Connecting grounds

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Projectitis

Projectitis

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Hi all,

I have a T_3.6 powered by USB.
I have a TFT display powered by an external source at 3.3v.
Should I connect the grounds of both together?

Cheers,
Peter
 
AFAIK- yes for data transfer lines to have usable value the devices need to have a common ground.

What other havoc might result I can't address should power paths cross - but data only lines and shared GND seem the norm.
 
It is not impossible to have power supplies with different absolute ground potentials. Before soldering them together it can be a good idea to measure the voltage with a multimeter and confirm something does not have a floating mains potential on it.

If you are not sharing grounds you will need to head down the optoisolator route which would be non trivial for a display interface given reasonable speed demands.
 
GremlinWrangler said:
... Before soldering them together it can be a good idea to measure the voltage with a multimeter and confirm something does not have a floating mains potential on it. ...
Click to expand...

Thanks GremlinWrangler - that measurement thought occurred to me ... but didn't want to imply I knew why to say that.
 
Thanks people. I think I should probably cut the trace and run both T_ and display from the same power supply, but I can't do that at the moment (trace not accessible in my setup).
Just checking - if I cut that trace to power the Teensy from an external source, can it still be programmed via USB?
 
If you cut the trace, that is only the 5V USB in, you can still program (assuming 3.3V power to run the Teensy is there) but you still have shared ground with the USB data and your power supply ground. It should not be a problem, but do have a dead USB port beside me right now because should is not the same as definite. Specifically the USB charge port on my desk power board floats up to around 100V AC (half 240 V mains), the USB on the desk computer is grounded to the building ground. Did not measure the current flow but was enough to kill the port, though thankfully not the rest of the PC.
 
Thanks GremlinWrangler. Would I measure between GND on Teensy and -ve on external supply to check for floating?
My external power supply is 240VAC to 12VDC laptop adapter, then plugged into a breadboard power supply that outputs 3.3v (or 5v).
Sorry - beginner question.
 
That's the idea

It is quite possible you will find up to 100 volts there, if you do second step is to add something like a 10K resistor between the grounds and see if the you just have capacitive coupling through the transformer (normal and will fall to close to 0 with a resistor there) or something more exciting. The voltage across the 10k resistor will give you an idea of how much potential current will flow if you actually connect things together directly.

If you are doing sanity checks on your power, also a good idea to check the no load output as well before going onto a teensy. Many PSUs are high on no load, which can mean if you have part of your circuit off you end up toasting the rest of it, like the Teensy which only needs a couple of 100mv over 3.3 before it's dead.
 
Voltage between external power -ve and Teensy GND (when powered from USB) fluctuates somewhere between 100mV and 15mV. This is just using a multimeter as I don't have an oscilloscope, but just waving the probes about in the air gives the same fluctuation! So I'm going to assume this is ~0. Certainly not 100V, so that's good.

The current between +ve and -ve of external power with no load is 0.00mA.
 
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