multiple regs

ed · Post by ed » Wed May 30, 2012 9:28 am

Can anybody suggest which is the best method here.

I've looked on the web and found conflicting views. I know at the end of the day it doesn't really matter but I'm keen to understand some reasoning. My guess is that series is better because theres less drop on the 5v reg, but in truth that really is a guess.

Mike H · Post by **Mike H** » Wed May 30, 2012 11:08 am

I've seen on datasheets 2 can be used in series "for better results", but I guess it comes down to how much Voltage overhead you've got.

Mike H · Post by **Mike H** » Wed May 30, 2012 11:10 am

Actually looking at it again, can't see anything wrong with the first one except the power dissipation as all the current is going through the first reg.

Neal · Post by **Neal** » Wed May 30, 2012 1:03 pm

If you are going to serialize 317 type regs then you are best to use the first as a pre-tracking regulator....see the 317 data sheet for more details...

ed · Post by ed » Wed May 30, 2012 2:04 pm

Mike H wrote:Actually looking at it again, can't see anything wrong with the first one except the power dissipation as all the current is going through the first reg.

I think current is negligeable:

5v
avr chip wants 0.5 - 10 ma
lcd wants 0.6 ma
12v
relay wants 16ma

I currently have a 9v 2va Tx which would ideally give me 12v ish unreg and 5v regulated but with these current draws the Tx thinks its unloaded and consequently gives about 19v at the unregulated point. Consequently giving the relay 19v equates to 0.5 watts instead of it's rated 0.2 which I think is why it's getting hot. If I've got my sums correct.(720R coil).

If I have got it right I see 3 choices : smaller Tx, i.e 9v 0.35 VA or 12v reg or drop volts the dirty way(divider 1k2 over 2k).

I think I'll try the 12v reg in series first. I was curious because the web search shows 2 groups who favour different methods but never explain why.

edit: I think I may have to adjust the voltage divider values above, because I think the relay load should be in parallel with the bottom resistor...also the relay is not in circuit for some of the time....oh pigspoo!

pre65 · Post by **pre65** » Wed May 30, 2012 2:20 pm

Ed, if the current draw is too small for that transformer can you not put a suitable resistor across the psu (after the bridge diodes) to draw a bit of current ?

ed · Post by ed » Wed May 30, 2012 2:30 pm

sorry Phil,

yes, there are 4 ways, the other 3 + the even dirtier method of inserting a heater...

and there I was getting all romantic about my newly discovered regulators

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multiple regs

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