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Old 5th February 2004, 10:51 AM   #1
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Default LM350 adjust pin

In Morgan Jone's book he should the 317 with a resistor and 15u cap on the pin to reduce ripple acceptance.

Later on he uses the standard 4.7u cap in a current sense heater supply circuit.

I have a few solid aluminium 15u caps, but no 4.7u's.

Is the regulator going to be okay with 15u caps?

I heard someone mention that such regulators may not start correct if they have too much capacitance on the adjust pin.

All the best,
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Old 5th February 2004, 10:58 AM   #2
joensd is offline joensd  Germany
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Hi John,
as always with such chips it´s recomended to read through the datasheet. You´ll find it here.
There is a circuit given that uses a 10uF cap.
It is also mentioned that caps bigger than 100uF or so might damage the chip if the output is shorted.
At a glance I´d say you are safe to use your 15uF cap.

What´s the regulator for?

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Old 5th February 2004, 11:05 AM   #3
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The heater supplies.

Later on Morgan uses a cap on the adjust pin of a 400v supply and, for the reason you mentioned, he connects a diode to the + plate of it. Should the load short out, the capacitor is meant to discharge thru the diode, back to an earlier section of the supply, where it will be electrically safer, rather than allowing it to discharge thru the regulator.

The regulator has a chain of resistors attached to it's output and then the adjust pin connects across these, instead of going to the ground rail. This way, you can sense the current flow and adjust the heater's supply so that stand-by mode actually puts the heaters at 63% of their nominal voltage.

Many thanks,
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Old 5th February 2004, 11:08 AM   #4
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Default Re: LM350 adjust pin

Originally posted by eeka chu
I heard someone mention that such regulators may not start correct if they have too much capacitance on the adjust pin.
I think this is just a rumour. A very large cap will only delay the voltage out. It will start at 1.25 V and then start rising.

The cap value determines where (which frequency) the PSRR will improve. 10 uF is a "normal" value but you can have both lower and higher values. Make sure though that the total capacitance has good HF properties. 10 uF AL + 10-100 nF polyester/ceramic isn't bad either.
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