2
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

I have built a wooden clock that is powered by an electromagnetically driven pendulum. Circuit given above was originally developed by a chap called Dick Bipes. In principle it works by a magnet attached to the end of a pendulum passing over a coil which detects the passing magnet and after a short delay, sends power to the coil to repel the magnet and keep the pendulum moving. An MSP430 processor is used to do some clever calculations and keep the timing accurate.

The circuit works fine when powered by 6V battery but ceases to work properly when I replace the battery with one of those plug in variable voltage power supplies! The processor doesn't initialise and I get no output to the coil. I have checked voltages and everything appears to be fine - correct voltage to processor, etc.

However, when using the power supply, if I connect the negative rail from my breadboard to Ground on my Launchpad which is connected to my laptop via USB and the laptop is plugged into the mains, everything springs to life! (Doesn't happen unless laptop is plugged into mains!)

So sounds like some sort of grounding issue but why does it work OK on battery and not power supply? (Have checked power supply and it is providing the expected 6V). Any thoughts or ideas on how I can resolve this frustrating issue?

\$\endgroup\$
  • 1
    \$\begingroup\$ Did you use decoupling capacitors for your processor? A wall wart produces more noise than a battery. \$\endgroup\$ – Mario Oct 6 '16 at 15:47
  • 1
    \$\begingroup\$ Is your negative rail properly connected to the negative of the power supply? Or is there a loose connection in the barrel jack etc. That's pretty much the only explanation for your "however" paragraph. \$\endgroup\$ – pjc50 Oct 6 '16 at 15:52
  • 1
    \$\begingroup\$ Is your wall connected power supply floating? (Not connected to ground.) \$\endgroup\$ – George Herold Oct 6 '16 at 15:55
  • \$\begingroup\$ Thanks for quick replies. Mario, processor is decoupled from input supply by LDO voltage regulator surrounded with a number of caps so I think it should be OK. pjc50, have double checked and as far as I can tell, all is firmly connected. George, power supply has only + and - wires, no earth/ground and therefore probably floating? \$\endgroup\$ – Garth Oct 6 '16 at 18:35
  • \$\begingroup\$ Try add large 470uF cap to battery location to remove dips if no cap in wallwart. \$\endgroup\$ – KalleMP Oct 6 '16 at 21:06
1
\$\begingroup\$

You have noise current pulses from D2 which creates large dI/dt * wire inductance (voltage noise spikes) and this can cause glitches to nearby cables and supply voltage.

If adding a Low ESR cap to PS out does not help, a large CM choke between Vdc out and in will help greatly to isolate glitch current noise. (100uH CM choke recommended or large ferrite clamshell around DC cable source.

| improve this answer | |
\$\endgroup\$
0
\$\begingroup\$

Your power supply provides power between the + and - terminals. The ground connection is there to give you the ground reference w.r.t. to the supply's ground.

So you actually need to do exactly what you did -- connect the ground terminal to the - terminal to make it a +voltage supply.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Problem is that the lead from the power supply has only 2 wires, + and -, no ground. I guess I need to find one with a ground as well. If so, bearing in mind low voltage, is it safe to tie -ve to ground? Since the power adaptor for my laptop only has + and - as well but seems to provide a ground, as given in my original post, I assume that it does exactly this? \$\endgroup\$ – Garth Oct 6 '16 at 18:45
  • \$\begingroup\$ Just answered my own little question. On my laptop power supply a continuity check between outer sleeve of connector and earth pin on plug shows they are tied or connected in some way. \$\endgroup\$ – Garth Oct 6 '16 at 19:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.