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Old 6th March 2012, 07:11 PM   #31
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Default Power Supply for the LM318

Where did you get the design of the +-14 volt power supply?

The LM317 and LM337 voltage regulators state that they need a
minimum of 10mA in order to work correctly. Your current sources
are only putting out about 18mA of current each. The formula in the
data sheet is I(out) = 1.25V/R1 = 1.25V/68 Ohm = 18.38mA.
That's only about 8mA above the minimum. Did you try R1=50 ohms
or R1=25 Ohms?

In addition, why didn't you do the following for the power supply
design:

LM317 & LM337 LM317 & LM337
Voltage Regulator ---------> Second Voltage Regulator ---> To LM318

Did someone do a simulation of your design? A recent article in Nuts and Volts
Magazine stated that dual LM317 and LM337 is better than a single set
of positive and negative regulators.

Where is you get the remaining part of the LM318 power supply from
(the zener's, transistors, etc.)?
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Old 6th March 2012, 08:51 PM   #32
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Default More about the LM318 Power supply

I'm not used to the editing tools on diyaudio.com. The suggestion for a
new power supply should be:

LM317&LM337 Volt.Reg Pair--->LM317&LM337 Volt Reg Pair---> LM318

In other words, two stages of LM317&LM337 in series. Each stage could
drop the voltage down a certain amount, thus each stage would drop 1/2
of the voltage. The output would still be +-14 Volts into the LM318 chip.

In addition, the LM317/LM337 data sheets state that a 10 microFarad capacitors
from the adjust pins to ground will reduce the ribble by about 15dB (see figure14 and 15) in the data sheet.

Maybe Linuxgurf cound simulate both designs to see which is best. Take care.
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Old 6th March 2012, 09:00 PM   #33
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Default Measure the voltage accross the 68 Ohm resistors

Quote:
Originally Posted by ClaveFremen View Post
Maybe...

I think that I've finally found where is the problem... LM317's power dissipation.

When idle temperature on LM337 is 44 C, on LM317 57C (23C room temperature).

I've replaced both LM317, the new ones heatsinked, problem seems gone.

What puzzles me is that when on breadboard this problem did not appear.

Just for the sake of precision I've measured also both unregulated rails (+33,6Vdc and -34Vdc with 2x24Vac transformers) so there is not a voltage difference...

So for all beta builders: buy TO-220 thin heatsinks for LM317s

This Mouser code should be suitable: 532-576802B00
This does not make sense, If the formula in the data sheet states the the output
current is 1.25V/R1 then there should only be 18.38 mA going thru each voltage
regulator. With a drop of =voltage from say 28Volts input to 14 volts output, that
a delta v of 14 volts, Power = IV = 18.38mA * 14 V = 0.2574 watts

That's why we need to know the voltage drop across each of the 68 Ohm
resistors to determine that exact current flow. Take care.
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Old 6th March 2012, 09:03 PM   #34
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Quote:
Originally Posted by mrsavage View Post
Where did you get the design of the +-14 volt power supply?
Basically is a pretty standard shunt regulated design with LM3x7 CCS.

There is plenty of similar circuits on the net.

Quote:
Originally Posted by mrsavage View Post
The LM317 and LM337 voltage regulators state that they need a
minimum of 10mA in order to work correctly.
...
Did you try R1=50 ohms
or R1=25 Ohms?
You're right, in fact the actual recommended value is 50R

Quote:
Originally Posted by mrsavage View Post
In addition, why didn't you do the following for the power supply
design:

LM317 & LM337 LM317 & LM337
Voltage Regulator ---------> Second Voltage Regulator ---> To LM318
While I'm sure the double stage regulator approach sounds better than a single stage one with LM3x7s I think the zener/transistor shunt sounds better.

In my tests the difference between a single stage LM3x7s PS vs the zener/transistor shunt (without CCS!) was pretty obvious.

Quote:
Originally Posted by mrsavage View Post
This does not make sense,
...
Power = IV = 18.38mA * 14 V = 0.2574 watts
I do agree, when the new LM3x7 will arrive I'll measure and compare again.

Probably the different temperature was due to a faulty BC639/BC640
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Last edited by ClaveFremen; 6th March 2012 at 09:09 PM.
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Old 7th March 2012, 05:53 PM   #35
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Default Breadboard problems

Quote:
What puzzles me is that when on breadboard this problem did not appear.

Remember that a breadboard is basically plates separated by plastic dielectric
material. Hence, breadboards have a lot of built in capacitance.

I would suggest placing four capacitors around the LM317 and LM337. The
input capacitors would be 0.1 microfarad. The output would be 1.0 microfarad.
The data sheet says use high quality tantalum. Try this before you replace the
voltage regulators. They could be oscillating, hence, that high temperatures. Take\
care.
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Old 7th March 2012, 10:53 PM   #36
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Quote:
Originally Posted by mrsavage View Post
Remember that a breadboard is basically plates separated by plastic dielectric material. Hence, breadboards have a lot of built in capacitance.
Interesting, never though about it.

Quote:
Originally Posted by mrsavage View Post
I would suggest placing four capacitors around the LM317 and LM337. The input capacitors would be 0.1 microfarad. The output would be 1.0 microfarad.
Both datasheet and this Jung's article don't talk about decoupling for LM3x7 CCSs...

Quote:
Originally Posted by mrsavage View Post
They could be oscillating, hence, that high temperatures.
Or simply a broken BC639 lower output voltage and thus raise voltage on LM317, paired with same current -> higher temperature.

In these days, waiting for the LM3x7s to arrive, I've made several measurements and experiments.

For example measuring currents with a multimeter leads immediately to a broken BC639/BC640 AKA lower output voltage.
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Old 8th March 2012, 07:25 PM   #37
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Quote:
Originally Posted by ClaveFremen View Post
Just a note: while 1N53xx sounds fuller they, maybe have a touch of hardness but as hours are passing it seems to lower.
That hardness never gone completely while BZX85 didn't have it.

Superfigone pointed out via email than he thought the 270R don't give enough current to flow through the zener.

In fact, since Vbe measures 0.638V, current is 2.36ma.

So I've tried 200R (current = 3.19mA) and hardness seems almost gone.

Probably 150R (4.25mA) could be better.

I did expected to fine tune values but I didn't expected that the initial values could be bad....
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Old 9th March 2012, 12:29 PM   #38
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Zeners are generally run where they are on the low dynamic impedance part of the I vs V curve.

If you run too low a current through the Zener you can be operating off the low dynamic impedance part of the curve.

I recommend that any Zener be run at 10% to 99% of it's max operating Power dissipation.
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Old 9th March 2012, 12:44 PM   #39
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Quote:
Originally Posted by AndrewT View Post
I recommend that any Zener be run at 10% to 99% of it's max operating Power dissipation.
Incredible, I was just reading some docs on dimensioning zener shunt regs... same recommendation.

Andrew, are you remote-controlling my mind?

Jokes apart, so it seems the 1N5352 should be operated at about 32mA...

In 1N53xx datasheet I can't find any indication on minimum zener current while on BZX85 one is clearly stated that minimum current is 2.7mA, quite lower than the 7mA found using the rule of thumb.

But even if we suppose that 1N53xx behaves in the same way (no guarantee about that) we still need at least 20mA...

Maybe going back to BZX85s could be wiser... I don't know...
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Old 9th March 2012, 12:49 PM   #40
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I seem to recall a warning about this subject when you first told us you "heard" something on swapping Zeners.

Stop guessing.
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