Inexpensive basic bench supply from MCM

Discussion in 'Electronics Resources' started by marshallf3, May 2, 2011.

  1. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010


    Ideal for any device drawing up to 2A
    Fully regulated to deliver constant DC power
    Rotary selector switch selects 3, 41/2, 6, 71/2, 9 and 12V
    Power projects or small battery operated devices
    Color coded binding posts makes connection easy
    Ultra-compact, lightweight design
    Power on LED indicator
    Dimensions: 5 3/16" (L) x 2 5/8" (H) x 3 7/8" (W)

    Not continuously variable and you might be able to go inside and tweak the 4-1/2V output such that it's 5V. Regardless, for the price it would be a handy addition to most people's benches that are just starting out.

    I went ahead and ordered one, I hope it will be easy to convert that 4.5 to a 5V. Worst case I can sneak a 7805ACT (good for 2.2A if well heat sinked) regulator inside and add an extra binding post.

    When ordering 7805 regulators I usually get these due to the high current capability: and they have the same 7V input voltage requirement as the supposedly low dropout versions: NCP7805TG
    Last edited: May 2, 2011
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    I have one of them in my classroom, it's 3 years old, and the students haven't blown it up yet.
  3. someonesdad

    Senior Member

    Jul 7, 2009
    I clicked on the link you gave and it shows the device for sale at $30.
  4. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    It worked this morning from the e-mail link I got, then I pulled it back to the product number and it still showed $12.99 which is all they charged my PayPal account. I went back to the e-mail link and it does show up as $12.99, perhaps if you register real quick they'll send you the flyer but it's only a Monday deal so you'd have to get moving:

    This link should show it:

    It's also possible if you sent them an e-mail immediately saying you had seen the $12.99 price on it earlier after a friend told you about it they'd still honor the price. Regardless I'd sign up for their newsletters, same thing if you aren't on Goldmine's mailing list. Neither sem to bombard you with e-mails like some others do.

    I've pretty well decided to leave the 4.5V output as it is and just add a 7805ACT regulator and an extra binding post for the 5V @ 2A output. Alternately I may put an SPDT switch to the binding post that switches between the 5V and the normal rotary selector as the output voltage, that way I'll only need to add a tiny switch instead of another large binding post.
  5. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    They had them on sale for $12.99 not long ago and probably will again some day.

    It's a true multi-tapped large 60Hz transformer design driving an LM317T regulator IC. Rotary switch selection of 3, 4.5, 6, 7.5, 9 & 12V output all at 2A. The output binding posts aren't true 5-way as there's no hole in the threaded part, you just have to squeeze any wire connection between the mating surfaces. Banana jacks seem OK though.

    With no load the worst case was the 6V output only being 5.96V, the rest of the voltages were off by a mere +/- 0.01V so that's pretty good. 2,200 uF main filter cap may be a little small for a 2A supply but there's room enough to wedge some additional filtering in there. Ripple would be fairly low since the adjustment leg of the LM317T has a small filter cap from the adjustment leg to the ground. I haven't tested it under any loads yet but in my experience that reg IC is usually pretty stable as the load changes.

    As to adding a 5V 2A output (which I intend to do) there's a 10mm x 80mm x 55mm area that would accept several different heat sink options from Aavid or Wakefield for holding an LM7805ACT regulator IC that's also good for 2.2A output if well heat sinked. Then you just sneak a mini SPDT switch in there such that in one position it worked with the factory rotary switch output or switch the main switch to the 7.5V range and have the positive output binding post change over to the +5V regulator IC output. Alternately you could change the resistor value that sets the reg IC to 7.5V and just lose that range, not much that uses 7.5V anyway. Now I'm leaning in that direction as it would be far easier and only involve a single resistor and a black Sharpi pen to black out the 7. part of the 7.5V that's listed on the case, leaving it showing up as 5V.

    Is it worth full price or waiting to see if they ever go on sale again? Well, I think it's a pretty good deal either way if you need a battery eliminator that will emulate anywhere from 2 - 8 cells and with a few additional parts and work becomes a +5V @ 2A supply I think either price is acceptable due to it having a true multi-tapped power transformer instead of being a switching supply. It has a small footprint, clean lines (looks professional) and appears to be right on the money as far as output regulation goes.

    My guess on the reason for using the multi-tapped transformer is that they can feed the LM317T regulator IC with just enough voltage to start with such that it can get by with a small heat sink. Let's face it, if that IC had to drop 12V down to 3V at 2A it would have to dissipate 18W.
    Last edited: May 6, 2011