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is it possible to use lm78xx to regulate electricity from the mains? Answered

What if i use capacitors to lower the voltage from the mains, then rectify it. Is it possible to use any member of lm78xx IC family to regulate it?

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user
lemonie (author)2011-08-29


No.
If you want to step-down from mains: use a transformer. Then then rectify and regulate it.

L

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codestroy7 (author)lemonie2011-08-29

but, is it possible to use a regulator IC without transformer. Or should i say, is there a transformerless power supply circuit that uses lm78xx?

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lemonie (author)codestroy72011-08-29


In the picture you post are there 2 big resistors "wasting voltage as heat"?

L

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codestroy7 (author)lemonie2011-08-31

its not my design, but i think they are just really wasting energy. :)

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lemonie (author)codestroy72011-08-31


Yes, it's fine for low-power stuff, but if you need a decent amount of LV current you need a transformer.

L

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seandogue (author)2011-08-29

Not, not really. None are rated for such high voltages. In fact, it's a good way to kill yourself or someone else.

Transformers suitable for use with the expected load currents derived from a 78 series voltage regulator are relatively inexpensive and can be purchased even cheaper by using an electronics surplus store or even cheaper by reclaiming them from discarded electronics items... Following the transformer, you'll want a rectifier. Rectifiers are cheap and can either be purchased as single device solutions or by using four discrete diodes.

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steveastrouk (author)2011-08-29

Don't mess around with direct on line circuits. Transform/isolate it.

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codestroy7 (author)steveastrouk2011-08-29

no i'm not messing or something. IM JUST CONFIRMING if it is possible because i saw a transformerless power supply doing it but i noticed a heat sink. i was thinking it may be a regulator IC like lm7805.

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steveastrouk (author)codestroy72011-08-29

It isn't, it'll be part of a switch mode powersupply.

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user
codestroy7 (author)steveastrouk2011-08-29

are you saying that this may be a switching regulator?

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steveastrouk (author)codestroy72011-08-29

Its certainly part of a direct on line powersupply. The whole system is a "the regulator" I suspect.

Steve

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codestroy7 (author)steveastrouk2011-08-29

yah, it is true that it is a power supply. what i am not sure is the thing in the heat sink. that size of heat sink is usually seen on linear voltage regulators.

Is it possible, in any way (without transformer), that a regulator IC can be used to regulate voltage from mains?

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steveastrouk (author)codestroy72011-08-29

No, because it will not produce mains ISOLATION. Only a very few systems (and none with external output cables) do NOT isolate the mains supply from the output.

Steve

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codestroy7 (author)steveastrouk2011-08-29

any example of this system?

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steveastrouk (author)codestroy72011-08-29

What you have there ? There is a high voltage rectifier, making high voltage DC. The circuit chops it back into high frequency AC, and, using a very small transformer, isolates and transforms it back to HF AC, at a lower voltage: this is then rectified again and smoothed.

Steve

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codestroy7 (author)steveastrouk2011-08-29

you may refer to the picture i added, sorry for it being too small. but that's the best image i have.

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