How to power a DSO150 when connected with UART to computer

[MrElectric]

I finally had time to go on with my DSO150 repair project, but got stuck when connecting it to my computer using a CP2102 UART.

I powered it on using my bench power supply, set to just 8 V to be on the safe side. The screen lights up, until I connect the USB UART, when a 3.15A fuse blows in my bench power supply. I tried this twice with the same result. I'm quite sure this is what happened last time I tried as well.

The current limiting was set quite low, and it was only consuming 50 mA when I checked right before connecting the UART to the computer.

The only connections between the UART and the DSO150 were GND, TX and RX.

Apparently, the computer and the power supply had different potentials.. Didn't think this would matter when the GND was connected between them.

I considered trying a simple power adapter, but I bet the same thing is bound to happen then.

The solution seems to be to use a 9V battery, to make sure it's electrically isolated. I don't have an isolation transformer, which might be another way to solve this. However, this idea struck me too late.

Foolishly, I decided to use the 3.3V output from the UART to power the DSO150. The DSO150 powers up ok, but the UART doesn't work anymore. It seems like the CP2102 has a 3.3V built-in voltage regulator that delivers the 3.3V, but it only delivers 100mA. I just assumed that the 3.3V was delivered straight from the computer. I should have checked the USB pinout, which clearly states that USB only delivers GND, +5V and two data signals. So, I think I fried the CP2102. Need to order a new UART.

Is this a problem with all UART's? I saw someone else recommend a FTDI FT232 based uart.

[ebastler]

Very strange. If you used a proper bench supply, its outputs should be floating (potential free). And if something goes wrong, its current limiter should kick in, rather than blowing a fuse.

Could you name the exact type of power supply, and document how you connected it?

[MrElectric]

Thanks!! Your reply made me start to think, so I did some measurements on my bench supply, and I found the problem! Measuring between the chassi (earthed) and the positive or negative terminals, there was some kind of connection, through a capacitor it seemed.

Looking at the circuit diagram, there should be no such connection. I had to hunt for a while before I found that the mica isolator between the chassi and the transistor was causing the problem. Disconnecting the transistor made the problem go away.

The bench supply is one I built myself almost 30 years ago. It seems I had not aligned the holes perfectly, so one of the holes had become larger, and when tightening the screws holding the transistor to the chassi, there was a connection between the collector and the chassi.

I have never noticed the problem before, as I mostly use it for powering up isolated circuits.

[ebastler]

Hey -- great to read that you found the problem, and that your trusty power supply is working as intended again!