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On board isolated power for driving relay coils and optos and ADC

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signalflow

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Hi, I am designing an industrial control board. I am optoisolating everything that goes to the outside world (field) which means I need isolated power/gnd as well. I have a 5V switching regulator (3 amp) that powers my board which is more than enough current (this is the power for the microcontroller and other ICs that do not interface the noisy industrial environment). However, since the optoisolators really need a separate Vdd and Gnd, I am having difficulty figuring out where to get this isolated this power from. 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 some other things that use about 240 mA, so total isolated power needs to be at least 800 mA.

I have considered using a DC-DC isolated converter such as MEE3 (Digikey PN 811-2360-5-ND) or (Digi PN: 445-2464-ND), but those only 600 mA. The outputs of these DC-DC converters are noisy as well and the voltage droops with increasing current load. And it says don't use these in parallel. Since I have an ADC that interfaces the outside world then it needs an isolated power as well but it needs to be clean and stable. 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.

Another option is to have a separate external power supply (such as a TDK Lambda) to power the relays and other isolated components, but this seems like a clunky design requiring 2 separate power inputs. Would be nicer if my board just had 1 power connection and was able to power itself and the noisy relays, etc.

What do you typically do to create on-board isolated power for relays, etc.? I have 24Vac incoming power which is rectified and then goes to a 5V, 3A switching regulator. My plan was to power the DC-DC isolated converter from the output of the 5V switching regulator.

This one has 1.2 A output (digi PN: 445-2491-ND), but needs 18V-36V input. I could pull off of the 24Vac rectified for that which is about 34 Vdc (could be higher with no load on the transformer) possibly exceeding the 36V max input and burning up the DC-DC converter.


Thanks.
 
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andre_teprom

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The relay array responds for about 80% of the consumption of the power supply, but the voltage that feeds the coil do not necessarily need to be of good quality. I believe that an option to consider is an isolated power source as you have found to feed the whole board, and for digital and analogical electronic circuits, use a low-power IC DC-DC converter on-board.
 

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