Amplitude modulate variable regulator

[Glenn Myers]

I have a few of these 0-30VDC variable power supply kits left over
from a previous project.

http://www.4shared.com/account/dir/6bfaqiMd/_online.html?rnd=24#dir=43926040

It uses a UA723 IC regulator.

Is it possible to modulate the output by inserting a control voltage
(via a function genrator), that would act the same way as manually
varying R10?

Any idea what the maximum modulating frequency might be?

I would like to know how so as not to damage the IC. Thank you for any
suggestions.

Glenn Myers

 

[mike]

I have a few of these 0-30VDC variable power supply kits left over
from a previous project.

http://www.4shared.com/account/dir/6bfaqiMd/_online.html?rnd=24#dir=43926040

It uses a UA723 IC regulator.

Is it possible to modulate the output by inserting a control voltage
(via a function genrator), that would act the same way as manually
varying R10?

Any idea what the maximum modulating frequency might be?

I would like to know how so as not to damage the IC. Thank you for any
suggestions.

Glenn Myers

link don't work, so no idea what R10 is, but it's an op-amp.

You can modulate the control voltage.
So, yes, you can modulate the control voltage to make it work like
manually varying the voltage set pot.

But is that what you want????

Be aware that the output is typically unidirectional.
You have active current in one direction only. The speed of
the other direction depends on the load and the output capacitor.
If you use a current limit, trying to slew too fast into a capacitor
will put
it into current limit.

 

[Glenn Myers]

I get "The file link that you requested is not valid."

Very (extemely) sorry about that. This one definitely works.

http://www.4shared.com/photo/tN1TX3ES/Power-Supply.html

The question was how to modulate the IC with a low frequency sinewave
.... and any downsides.

Many thanks for the comments so far, and for looking again.

Glenn

 

[Fred Abse]

[quoted text muted]

Very (extemely) sorry about that. This one definitely works.

http://www.4shared.com/photo/tN1TX3ES/Power-Supply.html

The question was how to modulate the IC with a low frequency sinewave ...
and any downsides.

Many thanks for the comments so far, and for looking again.

My experience of similar circuits using the 123/723 is that they have the
habit of going unstable in the low MHz range.

Commercial ones, too. The only cure in such cases was to reduce the loop
gain and accept a little poorer regulation.

Stay away from 723s ;-)

 

[Glenn Myers]

(1) Midpoint (setpoint) voltage?

Even though the linked circut is shown operating from a 30VDC supply,
I will be using a single 12V lead acid battery. So anything close to
that.

BTW I am not using this circuit to charge a battery. Only as a high
current signal source.

(2) Desired amplitude of modulation (at OUT)?

As high as is practical without re-jigging the entire circuit. 5 or 6V
would be acceptable.

(3) Frequency of modulation?

Between 1-60Hz is all I really need. But I would be interested to see
the upper limit.

(4) Load value, resistive, reactive?

Whatever current the circuit will comfortably sink ... preferably lots
given the TO-3 stage. Load, I assume, would be more or less resistive
at these low frequencies. Ceramic load resistor, solenoid, etc.

Thank you for your reply.

Glenn Myers

 

[Glenn Myers]

Too vague. Write down your requirements as if you were composing an
RFQ for something you would buy if it met your specs.

This is just an experiment Jim, not a military contract I'll
settle for anyone's best guess ... expect mine.

Glenn

 

[mike]

Without thinking I tried to use it as a cheap and dirty speaker
amplifier. But it only will source current, not sink it, so you get a
horribly truncated wave into the speaker through the coupling
capacitor.

Jim

Yep, but it does do what you asked..."it acts the same way as
manually varying R10."
As others have stated, just because it's not a military contract
doesn't relieve you from specifying exactly what you want it to do.
The hardest part of any project is specifying, IN DETAIL, the result you
expect.

 

[P E Schoen]

"Glenn Myers" wrote in message

This is just an experiment Jim, not a military contract I'll
settle for anyone's best guess ... expect mine.

If it's just an experiment, then you can just give it a try. You probably
won't damage anything, and if you do, and you have a bunch of these surplus,
it's no big deal. But as Jim points out, you should have some idea of what
you are trying to do, and for modulation up to 60 Hz into a resistor it
should work just fine. But a solenoid may be a different animal, and you
have to provide some way to deal with the inductive "kick" when the supply
is going low. A diode across the coil will probably be all that is needed.
But it is still a mystery what you have in mind.

If you really want a power amplifier or a function generator, then there are
some very inexpensive ICs that can do what you want MUCH better than the
723. It's been so long since I've used one I forgot they even existed. Even
for power supplies, ICs like the LM317 or LM2931 are much better and cheaper
(like 25 cents) and require less external components. And for about a dollar
you can get a 4 watt stereo amplifier TEA2025 in a DIP package:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000172.pdf

I know it is tempting to use parts you already have to build something, but
usually it is better (and cheaper) to get a more modern part that does
exactly what you want. And it's also futile to imagine possible alternate
uses for parts you have. I have thousands of surplus parts that are now so
old that they are obsolete, or damaged because of humid storage conditions.
I hate to throw them away, but I've had them for 20 years and I've only uses
a few of them and I've sold a few, and it really cost me more in time and
effort than I got from the sales.

