5V supply for BEC755 encoders on 7i77/6i25

Hello All,

in my retrofit, the x,y,z servos are SEM-MT22 with BEC755 encoders.

The datasheet shows a typical current consumption of 80mA per encoder.

So a minimum of 240mA must be provided.

Is it reasonable to withdraw this amperage from an ATX PSU through the 6i25 ?

Thanks.

In general its better to power the 7I77 from an external 5V source but
you can see if the cable power option will work by setting the 6I25/7I77 jumpers
for cable 5V power, and checking the 5V supply voltage at the 7I77 (at TB1)
when loaded.

Cable power
7I77 W5 LEFT
6I25 W2 UP
7I77 TB1 UNCONNECTED
External power
7I77 W5 RIGHT
6I25 W2 DOWN
7I77 TB1 --> 5V 1A external supply (or stolen from PC USB/Disk cable etc)

What do you mean by 'bettter' ?

Can you explain ?

What is the max amps that can be withdrawn from the 5V 7i77 without an external 5V power supply?

Its usually better if you have significant load to power the 7I77 5V externally.
This is because there is significant voltage drop along the DB25 cable.

This means the 5V supply at the 7I77 may droop significantly.
You can test the 7I77 with cable power and probably use cable power if you have low power encoders but its not optimum.

The 7I77 draws about 400 mA by itself from the 5V supply with no load on the analog outputs, if you add ~250 mA for encoders and load the analog outputs you could get up to perhaps 800 mA. Depending on you ATX power supplies exact output voltage
the drop along the cable with a 800 mA load might lower the 5V at the 7I77 to less than the specified 4.75V (this is why i suggested testing the voltage at the 7I77)

Thank you PCW for this clear explanation.