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Filter capacitor in power supply is increasing the voltage

I'm building an unregulated 12V power supply, but I've run into an issue I can't seem to resolve. The supply consists of a mains transformer that converts 120VAC into 12VAC. The resulting 12VAC is fed into a full-wave bridge rectifier, with the resulting output of ~12VDC.

Now obviously I need some filtering on that 12VDC, so I added a capacitor between the (+) and the (-) output of the bridge rectifier. The resulting output is filtered, but as a result the voltage at the capacitor is now ~17VDC.

I'm somewhat familiar with why this occuring, having built a voltage multiplier in the past, but is there any "trick" to prevent it from occuring, or possibly a way (other than a resistor network or a voltage regulator) to reduce it afterwards?

Thanks for any suggestions,

Jon

Reply to
Jon Danniken
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Unloaded, the cap will always charge up to near the peak voltage of the incoming 12Vac. (1.4 X 12Vac is 17Vdc). Other than inductors or regulators then the best option is maybe just use a

9Vac transformer. Bear in mind, the DC voltage can drop a few volts when you put load on the power supply. regards john
Reply to
john jardine

Why not use something like a 7812 regulator IC?

Ed

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Reply to
Jag Man

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Thanks, John, the voltage did indeed drop down to ~12.5VDC when I applied a

5A load. I'm curious about your statement regarding the inclusion of an inductor though. In what way would an inductor be applied to reduce the voltage?

Thanks,

Jon

Reply to
Jon Danniken
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Used in this situation the inductors are usually called "chokes". The idea is to connect a choke to the output of the main capacitor, then feed into yet another capacitor that supplies the load. It doesn't 'drop' the voltage like a switcher or regulator or resistor would. It simply acts with the extra capacitor to 'reshape' or 'mould' the nasty incoming waveform to something flatter and smoother. As ...

___ o/p V +-+--+--+------UUU-----+------o to load o-. ,--, A A | choke | AC in )|( |-+ | --- --- )|( -(-+ --- --- o-' '--' A A | cap1 cap2 | +-+-----+--------------+------o

The choke and Cap2 form a very low frequency low pass filter acting as a 'flywheel' energy store that can smooth out the effects of the peaky voltages that are charging up Cap1. Nett result is that the load voltage is a nice smooth voltage hovering somewhere near the average (not peak) of the incoming voltage. I.e near 12V. Problem is that to be effective (at mains frequency), the choke inductor needs lots and lots of Henries and ends up as some humungous, expensive lump of steel and copper that is similar in size and cost, to the power transformer. Hence at the high currents and low voltages of modern equipment, is a technique little used nowadays. regards john

Reply to
john jardine

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Thanks again for your reply, John. I've got enough now to play around with componenets and look for results.

Jon

Reply to
Jon Danniken

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