I want to build a 0 - 24V dual( + & - ) regulated power supply. My problem is that I am not sure which transformer to use. The purpose of the power supply is as lab power-supply.

I intend on using the LM317 and LM337 as regulators with all necessary filters and such. Just not sure about the transformer. The Primary Voltage is 230V @ 50 Hz.

Any suggestions?

Edit: Schematic enter image description here

  • 1
    \$\begingroup\$ Schematic?????? \$\endgroup\$ Mar 7, 2013 at 15:30
  • \$\begingroup\$ This is a really tall order for a linear regulator. First, you're not going to get 0-24V. The LM317 regulates down to 1.2V. Second, assuming your transformer and its filtering gives you 2V headroom, at the minimum voltage and max current, \$P=VI=(26-1.2V)(1A)=24.8W\$ Getting rid of that much heat is going to be a challenge. \$\endgroup\$
    – Matt Young
    Mar 7, 2013 at 16:16
  • \$\begingroup\$ @MattYoung That wasn't his question. Also, look at the datasheet carefully. The LM317 can't drop 1A down by 20 something volts. The canonical rating is power dissipation. \$\endgroup\$ Mar 7, 2013 at 17:36
  • \$\begingroup\$ @ChintalagiriShashank I am simply pointing out a problem that I see from experience. \$\endgroup\$
    – Matt Young
    Mar 7, 2013 at 18:00
  • \$\begingroup\$ @LeonHeller updated schematic. \$\endgroup\$ Mar 7, 2013 at 18:40

1 Answer 1


Unless you have specific requirements of isolation levels and such, any transformer will do. For almost all applications that dont include RF the deciding factors are availability, cost, and size, usually in that order.

Make sure that the one you get has its inputs at 220VAC, and a secondary of about 19-0-19 with sufficient current rating for what you need. The 19 comes from the fact that you need atleast 25.2 on the output to give the LM3x7 room to regulate, and after rectification you end up with 1.4 times the secondary AC rating. Best to also leave a little bit of margin on top of that. Don't go too high, though, or you risk inviting a heating problem at the regulator.

  • \$\begingroup\$ Could I use a 24-0-24? Or is that pushing it? Also, is it really necessary to have - as well for a bench supply? This is me building my first ever power supply. I was looking at this as an example. Not quite sure about the theory though \$\endgroup\$ Mar 7, 2013 at 16:10
  • \$\begingroup\$ You can, since the LM can drop upto 35V or so. Be prepared for it to heat up some, though. I would recommend some sort of heat sinking and maybe use the metal can package if your consumption is more than the order of about 100mA or so. Bench top supply is really a vague definition. What you're doing on the bench is a big part of understanding requirements. The transformer is not essential in general, but this is the easy way to go. The other ways are much more difficult to design, atleast design well, and generally include harder to find magnetics.For practical purposes,yes it is necessary. \$\endgroup\$ Mar 7, 2013 at 17:33
  • \$\begingroup\$ I calculated your filter caps need to be 2.651 milliFarads if you want a 5% ripple. Arcsin(.95)=252 degrees and 252-90= 138 and full wave rectification gives you a frequency of 100hz as there are twice as many peaks so if the sine wave goes 1 degree in 1/100/360=.00002778 seconds then ∆T=(252-90)*.00002778= .0045 and the Vmax is 24/.707=33.9463 making a 5% ripple 33.9463* .05=1.69731 and C=I*∆T/∆V so if you want 1 amp then C=.0045/1.69731=.002651254 Farads, so 2.2 milliFarads isn't enough. \$\endgroup\$ Feb 16, 2019 at 0:49

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