From: Tom Clifton kc0vsj@yahoo.com
Subject: [time-nuts] Parallel voltage regulators
Date: Wed, 24 Oct 2007 16:33:08 -0700 (PDT)
Message-ID: 329129.31442.qm@web37014.mail.mud.yahoo.com

Tom,

I would not do that, even with the equalizing resistors. The load-balancing
with resistors works when there is a common control-loop, but here you would
have four inter-connected control-loops.

It is not that hard to acheive 2 A at 24 V after all. The old uA723 and
variants would probably do the trick good enought for you with external
transitor(s).

I recommend to at least include fold-back for over-current protection, but
adding an over-voltage in form of a crow-bar setup isn't too hard either.
The point of the crow-bar is to cause fold-back and if that fails, blow the
fuse, so include a fuse on the unregulated supply side.

Powerbalancing can be very tricky indeed, and you might end up with either one
having the full load or oscillative behaviours.

Cheers,
Magnus

Tom Clifton wrote:

Hej Magnus

Since the output voltage range of a 7824 is 23 - 25V that arrangement
wont work very well unless you match all 7824's to within say 50mv or so
of the same output voltage and mount them on a common heat sink. If you
want to use a 3 terminal regulator why not just use an LM338 (plus a
couple of resistors to set the output voltage) or similar device?

Bruce

Hi Tom I dont think you need the resistors these are current limited and the
sense is inside the chip so the resistors dont do anything (you are thinking
of a negative feedback effect). All that happens in paralled operation is
that one may take the majority of the current til it current limits and the
rest is provided by the other. They will need heat sinking. You can get
Hi-power versions of these fixed regs as well or use one of them to drive a
big transistor (the 2N2955 PNP used to be a popular choice up to 5A) for the
series control. You may ned to lift the common leg with a diode to allow for
the e-b drop on the pass transistor if you use an NPN.
Cheers de Alan G3NYK

----- Original Message -----
From: "Tom Clifton" kc0vsj@yahoo.com
To: time-nuts@febo.com
Sent: Thursday, October 25, 2007 12:33 AM
Subject: [time-nuts] Parallel voltage regulators

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

Alan Melia wrote:

Nonsense look at the circuit.
The base emitter drop of the pnp booster just increases the composite
regulator dropout voltage.
No diodes required to compensate Vbe drops.
However this circuit is unreliable without current limiting of the PNP
collector current.
These days its simpler to use an LM338.

Bruce

Magnus Danielson wrote:

Hej Magnus

Using a 723 correctly configured with a low pass filter on the reference
is a very low noise regulator solution with its output noise at least
20dB lower than that produced by a typical 3 terminal regulator.

Most of the OEM open frame linear supplies use 723 regulators with
external (to the 723) series pass transistors.
These regulators have varying degrees of sophistication, some even use a
zener plus emitter follower preregulator for the LM723.
Some foolishly omit bleeder resistors across the reservoir capacitors
which can lead to damage when making connections after powering the
supply on and then off as the residual energy stored in the reservoir
capacitors is more than sufficient to destroy the 723 should the output
terminals be shorted.

Bruce

Hi Tom,
If you really want to regulate the oven`s supply voltage, my National
Voltage Regulator handbook shows that the LM317T will supply over 2 Amps,
with an input/output differential of between 5, and 12.5 Volts. A single one
of these should do the job OK.
Cheers!,.................................Don Collie jnr.

----- Original Message -----
From: "Tom Clifton" kc0vsj@yahoo.com
To: time-nuts@febo.com
Sent: Thursday, October 25, 2007 12:33 PM
Subject: [time-nuts] Parallel voltage regulators

Don Collie wrote:

Never rely on typical specs always use the minimum spec which is 1.5A
not quite enough.

