Isolated Supplies for driving Optoisolators, Relays and ADC

Thread Starter


Joined Mar 12, 2014
Hi, I am designing an industrial control board. I am optoisolating everything that goes to the outside world (field). I have a 5V switching regulator (3 amp) that powers my board which is more than enough current. However, since the optoisolators really need a separate Vdd and Gnd, I am having difficulty figuring out how to isolate this power. The reason is each opto LED needs about 20 mA of current and with 8 of these that's 160 mA. Then I am driving some relays which need isolated power too (isolated from microcontroller) and they need about 80 mA each x 6 = 480 mA. So the relay coils + opto LEDS are 640 mA alone.

I have considered using a DC-DC isolated converter such as MEE3 (Digikey PN 811-2360-5-ND), but that's only 600 mA. The outputs of these DC-DC converters are noisy as well. Since I have an ADC that interfaces the outside world then it needs an isolated power as well but it needs to be clean. That's my other problem. I thought about putting an LDO after the DC-DC converter to create an isolated and regulated supply for the ADC, but the DC-DC converter output voltage is only 5V and that's not enough for an LDO to generate 5V output.

Any ideas?