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[SOLVED] ~50-100V Voltage limiter

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Member level 3
Jun 2, 2009
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Lyngby, Denmark
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Hey guys.
I am scratching my head here.
I got a power source which varies from 20VAC to 70VAC.
I want to limit this to 12V.
Now my first attempt was to use a step down converter made from a LT1074 we have at work, but this was before I knew the supply could go above 45V.

How do I limit the input voltage to ~40V Max ? Is there any easy way to do this? The minimum voltage I need for my step down converter is about 10V.

When you say "limit", do you just mean converting the variable AC to a fixed DC output?

For that what is normally done is you use a stepdown converter which has a high enough input voltage range. This usually requires using a converter with an external switching FET, unlike the LT1074 which has an internal FET. Controllers with internal FETs generally can't withstand high input voltages. The converter itself needs to be powered with a lower voltage, but this can be done using a low power shunt regulator.

Well yeah, ultimately I would like 12VDC and able to drive atleast 250mA.

Could this possibly be achieved with a circuit like this:
Opamp I have MCP6272 and transistor I have TIP122 :S

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Maybe with a zener diode as the reference?

If you only need 250mA out, then a clamping technique might work, but it will be inherently power inefficient, and have poor regulation and high ripple. Do a search for threads on transformerless power supplies to get more info.
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I am marking this as solved, and going to try and solve the whole problem with the system giving such high power to begin with. It's the power from my moped I am rectifying, and the power is way higher than it shoud be :)

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