Yep, good idea. Good power supplies have load switches. Very handy.
Another good idea, and again, this is what the good supplies have.
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.
"quietguy" wrote in message news: snipped-for-privacy@REMOVE-TO-REPLYconfidential-counselling.com...
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).
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 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.
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
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
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.
I read in sci.electronics.design that quietguy wrote (in ) about 'Modifying power Supply - worth it?', on Sun, 6 Nov 2005:
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.
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.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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.
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
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.
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" wrote in message news: snipped-for-privacy@REMOVE-TO-REPLYconfidential-counselling.com...
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:
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In fact, you'll learn a lot if you read *all* his papers.
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!
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.
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.
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.
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