Recently my 8 year old grandson, Christian, has taken to hooking switches, lights, motors and other pieces of salvaged electronics together. When I discovered that he was using an old 12 volt wall wart adapter to power his creations I realized that I could bring some of my new found electronic knowledge together to make a better alternative for him.
My list of important features:
Safe and secure form line voltage
Overload and Short Protection
Reasonable Current at 0 to 12 Volts output.
Easy to read meters
Easy to use controls.
While the unit will be plugged into the wall I have used a grounded metal enclosure and a hospital grade plug. Additional protection will be provided by my son who has agreed to supervise the experimentation.
Thanks to Don Hersey I have the perfect component to use for the output driver. The LM395 Darlington IC can provide 2.2 Amps at my max target voltage of 12 volts. Furthermore the LM395 is thermally and overload protected by internal circuitry. This will give me peace of mind as there is no doubt that many of Christian's hookups will tie V plus directly to ground. Recent tutorials by Peter Oakes have given me the designs I will need to complete the rest of the power supply circuit. I will use a 6 volt Zenner diode to create a voltage reference and a TLE2142 rail to rail op amp to drive the LM395. Current limiting will not be necessary as the LM395 has this all built in. Pre-testing has indicated that the output leads can be shorted together with no damage to the LM395 or any other part of the circuit.
The base power will come from a 15 volt chassis mount transformer, a full wave bridge, and a filter capacitor. I am going to mount these items into a nice heavy gauge aluminum case that was originally a piece of dental equipment. In the cover of the case there will be room for a voltmeter and ammeter. The 2.75 mm thick aluminum case will provide an excellent heat sink for the LM395 and bridge rectifier.
Here is the schematic for the unit.
Here are some pictures of the finished unit.
This power supply should provide my grandson with a way to power his experiments and still protect him from dangerous voltages and currents. Thanks again to Don and Peter for their contributions.
John
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