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  • Author Author: jw0752
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Bench Suppy for Young Experimenter

jw0752

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.

 

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Here are some pictures of the finished unit.

 

 

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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

  • Let me continue my Illinois chauvinism by recommending PREM brand transformers for this sort of thing.  They make a good-quality transformer, are willing to vend small quantities, have good prices and customer-service.

  • Awww John!  I got a coupla grandkids, Pops says they are just a little young yet for this sorta thing, can hardly wait.  'SCRATCH' is a popular thing for kids on the computer these days, I'm told.

     

    Pilot light might be nice.  Darn!  Its nice already.  You should get a copy of the Motorola Zener Diode Manual.  It is written more at a practical rather than theoretical level, quite breezy and informative, comprehensive but brief.  Now, since my copy is at my other locale we will have to rely on my dimming powers of recall:  Clarence Melvin Zener*, a Saluki, discovered two types of Zener, a Zener-Zener and an Avalanche Zener.  The charge carrier in the Zener is likely to make it through the device, in reverse breakdown mode, as a soliton.  But in the case of the higher-breakdown avalanche zener, the mean free path of the charge carrier is longer than the channel length,  so a charge carrier is likely to collide with another, dislodging it from the lattice, on the way.  Turns out the significance of this (aside from a discussion of noise) is that the two varieties have opposite signums of their respective tempcos!

     

    What they will show you in the book is that you can stack a 6.2 V Zener with a pair of silicon diodes operating in forward-mode.  You would have to make a slight adjustment to your voltage ladder to accommodate the new reference, perhaps.  But through the range of Brewers' season to Packer's** season and beyond, your g'kids device would remain stable, indoors or out!  This is because the various tempco's would cancel each other out so the reference won't drift. The manual is full of other great tips and data sets that you could use to improve this and your other circuits as well.

     

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    *ZENr

    **Everyone down here, alas with a lump in their throats, is rooting for the Pack now!

     

     

    ==========

     

    I see the pilot light now.  Woke up bleary.

  • Nice work ... and REAL meters.

     

    One minor suggestion is an Output switch to instantly kill the volts, and allow setting before applying.

     

     

    Mark

  • in reply to michaelwylie

    Hi Michael,

    Thanks for checking it out. I made the schematic for Christian so he could see the relation between the parts in the unit and the symbols. I will have to wait for Saturday when he comes over again to show it to him.

    John