Bruce

Bollocs, Bruce! If National say it will do it, you can bet that it will. An
LM338K will do the job too, but in my opinion its overkill, and in the event of a short circuit on the output of the regulator the current for the LM338 will only be limited to [.......he gets the book......] 8 Amps [Typ], as against 2.2 Amp [Typ] for the LM317T. This would probably be too much for the transformer, rectifiers, and smoothing capacitor, effectively meaning that you would have no current limiting. If the input/output differential was kept in the range of 5 to 10 Volts, while the oven was stabilising, and the LM317 had an adequate heatsink, it would do the job nicely [and cheaper too!] Actually, it wouldnt matter if the oven supply went unregulated while
the temperature was stabilising, because you wouldn`t be using it for
measurements during this time anyway - or is that a bit radical!?
All the best!,..................................................Don.
----- Original Message -----
From: "Bruce Griffiths" bruce.griffiths@xtra.co.nz
To: "Don Collie" donmer@woosh.co.nz; "Discussion of precise time and
frequency measurement" time-nuts@febo.com
Sent: Thursday, October 25, 2007 5:42 PM
Subject: Re: [time-nuts] Parallel voltage regulators

Don Collie wrote:

Show me where that is actually guaranteed on the datasheet.
Only the inexperienced and the gullible fall into the trap of assuming
every regulator (or any other device) will meet its typical specs.
The designer of this particular regulator actually cautions against this
cavalier approach to design.

If you worried about the transformer a simple fuse (resettable or
otherwise) will surely cure that problem.

Bruce

----- Original Message -----
From: "Bruce Griffiths" bruce.griffiths@xtra.co.nz
To: "Don Collie" donmer@woosh.co.nz; "Discussion of precise time and
frequency measurement" time-nuts@febo.com
Sent: Thursday, October 25, 2007 7:20 PM
Subject: Re: [time-nuts] Parallel voltage regulators

Hi Bruce,
Ive got the National Semiconductor Corporation Voltage Regulator Handbook [1982]. On page 3-3, the leftmost graph shows the LM117/217/317 as having its current limit, with a junction temperature of 125 degrees Centigrade, at 2.25 Amps over the input/output differential of 5 to 10 Volts. The point being, that if you use a higher current regulator, you loose the advantage of the regulators current limiting, and perhaps, its thermal shutdown
protection as well.
A fuse might protect the semis down the line, but often its the
semi`s that fail before the fuse, and the peak current that might flow
before the fuse blows might be many times the current limit of the regulator
[which is nearly instantaneous], and if so, damaging, so it is wise to run
these regulators near their current limit, just as you would set the current
limit on a bench supply to just above the working current.
I find your use of the emotive words "inexperienced",  "gullible", and
"cavalier" saddening.
Wishing you well,................................Don.

Don Collie wrote:

The graphs are only typical, read the actual printed specifications.
You need to be more skeptical and question your assumptions.

If you want a lower current limit regulator use an LM350.
However you have identified one problem in applying 3 terminal
regulators, the rather wide tolerances associated with the current limit
circuitry.

A 723 regulator can be easily designed to have a much narrower spread
for the current limit.

Bruce

From: Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] Parallel voltage regulators
Date: Thu, 25 Oct 2007 13:51:47 +1300
Message-ID: 471FE8A3.1060304@xtra.co.nz

While the 723 isn't the highest degree of sofistication these days, it does
alow for building more or less complete linear regulators. In another hobby of
mine they are in plentiful use and provide stable enought regulation.

Cheers,
Magnus

From: Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] Parallel voltage regulators
Date: Thu, 25 Oct 2007 17:42:51 +1300
Message-ID: 47201ECB.6050702@xtra.co.nz

You want design-margin. Some of that toll will be in less than optimum heating,
some will be in less heating in the first place (compared to upper limit) and
for a power regulating aspect, headroom allows better regulations.

In one design we had to parallel the regulators since the regulator the
designer put in just barely was able to regulate the CPU core voltage.
It worked, but at just rebooted at some vauge point an the memory tests.
What actually happend was that as soon as it started to actually do anything,
the regulator was running at its limit and output voltage dropped as the
current was rising and the voltage supervision pulled the RESET.

That's what you get from reading the typical reading on the CPU current and
match that with the maximum rating of the power regulator. A no margin design.
That designer had a few more flaws which was creeping around in that design,
but let's not bring that can of worms open here. :)

The 5A LM338 will be just fine. Infact, you can pull 8A out of it under
certain conditions.

Cheers,
Magnus

The simple answer is if the heater supply in unregulated, and the
supply changes, it will take a little while before the heater control
loop can compensate.  While this is happening, the oven temperature
will be slightly wrong, and the crystal frequency will be offset.
Voltage regulation adds one more level of isolation to the oven temperature
from changes in raw supply voltage.

-Chuck Harris

Matt Ettus wrote:

Magnus Danielson wrote:

Hej Magnus

The LM338 thermal design is also much easier (it has a much lower
junction to case thermal resistance than an LM317) especially if the
circuit is intended to operate over wide temperature (0 -40C or more)
and mains voltage ranges (+-10% or more).

