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Old 26th February 2013, 12:05 AM   #31
JMFahey is offline JMFahey  Argentina
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The circuit posted on #10 has all that's needed, both high precision voltage source and safe current limiting, just a couple parts need to be moved around.
But no need to add extra regulators or switch chip type.
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Old 26th February 2013, 07:34 AM   #32
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marianb, what value would you use for R4 in your schematic?
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Old 26th February 2013, 08:17 AM   #33
MarianB is offline MarianB  Romania
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About 100 Ohm should be ok, it's role is to protect the BJT in case of shortcircuited output.
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Old 26th February 2013, 08:23 AM   #34
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thanks. what power rating should it have? same as R3?
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Old 26th February 2013, 08:30 AM   #35
MarianB is offline MarianB  Romania
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No, about 0,5W should be fine.
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Old 26th February 2013, 08:45 AM   #36
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Quote:
Originally Posted by JMFahey View Post
The circuit posted on #10 has all that's needed, both high precision voltage source and safe current limiting, just a couple parts need to be moved around.
But no need to add extra regulators or switch chip type.
I would actually move only one...
An other advantage with it is that you can also use the R3 as the shunt for an ammeter! Which is nice...
:-)
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Old 26th February 2013, 08:57 AM   #37
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by palstanturhin View Post
..you can also use the R3 as the shunt for an ammeter! Which is nice...
Didn't thougt of it but yes it woud work just fine that way
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Old 26th February 2013, 12:38 PM   #38
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by JMFahey View Post
high precision voltage source and safe current limiting,.
Depends on the meaning of "precision" and "safe": in this case, a worst case error of 50%, and a current that can reach ~4A.

There is also the unresolved issue of loop stability in the current limit mode.
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Old 26th February 2013, 01:26 PM   #39
MarianB is offline MarianB  Romania
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That worst case scenario of voltage regulation less precission would be at the minimum Vout/max Iout, i doubt this unit will power something that sensitive... for real performance lab power supply a much more powerfull transformer is rerquired along with much more complex regs... this unit i think it is just good enough for general testing. As for that 4A, that will only be at shorted output, witch u do not seam to get it that this circuit is not intended, a 2A fuse along with this circuit will make sure nothing bad will happen with it, the regulator will be able to witstand the stress for those few seconds untill the fuse blows up as for the power transformer, well, if a 50/60Hz transformer blows up at the first shorted output then the hell with it, it's not wort anything.

So thing is either you do not see, don't want too see that you are just looking for problems where they are not... as i sayd, this cind of behaviour it is sad...
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Old 26th February 2013, 02:10 PM   #40
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Elvee View Post
Depends on the meaning of "precision" and "safe": in this case, a worst case error of 50%, and a current that can reach ~4A.

There is also the unresolved issue of loop stability in the current limit mode.
You have no idea.
"Precision" refers to the built-in precision of the regulator.
Just check the datasheet.
Does it state 0.1% or whatever? Well, that's what you'll get.
50% error?
Not even an unregulated supply will be that bad
Current can reach 4A ?
Who says so?
Didn't I tell you the OP can current limit it to *any* value he wants?

Loop stability in the current limit?
Don't worry about that , all these regulators are *designed* to be able to be current limited.
No designer worth his salt will release an unstable design.

By the way, palstanturhin *instantly* understood what I am talking about.
And he also thought about another practical use, go figure.
No doubt many others did too.
Think a little and you will too
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