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Old 28th May 2012, 02:34 PM   #1
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Default 12VDC power supply

Dear all,
I have diy a 12VDC power supply using a 220V to 15V AC transformer. but the measured DC ouput is 19V dc.
Is there any simple way to correct the voltage to 12VDC?
Thanks
matthew
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Old 28th May 2012, 02:47 PM   #2
AndrewT is offline AndrewT  Scotland
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regulate.
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Old 28th May 2012, 03:37 PM   #3
Ar4 is offline Ar4  Latvia
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Check for LM317/LT108x for adjustable solution, or LM7812/LT108x-12 for fixed, 12V version. If you plan to use heavy loads, you probably would want to remove some windings, because linear regulators waste unneeded voltage in heat, but for low current applications small heatsink will do the trick.
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Old 28th May 2012, 04:52 PM   #4
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Why donīt to try a switching regulator as STīs L497x series? They come up to 10amp. output currents.
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Old 28th May 2012, 05:03 PM   #5
RJM1 is offline RJM1  United States
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#1, How much current do you need to get out of it?
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Old 29th May 2012, 12:01 AM   #6
gootee is offline gootee  United States
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With what load current did you measure the output voltage? And what was the AC Mains voltage at the time?

If the load current is to be variable while the voltage needs to stay at about 12 Volts, then a three-terminal regulator might be a good choice, or, a switchmode regulator.

You first need to figure out what the likely range of transformer output voltages would be, with the minimum and maximum likely AC Mains voltage, transformer regulation, and the minimum and maximum load current.

You can calculate the required minimum smoothing capacitance, based on the worst-case combination of the transformer output voltage and load current ranges, to make sure that the bottom of the ripple voltage waveform Never causes the regulator's input minus output voltage to get too close to the regulator's specified dropout voltage (otherwise the output voltage gets really ugly). i.e. Calculate how much smoothing capacitance is required to ensure that the ripple voltage waveform, before the regulator, can never dip down too near to a voltage equal to the regulator's output voltage plus its dropout voltage spec.

All of the basic equations and concepts that you need are at the following link:

Unregulated Power Supply Design

Cheers,

Tom
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Old 29th May 2012, 04:26 PM   #7
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Dear All,
I need at least 2A.
I measured the DC voltage directly at the output terminal without any load.
The input voltage was 220V at that time-stepped down to 15V via a toriodal transformer.
There is already a voltage regulator circuit in situ but the DC voltage was 19V instead of 12V. Is there any simple way to step down the voltage without rebuilding the whole circuit? Can I add resistors across the output terminals to bring down the voltage?
Thanks
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Old 29th May 2012, 04:48 PM   #8
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Try L4974, it is easy to make it work, and very reliable. I had used lots of them, and in some crazy ways...
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Old 29th May 2012, 06:04 PM   #9
Ar4 is offline Ar4  Latvia
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manner, what kind of regulator there is? If there is some DC regulator already, probably you can adjust voltage by simply changing a few resistors. For a common 3 leg reg you could even parallel one resistor to get desired output voltage.
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Old 29th May 2012, 06:23 PM   #10
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You just need to put a load on the output. 78 series regulators can show strange behaviour when they have no load.
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