The alleged problem with the high short circuit current is easily solved
by using diodes with adequate current ratings in conjunction with a fuse
to protect the transformer if it isnt rated to produce an 8A dc output.

The startup current of the load (rubidium standard) may also vary with
temperature and /or input voltage.
Either  find the manufacturers specifications or allow adequate margins.

Worst case design is desirable even for one off circuits especially if
the circuit is published.
When the design is  publicly available one is in effect transferring the
production run problems associated with a marginal design to many
individuals rather than a single factory or production line.

Bruce

Surely there will be an improvement in oven temparature regulation, by using
a regulated supply, but whether or not this improvment is marginal or
significant would have to be determined either by experiment [my favourite],
or calculation. As you say Chuck : it`s eliminating [or reducing the effect
of] one variable in the equation.
Cheers,................................................Don.

----- Original Message -----
From: "Chuck Harris" cfharris@erols.com
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Friday, October 26, 2007 12:29 PM
Subject: Re: [time-nuts] Parallel voltage regulators

----- Original Message -----
From: "Bruce Griffiths" bruce.griffiths@xtra.co.nz
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Friday, October 26, 2007 12:36 PM
Subject: Re: [time-nuts] Parallel voltage regulators

will protect both load, and regulator [and resovior capacitor, and diodes,
and transformer] Commonly available fuses won`t give much protection to the
load - especially the delay types often necessary with large filter
capacitors.
A precision, proven, high performance, low noise regulator like the 723
using an external pass transistor [or preferably a darlington], to avoid
chip heating, and a well bypassed reference would be a lovely solution.
Cheers!,............................................Don C.

Don Collie wrote:

One can always use more than one fuse (one in the mains wiring and
another between the reservoir caps and the regulator input), only one of
which needs to be a slow blow type.
An LM350 has a more tightly specified current limit than earlier 3
terminal regulators.

Powering a load from an unregulated supply as suggested also offers
little load protection, however if the load isnt faulty then close
current limiting wont be necessary.
However it is always useful when testing to have close control of the
short circuit load current if at all possible.
If this is required then a 3 terminal regulator by itself will not suffice.
However a low power small signal npn transistor to sense the voltage
drop across a current sensing resistor in the load return lead can be
used to pull down the regulator adjust terminal when the transistor
turns on limiting the maximum load voltage to around 1.2V in this
situation which will limit the current in most loads but not in a short
circuit. The only way to add control of the regulator short circuit
current being to use an electronic current limiter in series with the
regulator input, however this increases the regulator dropout voltage
significantly as well as the circuit complexity.

The only problem with the 723 for a beginner is they are much more
complicated to wire up and too easy to destroy when probing if care isnt
taken.
Also with modern high gain transistors a darlington pass element isnt
really necessary for a load current of a couple of amps.
The commercial open frame linear supplies  dont use them even with a 3A
load current.

Bruce

I do have some experience with temperature controlled ovens.  I found that a
long term plot of the temperature while using an unregulated supply on the
oven heater showed small random variations due to voltage variations.  When
the oven heater was put on a regulated supply the line became flat.  The
problem here seems to be during the heat up cycle.  The voltage doesn't need
to be regulated during that time.  You might try sensing the error signal
and when it is outside of the linear control range you could switch in a
parallel transistor and resistor to handle the warm-up current and it would
switch out when the oven has stabilized at the lower current.

Regards.

Max.  K 4 O D S.

Email: max@maxsmusicplace.com

Transistor site http://www.funwithtransistors.net
Vacuum tube site: http://www.funwithtubes.net
Music site: http://www.maxsmusicplace.com

To subscribe to the fun with tubes group send an email to,
funwithtubes-subscribe@yahoogroups.com

----- Original Message -----
From: "Bill Hawkins" bill@iaxs.net
To: "'Discussion of precise time and frequency measurement'"
time-nuts@febo.com
Sent: Friday, October 26, 2007 1:21 AM
Subject: Re: [time-nuts] Parallel voltage regulators

Poul-Henning Kamp wrote:

That is a recipe for disaster if one wants a really low noise oscillator.
This is particularly true if one is a beginner.
Reducing the output noise of a switching regulator to 100uV rms or less
is neither easy nor simple.

