Over the years of Tube Amp repairs, and old instrumentation restorations, I have always relied on an incandescent bulb current limiter, in conjunction with a Variac. I finally decided to build a piece of kit to make this cleaner, and add a much needed isolation transformer.
It consists of two circuit boards, a Relay controller (no software, just good old logic), and a Lamp board. It contains a large 1000VA toroidal isolation transformer for isolation. There are three duplex AC receptacles on the rear panel that are not isolated, one being a 20A GFCI. The other two are switched on/off by the front panel. There are two ground isolated duplex AC Receptacles on the front panel. One is directly connected to the Isolation secondary, the other is bulb filament current limited by selectable incandescent bulbs controlled by relays. The bulbs are visible through the front panel making it easy to see if any current limiting is taking place. There is an input and output for connecting a Variac to use in conjunction with the current limiting section. The Variac input and output is also isolated from the mains.
Since the transformer had low voltage windings, I rectified and filtered them to provide fixed DC of around ±24V. I just hate wasting windings. The variac is also connected to a pair of Banana Jacks in case I need an variable AC source.
There is a Chinese made Power meter that monitors the isolated outputs.
The build was fairly flawless. I handplaced the SMD parts using a solderpaste stencil, and reflowed them in a modified toaster oven. Everything soldered fine, but I had a couple of bridges on a TSSOP part that I had to clear. The LEDs are too bright at only 5mA, so I might go back and change all the LED resistors. I used RGB LEDs for all of them, and pinned out all three LEDs. I could make any color based on which resistors I placed. I mixed some colors by using more than 1 resistor on them. The LEDs are Right Angle Light Piped to the front panel.
The Lamp Board uses E17 sockets for the incandescent bulbs. I designed this with 20W and 40W E17 bulbs which will not be discontinued as they are oven light bulbs which are exempt from being discontinued. I did discover that a 40W bulb really only draws 32W bulb at 120V which kind of threw my nomenclature into error. However, I was able to kludge in "Calibration" bulbs to get it closer to the desired levels. You can see the kludged bulbs on the back of the lamp board in the open top picture attached. I added a 40W (really 32W) to the 160W bank which then totaled actually 160W. I added a 15W E17 to the 80W bank (two "40"W bulbs) which then actually totaled 79W. I'm going to live with the 40W setting being 32W. The 20W actually was darn close when I used a 25W bulb there. I should have anticipated that 40W bulbs are 20% low. This is consistant no matter what brand I tried. I think I will make a modification to allow an external lamp to be put in parallel for higher powered applications, but so far, it has plenty of power for what I'm doing.
The final build had a lot of wiring to hook up. Mostly wired with 14AWG Elastomeric Insulated hookup wire. I used ferrules in all the terminal block wiring. I used lever Terminal Blocks from WAGO. The picture showing the open top was before I dressed the wiring up neater. There is a lot of wiring in there!
The enclosure panels were supplied by the excellent SENDCUTSEND company and were laser cut. Front, top, and bottom panels are 100mil Aluminum, and the rear panel is 70mil aluminum (because some of the press-fit connectors I used on the back would not work in 100mils). The Front panel is powder coated, also by SENDCUTSEND. I brushed the other panels. The side panels are heat sinks provided by HeatSinksUSA. They are extrusions that are thick enough at their base to drill and tap all the holes that hold the whole thing together. I didn't really need heat sinks, but they were readily available in the size I needed. The price for doing all the aluminum was surprisingly low, and far more accurate than I could manage with drill press, not to mention I have no good way of cutting thick aluminum panels.
The labeling on the front and rear panels is Vinyl originally designed for transfers to things like cups or fabric. But it allows full color, quite detailed nomenclature. I'm really happy how that turned out and it seems quite durable. The labeling was made by Ninjatransfers with their UV DTF Gang Sheet product. I suppied art from Adobe Illustrator.
I have spare circuit boards. If anyone wants to make one (for private use only), I am willing to share all the files for the enclosures, and I have left over raw PCBs.
It consists of two circuit boards, a Relay controller (no software, just good old logic), and a Lamp board. It contains a large 1000VA toroidal isolation transformer for isolation. There are three duplex AC receptacles on the rear panel that are not isolated, one being a 20A GFCI. The other two are switched on/off by the front panel. There are two ground isolated duplex AC Receptacles on the front panel. One is directly connected to the Isolation secondary, the other is bulb filament current limited by selectable incandescent bulbs controlled by relays. The bulbs are visible through the front panel making it easy to see if any current limiting is taking place. There is an input and output for connecting a Variac to use in conjunction with the current limiting section. The Variac input and output is also isolated from the mains.
Since the transformer had low voltage windings, I rectified and filtered them to provide fixed DC of around ±24V. I just hate wasting windings. The variac is also connected to a pair of Banana Jacks in case I need an variable AC source.
There is a Chinese made Power meter that monitors the isolated outputs.
The build was fairly flawless. I handplaced the SMD parts using a solderpaste stencil, and reflowed them in a modified toaster oven. Everything soldered fine, but I had a couple of bridges on a TSSOP part that I had to clear. The LEDs are too bright at only 5mA, so I might go back and change all the LED resistors. I used RGB LEDs for all of them, and pinned out all three LEDs. I could make any color based on which resistors I placed. I mixed some colors by using more than 1 resistor on them. The LEDs are Right Angle Light Piped to the front panel.
The Lamp Board uses E17 sockets for the incandescent bulbs. I designed this with 20W and 40W E17 bulbs which will not be discontinued as they are oven light bulbs which are exempt from being discontinued. I did discover that a 40W bulb really only draws 32W bulb at 120V which kind of threw my nomenclature into error. However, I was able to kludge in "Calibration" bulbs to get it closer to the desired levels. You can see the kludged bulbs on the back of the lamp board in the open top picture attached. I added a 40W (really 32W) to the 160W bank which then totaled actually 160W. I added a 15W E17 to the 80W bank (two "40"W bulbs) which then actually totaled 79W. I'm going to live with the 40W setting being 32W. The 20W actually was darn close when I used a 25W bulb there. I should have anticipated that 40W bulbs are 20% low. This is consistant no matter what brand I tried. I think I will make a modification to allow an external lamp to be put in parallel for higher powered applications, but so far, it has plenty of power for what I'm doing.
The final build had a lot of wiring to hook up. Mostly wired with 14AWG Elastomeric Insulated hookup wire. I used ferrules in all the terminal block wiring. I used lever Terminal Blocks from WAGO. The picture showing the open top was before I dressed the wiring up neater. There is a lot of wiring in there!
The enclosure panels were supplied by the excellent SENDCUTSEND company and were laser cut. Front, top, and bottom panels are 100mil Aluminum, and the rear panel is 70mil aluminum (because some of the press-fit connectors I used on the back would not work in 100mils). The Front panel is powder coated, also by SENDCUTSEND. I brushed the other panels. The side panels are heat sinks provided by HeatSinksUSA. They are extrusions that are thick enough at their base to drill and tap all the holes that hold the whole thing together. I didn't really need heat sinks, but they were readily available in the size I needed. The price for doing all the aluminum was surprisingly low, and far more accurate than I could manage with drill press, not to mention I have no good way of cutting thick aluminum panels.
The labeling on the front and rear panels is Vinyl originally designed for transfers to things like cups or fabric. But it allows full color, quite detailed nomenclature. I'm really happy how that turned out and it seems quite durable. The labeling was made by Ninjatransfers with their UV DTF Gang Sheet product. I suppied art from Adobe Illustrator.
I have spare circuit boards. If anyone wants to make one (for private use only), I am willing to share all the files for the enclosures, and I have left over raw PCBs.
