Modifying power Supply - worth it?

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Bit the bullet and bought myself one of those 0-30V 2.5 amp power
supplies from DSE.  Nice unit, but two problems I didn't anticipate.

1. It has a power switch on the front panel - just on/off.  Doesn't seem
safe to me, as switching it on with an unknown voltage setting while
connected to one of my TTL/PIC projects might result in tears if the adj
knob was jarred etc while the unit was in the off state - and it seems
silly that I have to either disconnect projects every time I switch the
PS off, or fit a 7805 etc on every board.

2. The voltage control is rather 'iffy' in that it is difficult to set
the output at (say) exactly 5.0v - a twitch and its 4.5, another twitch
and its 5.6 (get the drift) fiddle some more and get 4.9 or 5.2 etc
etc.  I realise some variations may not be a problem, but I don't like
it.  Life should be easy.  V should be easy to adjust

So, I am thinking of

1. Fitting a load on/off switch - possibly by fitting a mains power
switch to the rear of the case, and using the current front panel on/off
sw as a load switch

2. If I can, changing the voltage adj. pot to a multiturn pot, assuming
I can find one that will fit in the original pots space - bit doubtful
there, but there is not really enough room on the front panel to fit
another (fine control) pot.

Any comments? Ideas? Anyone else done these (or other) mods?

David - who knows the mods will nullify the guarantee, but these PS's
should be easy to fix, so...


Re: Modifying power Supply - worth it?


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Yep, good idea. Good power supplies have load switches. Very handy.

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Another good idea, and again, this is what the good supplies have.

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Add a few fixed outputs too if you can, say +5V and +12V, you should be
able to tap directly off the filter caps. But watch out because a lot
of supplies will relay switch the transformer taps to avoid excessive
power dissipation. In this case you could add your own
rectifier/filter/regulator board directly on the transformer output.

Just a matter of space really.

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Yep, go for it.

Dave :)


Re: Modifying power Supply - worth it?




quietguy wrote:
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The adjustment pot is a feedback voltage divider to an error amp input.
Find the exact value resistance it takes to make 5.00V, get a 1%
resistor of this value and add a switch to switch pot out and your 1%
resistor in. Then just double check switch is in 5V position before you
turn power on to your ongoing breadboard. But I still wouldn't trust it
to stay within limits on overshoot. Another alternative is to restrict
the range of adjustment by paralleling pot with resistance. Find exact
value you need to limit to 6V, obtain 1% in this in this value
etc...this might be better because you can then turn supply on at 0V
output and fine adjust to 5V at your circuit.


Re: Modifying power Supply - worth it?


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A multiturn pot with integrated on/off switch:

1) always starts off at the lowest voltage
2) more precise control over output voltage

Do they exist?

Dave

Re: Modifying power Supply - worth it?


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Or another idea, replace the the pot with a multiturn and add a simple
logic cct to detect if the output voltage is above a certain threshold
at switch on, (say 2.5V).  If it is, power to the output is disconnected
via a mosfet/relay/etc.

Dave

Re: Modifying power Supply - worth it?


I read in sci.electronics.design that quietguy
'Modifying power Supply - worth it?', on Sun, 6 Nov 2005:

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Can you not find space on the panel even for a miniature toggle switch
for load switching? They have surprisingly high current ratings at low
voltages.

That would save you disturbing the mains wiring.
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If the pot is wirewound, consider replacing it with a cermet pot. They
have much smoother variations of resistance with slider position.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
We've slightly trimmed the long signature. Click to see the full one.
Re: Modifying power Supply - worth it?


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It's sure easy to make, or buy, a regulated fixed 5.0V supply.  If most of
your work is 5.0V logic, make or buy one.  You'll thank yourself in the long
run.

When I built my first bench supply in my youth, I looked at existing
supplies and thought the feature set was kind of dumb, and built one that
had a load switch and a fuse.  I quickly discovered that having adjustable V
instead of a load switch, and current limiting, was a much better approach -
turn on the juice quickly to a circuit, and one of two things happens:
either the circuit was good and the fuse blows while you're charging up the
various capacitors in the circuit, or the circuit was bad and the fuse blows
shortly after the circuit does, or the fuse was rated too high and only the
circuit blows :-)

So these days I find that I'm quite happy with a supply that doesn't have a
load switch, just a smoothly adjustable output voltage and a meter to
display it.

But if your output voltage isn't smoothly adjustable, that's worth fixing,
by replacing the existing pot with a better-quality one (cermet, or even
multi-turn).



Re: Modifying power Supply - worth it?


On Sun, 6 Nov 2005 17:19:20 -0800, "Walter Harley"

(snip)

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and/or consider putting a highish value resistor from the wiper pot to the
"cold" end, so an open wiper doesn't hit your load with max volts.

Re: Modifying power Supply - worth it?



"quietguy" <

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** Have you checked to see if the voltage jumps up to full at switch on ?

 One of the DSE  bench PSUs was notorious for that.



.........  Phil





Re: Modifying power Supply - worth it?


