Question on inputs to an LM317 or LM78XX

I have a 28volts AC source that I've like to be able to bring down to microcontroller usable DC level. I was thinking a bridge rectifier followed by a voltage regulator such as the LM317 or LM78XX.

My simulations show that AC at 120Hz at 28 volts, with a full bridge rectifer, 100 ohm load, and a 100uF filter cap produces about 19.5 to 26.5 volts on the output. I'm sure that the regulator won't like that and will echo these ripples on the output. I can go with a bigger cap, but I don't have any 10mF caps laying around that will filter that down to a reasonable DC level.

Can some one clue me in on how to bring down the variations here so I can run it through a regulator and into my uC.

Also, are there any suggestions on polling another 28volt line with a uC other than another bridge rectifier, BIG cap, and a voltage divider?

19 - 20V Dc into a LM317 is ok
100uF is prob a bit small go for a 1000uF and it would have to be rated at at least 35V
As far as the value of the capacitor goes.... rule of thumb is .... 1000uF for each Amp of current drawn
A LM317 TO220 style case is going to be 1A max when heatsunk

10mF = 10uF in the real world of capacitor values, mF designator isnt really used any more, so thats too small, your 100uF is much bigger
and as said you prob need at least 1000uF anyway.

Dave

28VAC has a peak voltage of 39.6V. The bridge rectifier reduces it to 37.8V which is too high as an input for an LM317 or a 7805 regulator.
Why use such a high AC voltage? It makes a lot of heat that needs a big heatsink to get rid of. Use 9VAC instead.

A microcontroller uses only a few mA so the filter capacitor can be small and the ripple will be small. The ripple is a few volts when 1A is drawn and a 3300uF filter capacitor is used. With a load oif 40ma then a 100uf filter capacitor has 2V p-p of ripple. The regulator removes the ripple if its input voltage stays above 2V or more than the output voltage.

maybe that just happens to be the supply voltage in the existing bit of equip. (was my assumption, right or wrong )

Yes Davenn, this is correct. I need to operate with a 28V AC source.

Am I going about regulating and stepping this down the right way, then?

Do you guys have any other suggestions?

What is the 28 VAC source? Why are you stuck using it?

I need to operate with a 28V AC source.

Am I going about regulating and stepping this down the right way, then?

Do you guys have any other suggestions?

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This will not work for the reasons AudioGuru pointed out. Your 28 volts RMS will have a much, much higher value following rectification and filtering as your filter capacitor charges to the peak value.

If you have a 2:1 transformer lying around like a 480 to 240 or 240 to 120 volt it would work to get the 28 down to 14 which would work for you.

I have a 12 volt 1 amp wall wart I use with a little 7805 circuit and power all my little pic experiments on it (those that I don't breadboard). Works fine and has lasted a long time.

Ron

The data sheet I looked at says the 317 has an input output differential of 40 volts, so it should be usable, but you will still need some heat sink. The regulator will take out most of the ripple. Check out the part of the data sheet that reduces ripple. Make sure your cap has a voltage rating for your peak voltage.

https://www.electro-tech-online.com/custompdfs/2011/03/LM117.pdf

The 28VAC is coming from a transformer that feeds the thermostat in my house. I have no intention of using a battery when I have a voltage source right there.

So what's the verdict here guys?

Small transformer or the full bridge and big caps?

Is there something similar to the LM317 or LM78XX out there where the input can be AC and it outputs a regulated DC?