Modifying power Supply - worth it?

[quietguy]

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...

 

[David L. Jones]

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

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

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.

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

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

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.

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

Yep, go for it.

Dave

 

[Fred Bloggs]

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...

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.

 

[Dave]

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?

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

 

[Dave]

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?

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

 

[John Woodgate]

I read in sci.electronics.design that quietguy

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

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.

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.

If the pot is wirewound, consider replacing it with a cermet pot. They
have much smoother variations of resistance with slider position.

 

[Walter Harley]

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.

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).

 

[Phil Allison]

"quietguy" <

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.

** 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

 

[quietguy]

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

 

[quietguy]

Thanks for all the ideas and info guys - I got inside the case for a short

time today and the mods may be harder to implement than I had
thought/hoped, as the OP terminals, meters, and V and A adj pots are all
mounted on a pcb which backs the front panel.

Will look more closely when I have time to completely disassemble the unit,
but at the moment I may have to...

Bridge the V adj pot to limit its range somewhat, if there is no room to fit
a cermet pot (don't know about these, have to research) - it wouldn;t bother
me if that limited the range to 0 to 15v or such.

The load switch may have to be external, as the connections to all the OP
terminals are PCB traces - but will see if there is a chance of breaking the
common earth to them

Adding fixed 5v is a good idea, but am not sure if I can do that and have
the OP metered - building a separate fixed 5V supply for my PIC and TTL
stuff might be the way to go

Cheers and thanks again

David

 

[Phil Allison]

"quietguy" <

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.

** 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

 

[budgie]

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

(snip)

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).

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.

 

[Frithiof Andreas Jensen]

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

*Always* -

With the Chineese gouging the earth for raw materials to change into IOU
notes, there is simply no point in buying components and building something
for ten times the cost of the commercial unit; for my hobby purposes it's
cheaper to buy something "almost-there" and then hack it to fit.

 

[quietguy]

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]

"quietguy"


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

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.

** 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

 

[John - kd5yi]

message

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:

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

 

[ehsjr]

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...

Modify the case to give enough room to add whatever controls/jacks/
meters you want. Just screw a project box to it, and run the
necessary leads into the project box. A fixed 5 V output makes
sense. For smoother and more precise control, add a selector switch
to select an appropriate resistance string for a range, and place
your existing pot in parallel with part of it to lower the resistance.
Or put a multiturn pot in series with a suitable resistance string
for the range to increase the resistance.

A nice "expanded precision" pot is this circuit:

-----+-----+
| |
| [R]
[R] |
| [POT]
| |
-----+-----+

Ed

 

[Walter Harley]

[...]

I'd bet you are seeing transients induced into the probe. [...]

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!

 

[David L. Jones]

[...]

I'd bet you are seeing transients induced into the probe. [...]

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.

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

 

[quietguy]

Did as you suggested, but couldn't see any transients, even on the CROs
sensitive settings - thanks for the suggestion though - however, due to my and
my gears limitations I don't discount what you say.

David