ericgibbs said:
Depending upon the current loading and voltage drop on the 4700uF the bridge diodes are going have a fast/ high current pulse when recharging the 4700uF's
Thanks for the advice Eric. My processor setup will probably draw 200mA max. If all the relays are on you could probably add another 200mA. So there should be no reason to exceed 500 mA overall at any one time.
The bridge rectifier can handle 1A, while it has a surge overload rating of 50A. The transformer can provide 0.875A on each secondary winding. Is this OK for the charging of the 4700 uF cap?
JimB said:
If you need complete electrical isolation between the 3.3v and 5v supplies, you will have to use an opto-isolator to couple the signal paths between the parts powered from 5v and parts powered from 3.3v.
Opto-isolators is a good idea.
JimB said:
You mentioned noise on the 5v supply, where do you think that will come from?
The relays should be OK if fitted with diodes across the coil to catch the back-EMF, and the relays themselves should provide good isolation from whatever they are switching, unless it is something very agressive.
This circuit is a redesign of a previous one that failed miserably. The relays are used to switch 250VAC coiled external relays which in turn switch 3-phase contactors which switch 3-phase pump motors. Each time I switched the relays off my processor would reboot. I tried various things and in the end it seems that putting MOV's on the AC side of the relays solved the reboots. It should be worth noting that I had groundplane covering my entire board. The relay noise probably coupled to the ground plane and caused a voltage drop in my processor. Needless to say, I am now doing as much as I can to eliminate as much noise as possible coming from wherever.
I am willing to throw caps around my board like confetti, but I don't want to overdo it unecessarily.
crutschow said:
Another method to isolate noise between two grounds is to use a small inductance to connect the two grounds together. For high frequency noise even a ferrite bead may provide sufficient inductance.
Interesting. What magnitude of inductance are we looking at here? I have some of those axial inductors (those that look like resistors) lying around. They are 100 uH, but I suspect that may be too much...
Thanks for all of your help. I am sucking all this up like a sponge at the moment.
If I connect the two grounds together I will do so only right where they leave the two bridge rectifiers. The shortest route (lowest impedance path?) for noise on the 5v's ground would then not be past the processor on the 3.3v side.
This is also the first time I am designing a mains power supply (mains-transformer-rectifier), so I am keen to learn where I am going to get caught out.
This piece of hardware will potentially be used in noisy environments (motors, radio signals etc), so I will be using an aluminium enclosure. The enclosure will be wired to the mains' earth. I have a question about this also, but posted it under a separate topic (
https://www.electro-tech-online.com/threads/connecting-enclosure-to-earth.115082/).