Nine year old grandson, Ivan, and I had two projects on the schedule for his Christmas Vacation. The first was to build a special Solder Iron Modification, See: Solder Iron for Ivan with a built in Timeout
With the solder iron complete our plan was to use it to build the second project, a small bench power supply. We wanted a supply that could deliver up to 15 volts at 2 amps. We also wanted it to be protected from shorts and easy to use. Ivan is just beginning and I want his initial explorations and experiments to be safe and fun. We began our build with an old Dental Composite Cure Light:
We removed all the internal circuitry of the Cure Light except the power switch and its wiring harness. I had previously built a small bench supply like this and so I wanted to use the same basic circuit design with some new twists and improvements. The heart of the control would be a single LM395 power darlington driven by a rail to rail TLE2142 Op Amp. I like to use the LM395 as it has a good internal overload protection circuit. Once completed Ivan will be able to short the outputs at full voltage and suffer no damage or ill effects. Here is the schematic for our power supply. The LED Volt Amp display is a Chinese special with 0 to 99 volts and up to 10 A as configured. I have it in the schematic as a black box component. If anyone needs more information on it let me know in the comments.
Our initial steps were selecting the proper Transformer and laying out the other components, in the room left over after the transformer took up half the enclosure. We also began to plan our control panel by making a cardboard template. Here you can see some of the controls as they will be positioned on the unit.
Once we had the template we marked and cut the aluminum of the enclosure. We also marked and cut a piece of white plastic which would serve as a front cover for the original markings on the Cure Light. Ivan used a coping saw to cut the Aluminum for the rectangle meter hole and we used the drill press for the round holes. We left the power switch in its original spot. After lots of cutting, filing, and adjusting here is the finished control panel.
When Ivan came over after lunch today the only thing left to build was the little control circuit board with the TLE2142 Op Amp and the related resistors and components. This was the part that Ivan was most excited about as he would be using his new solder iron. Here is a picture of Ivan soldering a connection. You can see his solder iron control in the back ground. During the 3 hours we worked today the solder iron faithfully shut off ever 30 minutes, each time with a little beep so we would know to push the start button as we wanted to continue soldering. Please note that I forgot to enforce the must wear eye protection rule. This is a serious oversight on my part and will be corrected in all future builds. No excuses. Too much excitement and too much to do in a short time.
I include also a closeup of the board we are installing.
Here is the top view of the unit with the control board mounted into its proper position
After the board was hooked into the circuit we made the first test of the unit. As is usually the case, all did not go well. The fuse of the unit popped and it took me a couple fuses before I realized that there wasn't anything wrong except the 8000 uF of filter caps were creating too much initial current. A one amp slow blow solved this problem. The red power LED while only drawing 12 mA appeared too bright and was distracting so we changed the series resistor from 1.5 K to 2.2 K. While we were getting a controllable output voltage, it would not go below 2.3 volts and the Green Active LED would not light. Careful inspection of the circuit board showed that Grandpa forgot one jumper tying the base of Q2 to the Vout. I was frankly pleased with the failures that we encountered. I can handle anything short of smoke and fire and it gave Ivan a chance to see the troubleshooting process. We improved the minimum voltage by putting a small 20 mA Max load on the output of the unit. This is the 2 watt 680 Ohm R10 on the schematic. With the load resistor in place the minimum voltage came down to 0.75 volts. Now we wanted to test the unit under an external load load. The first picture shows the unit set to 12 volts and no load. In the second picture we have put a 12 volt car tail light on the unit which drew 510 mA. We were pleased to see the regulation kept the voltage at 12 volts.
You may notice the switch on the left side of the control panel marked ENGAGE - OFF - TEST. This was an idea given to me by mcb1 when I built the first supply similar to this one. It is a SPDT switch with a center off, a momentary in the down position and an ON in the up position. This switch connects the output of the LM395 to the Red and Black output binding posts. The green active LED is only on when there is voltage on the binding posts. This arrangement will allow Ivan to hook up to a test circuit and to momentarily TEST the circuit to see what happens before switching to the ENGAGE position. Further testing showed that the unit was capable of 11.5 volts at 2 amps. For a burn in test we hooked up (2) automotive tail lights in parallel for a total of 1.04 amps at 12.1 volts and left it running for an hour. The case was found to be just warm to the touch after this time.
This was a great adventure to build the Soldering Iron and Bench Power Supply with Ivan over his Christmas Vacation. The next time he comes to visit we will build some test leads and put together some small bread board project circuits that he can take home and experiment with using the new power supply.
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