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Topic: Old educational power supply upgrade. (Read 5044 times) previous topic - next topic


I'm a high school teacher and have about 16 power supply  that i want to upgrade.  I repair them all but there is still a problem : they present, on a oscilloscope, ripples  with a load and they don't present constant voltage when varying the load.

They are very simple construction :  120V to a VARIAC, VARIAC to a transformer (27v AC max), tansformer to a bridge rectifier that have a 1000uF capacitor in parallel with the "DC" output. They are built like tanks!

I tried to add parallel caps up to 10000uF, it helps for the ripples but that is not perfect, expecially with high current.

These power supply are rated 30v 5A VARIABLE  "DC"

What can i do to remove the ripples and to regulate the variable output.  I'm looking for an upgrade circuitto add in series  that will do the job that i can put in the exixting box without too much mods.

In the old days we only power light bulbs, ceramic resistors and switches.  Not we uses LED, transistors, buzzers and "delicate" components, arduino's.  So these power supply are no more suitable for my teaching...



Feb 21, 2013, 02:40 am Last Edit: Feb 21, 2013, 02:50 am by retrolefty Reason: 1
That would be a neat project. It you can find them (not sure they are still in production) the LM138/LM338 is an adjustable 3 terminal 5 amp voltage regulator that might be a simple add on. It will need to have a heat sink to run at full rated power, but would seem an easy upgrade. The datasheet shows typical circuitry to get them going, typicaly just one fixed resistor and one variable resistor and a couple of small filter caps mounted close to the regulator terminals.

Check it out:



Kit version:



That last one has a schematic that you might be able to utilize if you DIY.



Feb 21, 2013, 03:32 am Last Edit: Feb 21, 2013, 03:45 am by CrossRoads Reason: 1
I'd be more tempted to add switching regulator to each, maybe a couple:
3.3V, and 5V.
You've found those inexpensive Chinese regulators Lefty? Put a couple of those on the output.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Feb 21, 2013, 05:30 am Last Edit: Feb 21, 2013, 05:34 am by retrolefty Reason: 1

I'd be more tempted to add switching regulator to each, maybe a couple:
3.3V, and 5V.
You've found those inexpensive Chinese regulators Lefty? Put a couple of those on the output.

Well he mentioned 5 amp capacity on his basic DC unregulated supply so I assumed he wanted full rating value for any value voltage he adjusted to. But your right, switching DC to DC regulator(s) would be another viable alternative.

But the fact that his DC PS has a variac adjustable input he can pre-adjust the input voltage to the regulator to be just a couple of volts above the adjusted regulated output voltage and run the regulator in a pretty efficient manner. Variacs are great (if not large and heavy!), I got a big 10 amp unit in the garage.



@ Lefty and Crew LM338 K (Steel) are or were available in June of last year, I bought 10 for $10.00... Yeah I know that there are switchers that do a better job for much less.. But those patrts are nostalgic too.. Made my first DC power supply with one for my CB radio base station... Docedison was my handle then too...

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