with the emissions from commercial dimmers and dimmable LED lights, that is. so there he decided to let it see on the diagram.
In these oscilloscope photos and spectrum analyzer shots, the probe is connected through the output / LED light fixture 555 at different LED intensities.
The LED Enemy
San Francisco contract and project engineer Bill Ruck was interested in Frank’s interesting dimming circuit, but more so in the super flexible silicone wire used. On Amazon.com, type “B07K9R9LBV” into the search box to find this 22 awg silicone electrical wire. Frank used the 22 caliber but other calibers are available.
Bill asked because most of the red / black “zipper” cables he saw are relatively stiff. He points out that most LED luminaires have a built-in regulated current switching power supply. If the fixture had just one current limiting resistor, it would gradually decrease.
Bill recounted a personal experience in which he replaced his circular kitchen “dome” light with LEDs. It turns out that the two round “Circline” fluorescent bulbs are no longer manufactured by GE. mjpa.com) with the appropriate diet. He then used the light box to build an LED based lamp.
The lamp worked great for about six months and then failed. This time he spaced the LED assembly under the ceiling with an extension box. This too failed in time. The repair saw Bill add heat sinks around the LEDs.
The moral of the story ? Heat is the enemy of long-lasting LEDs! Next time, Bill will rebuild the fixture and place the heat sinks on the outside of the box to better dissipate the heat.
John Bisset, CPBE, has 50 years in the broadcast industry, including 31 years writing Workbench. He handles radio sales in the western United States for the Telos Alliance and has previously received the SBE Educator of the Year award. Workbench submissions are encouraged and qualify for SBE recertification. E-mail [email protected].
