How much can a LM317 take?

bluebird

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I'm building some hamptone pres. The box I have already has a 120VAC to 35 VAC (no load) transformer in it.

If I used this power supply set up:

http://recording.org/users/kev/TKpowersup.pdf

is the 35 vac going to be to much for the regulator after it is rectified?

also I could use a voltage DOUBLER instead of TRIPLER for phantom right?


Thanks,
ian
 

Steffen

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is the 35 vac going to be to much for the regulator after it is rectified?

should be ok, you´ll end up somewhere between 50 and 55 V rectified voltage. the LM317 can eat 40V input to output diff, so 55-24=31V is ok. keep in mind that even small currents in this config produce lots of heat... 100mA ---> 3.1W !! so use heatsink.

also I could use a voltage DOUBLER instead of TRIPLER for phantom right?

yepp

steff
 

Tekay

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There's a new version at my page now with a pcb layout too!
http://www.vintagedesign.halmstad.net/diy/schem_mods.htm

I've forgot to 2 diods but they will be added as soon as I'll find time to add them to the layout! Diods shall be connected between the output and adj pin on LM317/337! Catod to out on LM317 and anod to out on LM337
This new layout uses a LM317 instead of the TL783 for phontom power!

There won't be enought voltage to regulate with just a voltage doubler!
 

bluebird

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Thanks fellas,

Got ya...Heat sink!!!

Tekay,

so where you have it marked as 240 ohms, is that R1 or R2?

And remember my transformer is putting out 35 volts AC. your saying that will not be enough with just a voltage doubler for phantom?
 

Flatpicker

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[quote author="Tekay"]There won't be enought voltage to regulate with just a voltage doubler![/quote]When I double/rectify/filter the voltage from a 24VAC wall wart, I get around 70V DC. Why would you want to triple it? Just curious.
 

Tekay

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If I understand ir right my unit also uses a voltage doubler!

The 240ohm resistor is the one between out and adjust! It's a 330ohm resistor in the phantom circuit!
 

Gus

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I would use a voltage doubler and a resistor between the "raw" supply and 317 to drop some voltage.
 

Tekay

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There is a 22ohm serial resistor on my board layout that forgot to add in the schematics! Maybe 22 ohm is a little low?
 

volki

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i was always wondering if a relatively high dropping resistor between the reservoir cap and the regulator would slow down the response of the psu when it has to deliver peaks of high current? with higher R's and a health dose of current peaks, you could even underrun the v.reg's minimum in-out voltage drop... :cool: ?!

as for me, i still need to build this multiple supply psu with about 400mA for +45v and 700mA for +24v and +/-20v each, and the dropping R question comes up here, too: i have a huge 28-0-28v xformer, plus another smaller 0-24v one which i could stack up on the former so that i get 28-0-28-52. i'm thinking about using a tl783 for 45v and lm317/337 for the rest. the dc voltages at the reservoir caps would be about 39v and 73v respectively. from 39v down to 20v, the lm317/337's would have to dissipate rouighly 14W each for max current. from 73 to 45, that would be 11W for the tl783. if i mount all these 4 v.reg's on a hefty heatsink of about 40x8x1 cm (psu sits in a rack), should that be fine? or should i rather use dropping R's in order to distribute heat dissipation? if so, can someone suggest values?

oh yeah - the tl783c is rated at 700mA max - will it still deliver 500ma over a 19v drop?
as for the lm317/337: the to220 pack is rated at 15W, will that still be fine, or should i go with to3's, rather? or (stupid question maybe), what about two to220 v.regs in parallel, anyway? never seen such a design though, so i guess that wouldn't work for some reason :cool:

hey tekay, why did you change your design from tl783 to lm317?
 

Flatpicker

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[quote author="volki"]i was always wondering if a relatively high dropping resistor between the reservoir cap and the regulator would slow down the response of the psu when it has to deliver peaks of high current?[/quote]Yes, typically use <5 ohms with a high enough power rating (which you'll have to calculate according to the voltage/load).
 

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