Power supply transformer

Discussion in 'The Projects Forum' started by dedalus, Jul 24, 2014.

  1. dedalus

    Thread Starter New Member

    Jul 24, 2014
    I wish to construct a 0 - 24 V variable DC power supply capable of delivering 1.5 A. I have a schematic. Problem: it specifies a 24 V transformer. Radio Shack (about the only option here in NYC) doesn't sell them.

    I do, however, have 2 center tapped 12 V transformers. I believe there is a way to hook them up together to deliver 24 V. But I don't know how. Can someone help me out? I really don't know much about electronics (no kidding) and I'd like to get on with what I want to do with it (aluminium anodizing) rather than learning. I know this is kind of lazy, but perhaps someone could take out a moment to guide me? Thanks in advance.
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Always use a higher voltage output than you need,like a 30v transformer as it will maintain the voltage at higher currents.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    If they are identical, then do this.

    Ignore the CTs on the secondaries. Wire one end of transformer A's secondary to one end of transformer B's secondary. Call that your new center tap (if one is required).

    Hook the primaries in parallel, and temporarily apply 120V power. Use an AC voltmeter to measure between the unconnected ends of the two secondaries. It the voltage is near zero, reverse the primary leads to either transformer and try again.

    If connected right, you should have about 24V between the two secondary wires.
  4. to3metalcan


    Jul 20, 2014
    MikeML's suggestion is a good one...I've done this exact thing with RadioShack transformers! A repair customer blew the primary in a set of portable powered speakers. He was in a hurry, and the neighborhood 'Shack didn't have a 24V that was up to spec.

    BTW, are there really no indie electronic stores in NYC???
  5. dedalus

    Thread Starter New Member

    Jul 24, 2014
    Most sincere thanks! Anyone ever need to know about electroplatiing, or that kind of chemistry, feel free to ask me.
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Is this for electroplating?

    Why a variable DC power supply? Why not use a constant current cupply?
  7. Lestraveled

    Well-Known Member

    May 19, 2014
    I have a question about electroplating. In the 90s I built a competition pistol. I wanted to hard chrome the receiver so I took it to a local electro-plater who assured me he could do a good job. A few days later I had a great looking receiver. A few months later it started cracking and pieces started breaking off. Long story short, hydrogen embrittlement from too much current during the plating process. I sent the replacement receiver to a company that specializes in gun hard chroming and all was fine.

    What really happened? Was the hydrogen embrittlement from too much current? Could you shed some light on this. I would really like to understand this better.


    At this point in the thread, I hope everyone does not mind a minor deviation in the topic.
    Last edited: Jul 26, 2014
    #12 likes this.
  8. crutschow


    Mar 14, 2008
    If you rectify the voltage from a 24VAC transformer its peak voltage is about 33V so I see no reason for a higher voltage transformer.
    #12 likes this.
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Actually, it is a combination. Not enough chrome in solution can cause the water to be reduced to hydrogen instead the electrons reducing the dissolved chromium ion. Too high of voltage could cause everything to start reducing (water and chromium ion), and, a substrate that conducts poorly (iron/steel) can cause problems. Also (any surface treatment (phosphating or even residual oil) can screw up the plating.
    Last edited: Jul 27, 2014
  10. dedalus

    Thread Starter New Member

    Jul 24, 2014
    Sorry, I haven't been here for awhile.

    Chrome plating: the kind of steel your firearm is made of requires post plate baking to get the hydrogen desorbed. Chrome plating is the hardest kind of plating there is. The plating doesn't like to go into low current density areas (this is called 'throw.') and only about 20% of the juice goes into deposting the metal; the rest is blown off as hydrogen.

    It does sound like you solved your problem, you found someone who could do the job right. A good, properly done chrome deposit is a great finish.

    There is an excellent website, finishing.com, that I contribute to a lot. It's a great resource. Take a look. You can often get really good, detailed answers to plating questions from people who've been plating longer that I've been alive.
    Last edited: Oct 9, 2014
  11. dedalus

    Thread Starter New Member

    Jul 24, 2014
    Oh, my power supply. I wired it up! The light lit! It worked...for about 15 minutes. Then, like an idiot, I shorted it out by accident and fried the voltage regulator.

    Two questions: is there a solid state, single package regulator that's a little...sturdier? 1.5 A is not very much. Is there one that will do 3 - 4 A? or do I have to build it out of discrete components? And, is there a socket, or something, like they sell for 8 pin IC's that I can just plug the 3 pin regulator into, so I don't have to solder it directly?

    Oh, and one more thing: I got a fuse assembly (like I should have done to begin with) and I wonder if it should go upstream or downstream of the regulator.

    Thanks in advance for any help, most sincerely.
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
    Here is a good one that should be bullet proof:
    Hook it up like fig.17 with a .1uf close to the input.
    If you use a fuse, put it between the big capacitor, after your rectifier, and the regulator then it will protect the most "stuff".
    It will need a large heat sink if it is to run high current at low voltage.
  13. ISB123

    Well-Known Member

    May 21, 2014
    What regulator did you use?I shorted LM317T few times and it would just go in short circuit protection mode which will generate a lot of heat.So if you had a heatsink it shouldn't been fried.
  14. #12


    Nov 30, 2010
    I recommend NOT to use sockets. An electroplating room is corrosive. Anything that is not soldered is going to be trouble. Besides that, 3 amps is too high to trust to friction on the tiny pins of a transistor.

    So, do your best to keep the atmosphere out of the power supply box, and solder everything anyway.