Dual PS and GND current problem

[gmorfeus]

Hi,

I have a problem using external PS and programmer I use PicKit3 but, as the device I am developing needs more than 500mA current, I must use external power supply. Well, all that was working until yesterday when PicKit3 GND wire was completely melted out After testing everything on my device and the PicKit3 itself (without external PS), I connected all together with external power supply powered up and then connecting USB of PicKit3. I noticed spark on shielding so I disconnected all. Then I took out cable from PicKit3 and measured voltage between shielding of USB cable and my external power supply GND. Result was 50V:shock: Also, measuring USB shield and Vdd of external power supply resulted in the same voltage. So, apparently, power supply went wrong. I also tested connecting USB to another computer and the result was the same.
Now I soldered power supply using MIC29302 and the output voltage from new power supply is as needed (4V). Now, when I measure voltage between new power supply and USB shealding, I get 0.07V. I think that won't result in melting GND again but I am a little curious:
1. Why that happened in first place? That is: current somehow closed between my laptop charger and external power supply over 220V
2. Is there any thing that should be used in account when designing devices that are connected to different GND?

I must notice that Vdd from PicKit3 and external power supply never get "together": I separated that with a jumper! So the only "interconnection point" was GND.

Thanks,
Gabrijel

 

[D.A.(Tony)Stewart]

Both DC and return ought to be isolated from ground unless the unit is grounded externally.

The chargers may have a grounded filter and have some ac leakage at SMPS f rates to the unit. ( but this is in uA and high impedance)

If one unit does not use an ac line filter but some connection from primary line to secondary return, something is wrong.

Therefore measure off-state resistance to earth ground and on-state voltage to earth ground and to other unit before joining.

 

[gmorfeus]

SunnySkyguy,

thank you very much! Your explanation sound very reasonable. Something's wrong with PS and shall be replaced or repaired.