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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.

 

image

 

Here are some pictures of the finished unit.

 

 

         imageimage

 

 

        imageimage

 

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

  • What a nice box!

     

    It is hard to demarcate the boundary to the rococo with these things.  I had a philosophy teacher use an ancient Greek term for 'tumor doctor' to call the process by which an artist quits working on a particular work.  If I ever find that word again, I will be elated! He held that if you kept improving something ad infinitum you will come to sink the project.  That said:

     

    You could put PBs by the meters that would verify that the needles weren't stuck.  They would mid-scale the meters when depressed.

     

    You could have an OVP crowbar:

    http://www.ti.com/lit/ds/symlink/lm431.pdf

    [fig 20.]

     

    For current-sense, we could put a 10 - 25 seimens conductor in the collector leg of our power Q.  We would be neglecting our base current contribution, to fix this we could put an NPN in front of the power Q in darlington configuration, or we could change the power Q to a N MOSFET on a generously-scaled heat spreader.

     

    This would generate a (in the case of the ten seimens conductor) 100 mV drop from the top at the ampere of flow at which you want to clip.  You need to use a fuse of slightly greater current capacity so it doesn't trip first.

     

    To detect this you would need a RRIO comparator.  There could be a switchable reference from the top to set the trip point.  Upon the crossing of the comparator's threshold, we trigger a F/F which 1)  Floods the inverting terminal of the op-amp with current so as to saturate it and shut down (or by another method) the output stage.  2) Turns on a red light.  The other side of the F/F is a PB that won't need much debouncing.  You might need to provide some dynamic de-sensing (likely a capacitor) to not be hyper on spikes.  Of course, provisioning will have to be made to reset the F/F on power-up.

     

    Use comparators or si-gate CMOS for the F/F and it won't need a local regulator.

  • in reply to D_Hersey

    Thanks Don, As usual your post is full of good information. I will followup and check out the Zenner Manual and PREM Transformers.

    John

  • in reply to mcb1

    Thanks Mark, This is a good suggestion and easy to implement. I will make it happen.

    John

  • I like it, I recognize the basis of the circuit, nice to see you applying what we are learning on our journey together, a nice addition might be a current limit, not sure there the pot would fit though image, this way he can protect his projects when not sure of the current requirements

     

    Well done

     

    Peter

  • Nice! I bet there are not many lucky 8-year olds with equipment with analog meters : )