If you want some parts really cheap, they are listed at:
http://www.pstech-inc.com/surplus.htm
and an even more complete listing at: http://www.pstech-inc.com/PARTS.txt or
http://www.pstech-inc.com/PARTS.xls

No guarantees on what I actually have or what condition, but I'd be happy to
find good homes for not much more than shipping costs from 21030.

Paul

 

[Phil Allison]

"Glenn Myers"

I have a few of these 0-30VDC variable power supply kits left over
from a previous project.

http://www.4shared.com/account/dir/6bfaqiMd/_online.html?rnd=24#dir=43926040

It uses a UA723 IC regulator.

** The minimum output voltage is 2.2 volts - not zero.



.... Phil

 

[Phil Allison]

"Glenn Myers"

http://www.4shared.com/photo/tN1TX3ES/Power-Supply.html

** The value of C1 is WAAAAYYY to low at 1000uF.

At 3 amps the ripple would be 25Vp-p.

10,000uF is more like it.



.... Phil

 

[Phil Allison]

"Glenn Myers"

Whatever current the circuit will comfortably sink ... preferably lots
given the TO-3 stage.

** The TO-3 stage cannot sink ANY current, you will have to rely on the load
to do that.

The main limit on the voltage rising is C3, the 100uF cap - with a 3A
current limit, the max possible rate is 30,000 volts per second. This
equates to a 6 volt peak sine wave at 800Hz.

There is no simple way to make it voltage controlled - but maybe an LDR
with LED drive as one arm of VR1 would allow rapid adjustment.


.... Phil

 

[Glenn Myers]

Thanks guys. I think I've got the picture now .. or at least enough to
try a few things.

Glenn

 

[Phil Allison]

"Dill Slowman"

The particular virtue of the 723 is that is uses a voltage reference
based on a avalanche breakdown (at 7.15 +/-0.35V), which is quieter
than the band-gap references used in more modern regulator chips, and
some people keep on using the part for this reason.


** How dopey.

Vref is accessible for RC filtering of the noise - so it can be easily
reduced to the uV level.


In every other respect it's pretty diabolical.


** Like hell it is.



.... Phil

 

[Phil Allison]

"Dill Slowman"

The particular virtue of the 723 is that is uses a voltage reference
based on a avalanche breakdown (at 7.15 +/-0.35V), which is quieter
than the band-gap references used in more modern regulator chips, and
some people keep on using the part for this reason.

** How dopey.

Perhaps. I'm just reporting facts, not expressing an opinion.


** You just expressed another wrong opinion.

Vref is accessible for RC filtering of the noise - so it can be easily
reduced to the uV level.

In every other respect it's pretty diabolical.

** Like hell it is.

So how do you like it's mechanism for implementing a current limit?


** LOL !!

The man's about as a sharp as a bowling ball.....



.... Phil

 

[legg]

Very (extemely) sorry about that. This one definitely works.

http://www.4shared.com/photo/tN1TX3ES/Power-Supply.html

The question was how to modulate the IC with a low frequency sinewave
... and any downsides.

Many thanks for the comments so far, and for looking again.

Glenn

You can modulate the output of this circuit by disconnecting R13 at
the pin six connection, setting the pot for maximum, then applying a
0-7v5 dc signal to the open end of R13.

The output will be a Vmod/7v2 fraction of the pot's original setting,
and the pot can still be used as a manual full scale adjustment, while
the external modulator is in effect.

This modulating signal could be the output of a filtered PWM, PAM, FM,
DAC or whatever.

RL

 

[legg]

Some improved survivability of this circuit is provided by:

An emitter-base resistor (1K-4K7) across TR1.
A series collector resistor for TR1.(~100R)
A fuse in series with the line input.

TR2, not being diffused mesa (as it would have been at the time of the
original design) needs foldback current limiting:
A series base resistor for TR3. (~5K)
A Vce-dependant offset provided by a resistor from base of TR3 to the
output. (~100xRB of TR3) This should ~halve the output short circuit
current. If you still want 3A of short circuit current, you should use
a heftier pass transistor, or two 2n3055s in parallel, with ~ 0.3R
emitter ballasting.

At 3A, C1 will likely also need to be larger.

A 1N5401 across the output terminals may ease series/parallel
operation of multiple units and is pretty standard in even the
cheapest products.

RL

 

[Glenn Myers]

You can modulate the output of this circuit by disconnecting R13 at
the pin six connection, setting the pot for maximum, then applying a
0-7v5 dc signal to the open end of R13.

The output will be a Vmod/7v2 fraction of the pot's original setting,
and the pot can still be used as a manual full scale adjustment, while
the external modulator is in effect.

This modulating signal could be the output of a filtered PWM, PAM, FM,
DAC or whatever.

Thanks for the expert technical advice. This is something to actually
work with.

Glenn