Thanks for that Phil - I haven;t seen that at switch on - my concern was
that the adj might get bumped while the PS is off.  However, I noticed that
at switch off the V meter jumped to a high reading (20+V) - but with my DVM
on the OP terminals I could not see that high reading - but I need to check
with the CRO as my DVM doesn't have a sample? facility and may not have
'seen' the transient.

David

Phil Allison wrote:

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Re: Modifying power Supply - worth it?



"quietguy" <
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 **  The CRO is the go.

Most DMMs with "sample" only store ( max or min) readings from the display -
not input transients.

The actual sampling rate is only 2 or 3 times per second  - so events that
last less than 300 mS are not captured accurately.




........   Phil







Re: Modifying power Supply - worth it?


I can 'see' some transients on the CRO at sw on and sw off, but they are so
brief that it is hard to see the amount of deflection etc - probably need a
storage/digital scope for that.

I am discussing some of this with the mfgs engineer - he is sending me a
circuit diagram and seems interested in my comments - will update.

David

Phil Allison wrote:

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Re: Modifying power Supply - worth it?



"quietguy"


 ** Gee I wish you would  **NOT** top post -  it really stuffs up writing
replies.


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** Turn the horizontal ( time base) off.

 Centre the dot and make it nice and bright.

 Amazing what brief transients you can see and measure then.



............  Phil



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Hi, David -

I'd bet you are seeing transients induced into the probe. Run a null test.
Leave your scope ground lead connected where it is but connect the probe
tip
to the probe ground wire (that's right, short it out). Run the test. Do you
still see a transient? If so, it may be the transformer inrush inducing
noise into the probe. Could be radiated or induced via grounds, inductive,
or capacitive coupling. It sometimes takes heroic efforts to make sure
you're seeing a valid signal in the presence of noise.

You can read about the shorted probe thing here:
http://emcesd.com/tt070199.htm

In fact, you'll learn a lot if you read *all* his papers.

Good luck.

John



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Or a transient caused by the rising edge of the power supply ringing against
the inductance of the probe leads.  Do you have the probe adjusted so that,
when a square wave is applied to it, it looks like a square wave on the
scope?  (That's partly what the 'calibration output' of the scope is for.)

But either way, this is just more reason (IMHO) why switching the power
supply on abruptly, whether with a load switch or the mains switch, is not
how you want to power up your circuit.  The bypass caps in your circuit are
doing their best to look like a short circuit; the power supply is doing its
best to look like a zero-impedance voltage source; so you're whacking it
with a heck of an initial current spike, depending on how fast the current
limiting in the supply (if it has any) can kick in.  Much better to ramp the
voltage up slowly with the voltage adjustment pot.  If this adjustment is
jerky or uneven, fix the problem, don't work around it.  Wait till you know
your circuit works normally, before you go testing its ability to withstand
power supply transients!



Re: Modifying power Supply - worth it?


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There is a lot of stuff that doesn't like being powered up slowly, it
can cause latch-up and other problems. Something to be careful of.

Dave :)


Re: Modifying power Supply - worth it?




Walter Harley wrote:

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Yep, shows a nice square wave from the cal terminal

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While I wouldn;t want to challenge the 'rightness' of what you say Walter, my
initial thoughts are that to provide such an artifical power source may lead to
probs when the circuit has to eventually face the real world.

Isn;t it better to design so that the circuit handles the real world of power
surges etc?  Rather than find this out later?

David



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IME, the first time you power up your circuit, the question is not usually
"how well does this handle real world conditions", it's "did I wire this
thing up the right way or not".  If the answer is "or not," you'd rather
find out when the voltage on it is about a volt, not when it's 5v and too
late.

David's right that there are circuits that don't handle a gradual ramp up in
power; latchup, oscillation.  But personally I'd worry less about that than
about zapping an untested circuit with full juice.

Current limiting is obviously a very important feature in a power supply.
With good current limiting many other sins are forgiven.  Sometimes I start
the circuit at full voltage but with the current limiting set to zero, and
then ramp in the current instead of ramping up the voltage.



Re: Modifying power Supply - worth it?


OK thanks Walter.  Having just got back into electronics after a 30 year break,
I am not used to all the luxeries that are around today - Back in those days
playing with my Exidy scorcerer and a bunch of TTL interfacing stuff it was a
matter of getting right 1st time!

But times have changed, and I need to change too

Thanks for the good advice

David

Walter Harley wrote:

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Re: Modifying power Supply - worth it?


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Yes, adjustable current limiting is an essential feature of any good
power supply.
Gotta watch it sometimes though, one setting might be OK for the steady
state current requiremens of the circuit, but the power on current
requirement is often much larger. So the current limiter will kick in
for a brief period and can cause latch up problems as I mentioned. Can
be really hard to track down if you don't know what's happening.

If you have a current limited supply then the general rule is as you
mention - set the volts you want and ramp up the current.
If you don't have a current limited supply then you would do the
opposite and ramp the volts up and watch the current meter.

Dave :)


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