Bruce

http://www.linear.com/pc/downloadDocument.do?id=4159

In this document, Linear Technology's Application Note 70, a
switchmode power supply design is presented with less than 100uV
output noise, and little RFI. This is achieved by using a switch
controller which has controlled output switch slewing, allowing a
trade-off between EMI/RFI and efficiency. Methods to measure and
control PSU noise are also discussed (IMHO, the EMI sniffer probe in
Section J is particularly interesting).

BTW: contrary to popular belief, linear PSUs are NOT inherently
noise-free. The cap charging current spikes through the rectifier
diodes can produce plenty of EMI/RFI.

JDB
[not claiming it's easy, just pointing out one route that could be taken]

LART. 250 MIPS under one Watt. Free hardware design files.
http://www.lartmaker.nl/

J.D. Bakker wrote:

However  this isnt a recommended approach for a beginner and you can
easily do much better with a well designed linear regulator.

The EMI due to diode reverse recovery and charging the reservoir
capacitor is exacerbated by the modern tendency to leave parts out of
designs, particularly if one doesn't understand their purpose.
A shunt capacitor across the input of a bridge rectifier or a capacitor
in parallel with each of the rectifier diodes can help by reducing the
area of the radiating EMI loop. However some damping may also be required.

Another problem with using switching regulators can arise when using an
input filter with the regulator.
Since the input power is to a first approximation independent of the
input voltage, for frequencies from dc to an upper limit determined by
the switching frequency the regulator has a negative resistance input
characteristic. This means that undesired oscillations are possible if
the input filter design doesn't take this into account. This situation
can easily arise when the load is another switching regulator (eg when
powering a Z3815 from a switching regulator usually Ok when powered by a
switchmode power supply, but if one decides to add a filter between the
switchmode power supply and the Z3815 one may just create an interesting
power oscillator instead of a quieter input supply).

Bruce

Pete wrote:

Using a series resonant topology for the switching power supply can be
helpful in reducing EMI.
However even then one has to be careful to use the correct form of
resonant converter to achieve low noise.
Whilst such regulators can easily be designed they are not commonly
available off the shelf.
The design and construction of such a regulator isnt a task for a beginner.

Bruce

Great idea!!,....................................Don C.

----- Original Message -----
From: "Max Robinson" max@maxsmusicplace.com
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, October 27, 2007 10:07 AM
Subject: Re: [time-nuts] Parallel voltage regulators

Well, there is another trick of the control trade, used to bring
batch process reactors up to the desired temperature in a hurry.

At first, you turn on full heat for an experimentally determined
time. You turn it off at the end of that time, before the oven has
reached final temperature. You leave it off for an experimentally
determined time while the oven coasts up to the peak temperature.

If you've done it right, the peak temperature is the oven setpoint.

Then you preset the oven controller to its normal output. This is
the value that you read when the output is settled out from a plain
old startup.

Now the controller is at its setpoint, and the output is at its
operating point. Turn on the oven supply regulator and enjoy
relatively stable control. This method is considerably faster
than letting the controller respond to the turn on transient by
oscillating its way into steady state.

Bill Hawkins

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Max Robinson
Sent: Friday, October 26, 2007 4:08 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Parallel voltage regulators

); SAEximRunCond expanded to false
Errors-To: time-nuts-bounces+bill=iaxs.net@febo.com RETRY

I do have some experience with temperature controlled ovens.  I found
that a long term plot of the temperature while using an unregulated
supply on the oven heater showed small random variations due to voltage
variations.  When the oven heater was put on a regulated supply the line
became flat.  The problem here seems to be during the heat up cycle.
The voltage doesn't need to be regulated during that time.  You might
try sensing the error signal and when it is outside of the linear
control range you could switch in a parallel transistor and resistor to
handle the warm-up current and it would switch out when the oven has
stabilized at the lower current.

Regards.

Max.  K 4 O D S.

Email: max@maxsmusicplace.com

Transistor site http://www.funwithtransistors.net Vacuum tube site:
http://www.funwithtubes.net Music site: http://www.maxsmusicplace.com

To subscribe to the fun with tubes group send an email to,
funwithtubes-subscribe@yahoogroups.com

----- Original Message -----
From: "Bill Hawkins" bill@iaxs.net
To: "'Discussion of precise time and frequency measurement'"
time-nuts@febo.com
Sent: Friday, October 26, 2007 1:21 AM
Subject: Re: [time-nuts] Parallel voltage regulators

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