Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to tell current drawn from a schematic

  1. Aug 19, 2012 #1
    Hi all,
    I am relatively new to the EE field, and while looking at the schematic for a VCO, I had the following question; if i wanted to build a bipolar power supply using a transformer for the VCO, what current should said transformer be able to draw? More specifically, how can I tell the maximum current the VCO will draw by looking at the schematic? I got the schematic from birthofasynth.com, and I will post a link to it below:


    Any comments/help would be greatly appreciated. Not to be picky, but I would prefer an explanation to an answer, that I may be able to learn and use the knowledge in future.

  2. jcsd
  3. Aug 19, 2012 #2


    User Avatar

    Staff: Mentor

    Hi asteinhorn! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    If you do build a regulated power supply, you'll be using a published design so won't it specify the transformer to use?

    If I were constructing from scratch, I think I'd just allow, say, 10mA per amplifier (the triangles), then double that to allow oodles of margin.

    But if you want to be more precise, have a look at the data sheets for those ICs and see what you are told about their currents. You have a quad amp, a dual, and the PLL chip. The transistors will use power, too, of course, at a guess, 40mA total for them should be plenty.

    Sorry if I'm too rough 'n ready for you were hoping. If it's important not to overdesign, your best bet might be to build the circuit then measure its current needs, before looking for a power supply.

    It seems an ambitious first project, for someone new to electronics.
    Last edited by a moderator: May 6, 2017
  4. Aug 19, 2012 #3
    Hi NascentOxygen,

    Thanks for the quick reply! By your suggestions, it would appear I'd need about 370 mA to be safe. Should I take into account any amplification of the output signal, or will that process be entirely external (i.e. feeding it through a dedicated amplifier)? Thanks again.

    By the way, I'm aware that this project is perhaps overly ambitious for a novice such as me (this isn't my first foray into EE, but it is my first significant one), but at least I'll learn something if I fail. That's all that counts, right? :)
  5. Aug 19, 2012 #4


    User Avatar

    Staff: Mentor

    It sounds a lot when you put it that way, I'd be surprised if it drew over 100mA, but I did say to err on the high side. The 4046's are available in various flavours, including some low-power versions which one would choose for a gadget to be powered by battery. The main current users will be the IC amplifiers.

    The VCO will draw a bit more current when you connect it to an amplifier, but that amp will have its own power supply with a lot more grunt.

    Power supplies are always handy to have, maybe you could instead make your first project the assembly of something like a 0-20v 2 amp bipolar regulated power supply kit, where everything is provided, including the board on which the parts need to be assembled? (No, I don't have any particular kit in mind.) You could then assess whether you are up to tackling the more ambitious project.
  6. Aug 19, 2012 #5
    That's not a bad idea at all. I've done some smaller EE projects and I know my way around a schematic (rudimentarily, at least), so I think I'll try to tackle the power supply project, either via kit or with perf-board and an online schematic. Thanks very much for your help and suggestions. I really appreciate it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook