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Old educational power supply upgrade.

Discussion in 'Troubleshooting and Repair' started by geologie, Feb 21, 2013.

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  1. geologie


    Feb 21, 2013
    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. So these power supply are no more suitable for my teaching...

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The ripples will diminish with bigger filter capacitance, but will never vanish when a load is applied. A rule of thumb is 1000µF per 1A of output current, so for 5A at least 4700µF are recommended. I suggest using 2*3300µF in parallel.

    Here is a rather simple 5A linear regulator IC. It can be used for regulating and adjusting the output voltage.
    If you're going to use this, remember to add a hefty heatsink.

    Or get a kit like this . You may have to look around a while for a 5A kit. The ones I have found doing a quick search were at most rated up to 3A.
  3. harveyradar


    Feb 13, 2013
    I have a number of similar units currently - I repair science equipment for schools here in Australia.

    I can do a little experimentation for you if you care.

    Can you give me make and model details of the units you have?

    What country are you in?

  4. shrtrnd


    Jan 15, 2010
    I'm with Harald, a voltage regulator will get you what you want.
    They're not all that expensive.
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