Issue with floating power supply

[CRi]

Hello everyone,

I have just bought a Dataman-40Pro eprom programmer and I'm having some problems with it.

The first time I connected the programmer, everything went fine. When I disconnected it, I felt an electric shock (I was touching the case of the computer and the USB cable coming from the programmer). The second time I connected the programmer, it was dead, not recognized by Windows (USB device unknown) !

Upon further investigation, I discovered that the negative side of the power supply is floating at 85V AC above earth potential!
I measured a 85v AC potential between the ground of the usb cable coming from the Dataman and the case or usb ground on the computer side.

I ordered a second unit and the symptoms are exactly the same !
Dataman support told me that's how it's supposed to work... but what can a do to prevent it to break again?

Thanks in advance !
CRi

[Dataman (Neil Parker)]

Hello CRi,

I don't have enough information about this situation. What is your "PC" (desktop, notebook, tablet, ... mainly: is it connected to GND or not?) Voltage is voltage, but this alone is not enough, the current is more important. What is the current going from PSU to GND? And importantly, GND is the reference point here.

The "electric shock" you are feeling is caused by the two-wire connection between the mains lead and the power adaptor which is not earthed due to the lack of a third pin, which is common for switch mode PSU's.

The programmer therefore exhibits an electrical potential (voltage) between its exposed metal parts (USB shield) and earth ground. Since there is no earth, your body basically acts as a wire that can conduct electric current, hence the tingling/jolting sensation.

This can be exacerbated depending on the conditions of a person's skin--whether it's oily or dry--and the surrounding environment. Humidity levels, flooring types or desk material all play a role here.

The electric current on all Dataman products have been measured and found to be well within the safety limits per safety standards--IEC950, EN60950, UL1950, etc., even with an input voltage of 240 volts. The voltage (tingling sensation) does NOT present any risk of injury to the user.

We would recommend that you unplug the AC adaptor from the programmer before attaching the USB cable to the PC, as this eliminates the possibility of experiencing the tingling sensation.

This situation should not have caused any damage to the programmer or host PC's USB ports, this would indicate a different problem.

If you have any questions, please contact me.

Best regards,

Neil Parker
Dataman Programmers

[CRi]

Hello Neil,

My PC is a notebook with three prong plug, the ground is connected to earth.
I measured about 0.1mA going from the Dataman ground to the PC ground (earth). I only have a cheap multimeter on hand so it might not be very accurate, I can bring a good one if it helps.

Thanks to your detailed reply, now I understand better why the electric potential is a non-issue... but that doesn't explain why my first programmer died.

I will make sure to always connect the USB plug before the PSU... but no-one is perfect and I (or a colleague) could connect it the other way around one in a while, and I want to be sure it won't break a second time !

Would using a three prong plug PSU be better, or could it cause other issues like ground loop?

Thanks in advance,
CRi

[Dataman (Neil Parker)]

Hello CRi,

>but that doesn't explain why my first programmer died.
I am afraid that I don't have a explanation for this. The protection diodes inside of the programmer can withstand 100V/0.1A without problems. Therefore the reason for the damage (if it really is damaged) must be different.

The "USB device unknown" message might be simply due to a bad software installation .. or to could be due to many other reasons.

Without having the programmer in question back here in our lab for analysis it is difficult to say any more.

Best regards,

Neil Parker
Dataman Programmers