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From: "Helmut Sennewald"
Subject: Re: op-amp for a regulated power supply(linear,series pass)...
Date: Fri, 15 Nov 2002 23:12:47 +0100
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"KILOWATT" schrieb im Newsbeitrag
> Hi everyones...thanks to read!
> I'm designing a power suppy/battery charger for
> a friend's camcorder. He then will be able to
> power is camcorder at home as a webcam or wathever
> when charging a battery for it at the same time, with the
> same unit. I have an op-amp remaining from an LM324 IC
> that i want to use for the camcorder's 6.5V regulated power
> supply section. Since i did a lot of power supplies for my
> projects,but like many ones i've used a regulator IC,
> especially the 78XX series,(the easy way!) i did not really
> gained experience and basics. I breadboarded the thing
> and tested it, but as i expected i had some stability problems.
> (see the link lower on this message)
your requirements are so easy, that the most simple circuit
with an adjustable LT1085 regulator will do the job.
This IC is a widely used 3A regulator.
> I've checked on Google with tens of keywords combinations
> for "power supply" "regulated" "op-amp"...ect...
> i've found a lot of infos,but not exactly what i wanted. Found more
> about Low Dropout regulators(LDO) ICs, and switching power
As I stated before, you don't need a circuit with an opamp and
a discrete power stage.
> But with the few infos i've found...i think i did something
> that is performing. I've also added a capacitor in the feedback
> loop and things are more stable than ever.
> However since i'm
> not 100% shure about the reliability of what i've done...i need
> (please?!!) some comments about my schematic and if possible
> a link where i can find infos about regulators made of op-amps.
This circuit has some serious design flaws especially in the output stage.
* Overall to much unpredictable gain.
* The 100kOhm at the +/- terminal of the opamp are one order
of magnitude to high, use 10kOhm.
* Unwanted low pass action due to the high resistor feeding the
base of Q1.
* Uncontrolled gain of Q1
* The same for Q2
* No short circuit protection
* No thermal limiter for long lasting shorts.
How could it be improved:
Simply add no extra voltage gain in the output stage Q1, Q2.
You only need one NPN darlington Q2 connected to the output
of the opamp directly. But still you have no current limit
and also no thermal limiter for short circuit protection.
That all is integrated in the mentioned LT1085 voltage regulator.
Take care when you order the LT1085 that you get the adjustable
version and not any fixed version(LT1085-3,4,5).
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