You can connect the inputs of all the regulators to a single smoothing circuit.
If the regulators will provide different output voltages, you may have to worry about excessive power dissipation on the lower-voltage regulators.
-- Peter Bennett VE7CEI email: peterbb4 (at) interchange.ubc.ca GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html Newsgroup new user info: http://vancouver-webpages.com/nnq
I want to make a regulated bench PSU with three adjustable outputs (LM317/337). Do I need to use a separate smoothing circuit for each output, or can I connect the inputs of the regulators to a single smoothing circuit?
-- Wally www.artbywally.com www.wally.myby.co.uk
Righto.
In the sense that they might be damaged, or that power is being lost? The transfomer is rated at 24-0-24v, 1A, and the spec of the regulators is 20W,
1.5A.-- Wally www.artbywally.com www.wally.myby.co.uk
Power will be "lost" in the regulators in the form of heat. The power dissipated in each regulator will be the voltage across it, times the current through it.
Full-wave rectification from that transformer will give you about 33 volts DC. If a regulator is set to give you 12 volts out, it will have 21 volts across it. If it has to deliver 100 mA, it will be dissipating 2.1 watts. Without a heatsink, it will get HOT!
Although the LM317 is spec'd for 20 watts, you do have to provide an adequate heatsink to achieve that performance.
-- Peter Bennett VE7CEI email: peterbb4 (at) interchange.ubc.ca GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html Newsgroup new user info: http://vancouver-webpages.com/nnq
Aha.
So, if I want the transformer to be able to deliver its maximum current of
1A, then I might be approaching the limit for the regulators. The (vague) plan is to have panel voltmeters on the outputs and an ammeter between the rectifier/smoothing and the regulators - maybe a current limiting circuit here would be a good idea.-- Wally www.artbywally.com www.wally.myby.co.uk
output,
circuit?
You can connect the inputs to the same filter cap, but it's *MUCH* more useful to have three electrically isolated outputs. i.e. each regulator circuit has it's own transformer (or transformer tap), it's own rectifier and filter cap, and it's own adjustable regulator. The grounds are not connected and the three circuits are totally isolated. You could veen put in a switch to connect the ground when needed.
It's also very useful to have a direct adjustable AC output. Simply put a muti-tap transformer in the box and feed the output straight through through a selector switch.
Dave :)
Is having three rectifier/etc circuits hanging off the same secondary a no-no? Methinks a multi-tap transformer might be more expensive than three of the single centre-tapped secondary that I have in mind (I'll do some searching and see what I can find).
A ground switch had already occured to me.
How is the AC made adjustable? Multi-tap xformer, some sort of resitive divider, active components? (I don't know much about this stuff, just conversant with the bits and putting them together.)
-- Wally www.artbywally.com www.wally.myby.co.uk
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