Elenco xp-720 kit / oscilloscope question(s)

[EricBNYC]

Hi all,

Pretty much of a noob here, proud to have completed the Elenco XP-720 power supply kit.  It looks like i did a pretty good job, the outputs are what they should be according to my multimeter, and the dc output looks nice and flat on my oscilloscope like I'd expect to see.  I have a couple of questions for anyone with a moment:

Page 14 of the attached manual shows different waveforms for different points of in the circuit.  As the proud new owner of a (used, from ebay on Dave's suggestion . . .) Tektronix 2225 I'd like to use it to view these waveforms at the correct points in the power supply, but i don't know exactly where to attach the ground and probes and I'd like to avoid blowing anything up.  Anyone care to make suggestions?

Also, the manual mentions that this should be a mins-isolated power supply, can anyone confirm this?  Schematic is on page 18 of the attached manual.

Thanks to anyone who care to help,
Eric

[MikeK]

Make sure you watch EEVblog #279.

Also, make sure you scraped away the paint from the case where the mains ground strap attaches (next to the transformer), you want the case to be a good earth ground (for operating safety).

I have this same supply and, yes, the output ground is isolated from mains ground.

[w2aew]

Yes, it is mains isolated.  Any voltage waveforms shown in the regulator circuit would me measured with respect to the COM terminal. You can safely connect your scope probe ground to the COM terminal.

[EricBNYC]

Thank you both.

Shame on me - I noticed that the ground wire was attached to a painted piece of metal but just blindly hoped that "they know what they're doing".  I'll scrape it clear tonight 

[Monkeh]

Thank you both.

Shame on me - I noticed that the ground wire was attached to a painted piece of metal but just blindly hoped that "they know what they're doing".  I'll scrape it clear tonight 

Test the connection. You might be surprised.

[MikeK]

Eric,

Once you've used your power supply a few times you'll get tired of using a multimeter to set the variable outputs.  I built a simple display for mine.  It's a good project.  And cutting the case for the LCD took longer than building the circuit, which is nothing more than a PIC, an LM324, an LCD, and some precision resistors.

Mike

[grenert]

Thank you both.

Shame on me - I noticed that the ground wire was attached to a painted piece of metal but just blindly hoped that "they know what they're doing".  I'll scrape it clear tonight 

There's still the bolt +/- washer, which is screwed into the case, so you're probably grounded despite the paint.  But as Monkeh says, just test the connection and see.

This power supply can become a project in itself.  Besides adding a meter, you can also do other improvements:
10-turn pots to adjust voltage.
Relay to switch on/off the outputs.
Current limiter.

Others might think that you might as well started with a better supply design, but I think the benefit of this one is that you can get up and running quickly, then make it better later as you have time, and you learn thing along the way.

[EricBNYC]

Mike,

Are there instructions and/or a parts list somewhere?  I'd like to try adding a display to the power supply.  I also figure on adding some finer-control pots to dial in the voltage, too.  Those, I've seen part nubmers and photo's by other's who have done that mod.

[T4P]

These "DIY" power supplies ( I hope they make higher power versions, will be very interesting )
are good for "purely botching your way through"

Feel free to modify the entire thing

[MikeK]

Are there instructions and/or a parts list somewhere?  I'd like to try adding a display to the power supply.

Nope, it's all homebrew.  A simple and worthwhile lesson in op-amps, though.  One OA in the LM324 is used for inverting and scaling the power supply's negative variable output.  Another is used for buffering the output from a voltage divider that scales the power supply's positive variable output.  The two unused OAs in the LM324 can be used for current measurements (a future project).  The OA outputs go to the PIC's ADC.  Or microcontroller of your choice.  You just measure the ADC pins and output the values to an LCD.  If you have no uC experience this might be a steep learning curve.  The op-amp stuff is straightforward.

[grenert]

Here's one way to add a digital readout:
www.n5ese.com/xp720.htm

[MikeK]

Another mod this power supply needs is to do something useful with the AC outputs.  It must have been designed a long time ago when there were vacuum tubes.  I'll probably rectify it and make a 3.3V fixed output.

[ivan747]

It also needs a good earth terminal, of course.