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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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lemonie
lemonie

9 years ago


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

L

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codestroy7
codestroy7

Answer 9 years ago

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
lemonie

Answer 9 years ago


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

L

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codestroy7
codestroy7

Answer 9 years ago

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

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lemonie
lemonie

Answer 9 years ago


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
seandogue

9 years ago

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
steveastrouk

9 years ago

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

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codestroy7
codestroy7

Answer 9 years ago

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
steveastrouk

Answer 9 years ago

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

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codestroy7
codestroy7

Answer 9 years ago

are you saying that this may be a switching regulator?

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steveastrouk
steveastrouk

Answer 9 years ago

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

Steve

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codestroy7
codestroy7

Answer 9 years ago

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
steveastrouk

Answer 9 years ago

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
codestroy7

Answer 9 years ago

any example of this system?

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steveastrouk
steveastrouk

Answer 9 years ago

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
codestroy7

Answer 9 years ago

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