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cheap 60VDC from 120VAC mains? — Parallax Forums

cheap 60VDC from 120VAC mains?

henry99henry99 Posts: 67
edited 2008-09-17 20:27 in Propeller 1
It seems that I can I just use a full bridge rectifier and a cap to get 60VAC at 20Amps from 120VAC without a bulky transformer.

I want to use this power source to power a few big stepper motor with the BS.

Does anyone know of a place I can just buy the above circuit cheap?· Most 1000W 60VDC power supplies seem rather expensive.· Can I just use 4 diodes + a big cap?· I calculate I'd need about a 2.3F cap for 20Amps.· Does this sound right?· Thanks.

Comments

  • evanhevanh Posts: 16,101
    edited 2008-09-14 04:42
    You're gonna get plenty of (expletive) about wiring direct to mains with no knowledge of what you're doing ...
  • RickBRickB Posts: 395
    edited 2008-09-14 04:45
    DON'T DO IT

    Don't know where you got this advice but it will likely kill you and destroy everything you have hooked up to it. If you have to ask, don't do it. How many steppers do you plan on running?

    Rick
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-14 05:18
    henry99,

    120VAC is actually 240 Volts Peak-to-Peak. In reference to 0V or Ground the standard AC voltage goes above ground by 120V, and then below ground by 120V. The potential you "see" of 120V is from a relative perspective of being at ground.

    You are just asking for trouble if you attempt what you are requesting... There is good reason why 1000W 60VDC power supplies seem rather expensive, and a bridge rectifier capacitor combination of any flavor simply won't cut it.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
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  • Beanie2kBeanie2k Posts: 83
    edited 2008-09-14 05:43
    Beau Schwabe (Parallax) said...
    henry99,

    120VAC is actually 240 Volts Peak-to-Peak. ....

    Actually 339.36V. Vpeak = sqrt(2) * Vrms. So 120 X 1.414 = 169.68Vpeak. Vpp = 2 * Vpeak = 2 X 169.68 = 339.36V.

    An unloaded cap connected to a simple bridge rectifier will charge to Vpeak. However it is of course possible with 2 diodes and 2 caps (actually 1 diode and 1 cap if you don't mind a ton of ripple) to create a voltage doubler which will give Vpp in an unloaded state.

    Regardless the basic point is true. PLEASE PLEASE PLEASE if you do not have a thorough knowledge of high power sytems leave their design to those who do. It will save you and those who care about you a lot of grief.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-14 05:51
    Beanie2k,

    "Actually 339.36V. Vpeak = sqrt(2) * Vrms. So 120 X 1.414 = 169.68Vpeak. Vpp = 2 * Vpeak = 2 X 169.68 = 339.36V." .... Correct, I was just trying to get the main point across.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2008-09-15 23:10
    Oh, it all sounds horrible, horrible.

    120VAC is actually 120V RMS (Root Mean Square) which is a way of measuring the "average" voltage of a sinusoidal waveform. Whereas the average of a square wave would be 50% of it's peak voltage as it has a 50% on and 50% off duty cycle. Filter the square wave with an RC circuit and you get "half voltage" but filter it with a diode and a capacitor and you get close to the 5V at light loads.

    Back to sine wave. Once rectified and filtered the resultant voltage will be the peak voltage (but not the peak-to-peak voltage). What you end up with are all the half cycles rectified so that they are no longer a half-cycle positive then half-cycle negative but half-cycles of one polarity (I could say positive but that's like saying us Aussies are "down under"). The "average" voltage is still 120V but now it's DC, once you filter that with a capacitor and provided the circuit is not too heavily loaded you will actually have the peak voltage which is 120V * 1.414 = 170VDC = lot's of smoke in Henry's circuit.

    Henry, everybody is right when they say DON'T DO IT, especially since you had to ask. How expensive is the proper power supply compared to someones life or even the motors themselves?

    BTW, I think your stepper calculations are out somehow as 1200W is a lot of power and they must be EXPENSIVE motors then. Don't get confused with the peak power required when stepping vs holding etc. Best thing is become familiar with small steppers first, do the 101 before you tackle postgrad stuff.

    Can't you just flash some nice colorful little LEDs instead?

    *Peter*
  • TeslaTesla Posts: 55
    edited 2008-09-17 20:11
    Just as an option, you could use a 1kw variac to drop the voltage down.

    For power supplies that large if your going to try always put a fast acting breaker right into the power supply.

    I have some 1.5kw transformers that have multi taps on the secondary side but they take 220Vac in.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-17 20:17
    I wouldn't recommend a Variac. They're not isolated, so still present a shock hazard.

    -Phil

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    'Still some PropSTICK Kit bare PCBs left!
  • TeslaTesla Posts: 55
    edited 2008-09-17 20:27
    Ok, I agree I take back the comment on the variac.

    Henry, before you try anything read up on power electronics. It is a diffrent world than micro controllers all together.
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