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110 to 12v converter?

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  • #16
    Originally posted by Fasttrack View Post
    A 12 volt peak input would result in an output that is 12V - 2*Vf where Vf is the forward voltage drop of the diodes. Typically values range from 0.7V to 1.5 volts per diode. If you measure RMS voltage, than you can convert that to Vp by multiplying by ~1.4 (or the square root of 2).

    So, 12 Vrms = ~ 16.9 Vp. You then subtract the two diode drops. Lets assume a Vf of 1.2 volts. Then the output would be ~14.5 VDC.

    Couple the 14.5V with a PWM controller meant for 12V and you'll never know the difference.
    I knew that.


    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton


    • #17
      Regulated power supplies aside, most 'brute force' ones actually deliver from 14 to about 16 volts with no load. If you are going to regulate the output voltage, it makes sense to start with 20 volts, but if you just want to make sure you have at least 12V under full loading conditions, then start with 14-16 volts. None of the motors we're talking about here will care if the voltage is a little high. In a pwm or chopper circuit, the output device when fully on won't waste more than a volt or so, so there's essentially full voltage available to the motor when the output device is on. There may or may not be the need for a higher voltage to begin with, depending on the particular design. I'd suggest a good design of controller won't have this 'extra' voltage requirement.

      That being said, you can then select a transformer with a secondary voltage rating of about 11 vac. That will be under load, so what you end up with is about 15 vdc after rectifier losses. That will drag down to about 12v under load, so it gets you into the right ballpark. If all you can find is a secondary rating of 12vac in a suitable transformer, that's fine.

      If I recall, the power pack that used to come with rv's outputs about 16 volts with no load. It would be perfect for your application. If there's a failure mode in those, for the most part it's going to be in a relay contact or charge controller pc board- usually dirt, heat, or water related damage. I'm suggesting here that even a dead unit (which you could probably get free or cheap) would probably have a working transformer, and probably a good rectifier also. It may need a filter to make 'real' dc out of rectified ac.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-