Consulting engineer and Radio World colleague Charles “Buc” Fitch, P.E., recently had one of those ubiquitous Sine Systems remote control panels on his workbench. He decided to forgo the wall-wart power source and convert it to a plug-in device.
In the broadcast engineering business, you may hear mention of GEP, which stands for Good Engineering Practice. In working on this project, Buc was reminded of one of its more basic tenets: the treatment of the line voltage input.
When 120 volts single-phase enters a unit, the phase line — the 120 volt potential referenced to neutral and ground — should go first to the fuse or circuit breaker, then to the on/off switch, and from there onto the transformers and circuits that use this power.
More specifically, the phase line goes directly to the center pin (the deepest contact) of any bayonet/twist-type fuse holder. The goal is to keep this potential as far as possible from anyone changing the fuse who might touch the capture ring just under the lip of the fuse holder.
GEP also expects that any appearance of this killer voltage will be covered by some sort of insulation or barrier, to minimize any shock.
Obviously if you must have a bare appearance of 120 volt, such as a voltage selection terminal strip, you should mark the presence of the potential on the strip or chassis.
The main board of the Sine Systems RP-8 is undersized for the 2 RU rack panel, which leaves just enough space to install a transformer, fuse and other components on one end and an LED power indicator on the other.
Buc only needed to drill four holes: two for #4 screws to mount the transformer, a 1/2-inch hole for the fuse holder on the right side of the panel, and a 1/4-inch hole on the left for the LED. The positions were chosen so the holes wouldn’t cover the informational lettering on the front of the panel.
Buc harvested the AC cable from a computer. The fuse holder is a Bussmann GMA type (fast-blow) to minimize size. The LED was not critical; Buc chose a Visual Communications L10005 (green). Any diode that can handle better than 20 mA will work.
The Sine uses 12 to 16 volts AC, so this little Stancor 12.6 volt 250 mA transformer (or equivalent) is perfect. The LED is powered by a diode — in this case a 1N4004 because he bought 100 of them for about five dollars — with a series voltage drop/current limiting 470 ohm 1/2 watt resistor. Buc reminds us to observe polarity or the LED won’t light.
The first image shows the entire assembly covered with shrink wrap.
The only item of concern is the exposed AC, and as Buc has noted in the past, those connections on the fuse holder can be covered in GE silicon caulk, seen in the third photo. Shrink wrap buries all other sins.
The final current flow is about 18 mA. Because this LED (like the Sine itself) will run continuously, it’s good to hold down the current. As mentioned, all items are dressed tightly so that the panel can continue to be rack-mounted.
A simple project, using junk box supplies, that gets rid of the wall wart.
PS: If you like modifying equipment, Dan Slentz, our roving internet explorer, shares a fun clip of a Roomba vacuum clear modified to run at 35 mph, affectionately known as the VRoomba! On YouTube, search “Building the World’s Fastest Roomba.”
Don’t forget to send me your list of three things that you absolutely “must check” when visiting a transmitter site. We’ll summarize your suggestions soon including some surprises for best entries. Email your list to [email protected].
Workbench submissions are encouraged and qualify for SBE recertification credit. Email [email protected].