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Using different voltage supplies in the same board

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dkace

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Hi again,
I have a pic that controls 4 relays and communicates via RS485 interface with a main control. The 485 int is optocoupled with the pic in order to be isolated. If I use the same P.S then I can't avoid interfierence. SShall I use a dual power supply? Shall I just change common?
Thanks,
D.
 

Moss

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You shoud another supply. Probably, the best solution is to use somе DC/DC converter, witn galvanic isolation.
 

nicleo

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(RS485 circuit + PIC) is isolated from (relay circuit). The optocoupler should be placed between (RS485 circuit + PIC) and (relay circuit).
 

dkace

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Optocouplers are allready put there. Ok guys thanks, I found a way to arrange things on the schematic and on the board. But do you think I have to use capacitors between the output and the ground after every voltage conversion? Is that really neccessary and if yes can someone explain me this with electronic terms?
I know it is silly to ask but sometimes you get this anoying altzheimer problem....
D.
 

Moss

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I don't understand, where you want to put this capacitors? After DC/DC converter with galvanic isolation you have two EARTHs - one for RS485 interface and separate EARTH for processor.
 

dkace

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Between DCDC output and uC earth. Afterwards I use a 5 to 5 for isolation and put a cap between 5 and 0. Is it correct? Don't ask why I am doing this, there are some standarts I have to meet. But is it correct?
How big the capacitors should be? There value depends on the voltage or on the current? I lost this class for power supplies and I have BIG questions as you see(lol!!!!:))) ) THat's why I am asking:
Let me ask again:

+24VAC->Bridge Rectifier->Electrolitic capacitor(Value?Why?)-> 24Vto5V->Capacitor?(Type?Value?EARTH of µC? Earth of RS485?Why?) ->5Vto5V->Earth of µC. Capacitor? (Type?Value?Why?)

Thanks,
D.
 

Moss

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In general capacitor value depends on current and working frequencies. Why ? Because this is writen in the Book :)

www.ljplus.ru/img/ichera/Resize-of-01b_0013.jpg
www.ljplus.ru/img/ichera/_Resize-of-06b_0003.jpg

For your design approximate values for 24V after rectifier is somewhere between 2200 and 4700uF. Smaller vaue are exaptable for smaller current. It is highly recomended to put smal ceramic capacitor ( 100n is generaly used) in paralell to the big electrolitic one. After 5V regulator only tantalium capacitor somewhat about 47uf and several 100n for each IC on the board and on regulator terminals ( closer as posible in all casses). Pre and after DC/DC unit also, but check manifaturer papers for details.

P.S. If you don't see pictures I will upload them on diferent host.
 

    dkace

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