DC coil/rectifier Help

I have some DC coils I want to energise from my PLC.
They are: 110v dc
20 W/VA
I would like to energise them via the PLC and a relay at 110VAC and then have a rectifier locally to the dc coil. I was thinking of having a rectifier for each solenoid.
My questions are:
Do I need to worry about back emf? Or will the diodes in the rectifier cope with this?
What size of rectifier should I use?
Should I run a spice simulation to test it works before ordering equipment panels etc? Can anyone recommend one?
Perhaps this is the wrong place to ask these questions? Could anyone offer another web site? Possibly an electronics site?

Thanks in advance.
Ben

Hi Ben

What size of rectifier should I use?

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You Should size your rectifier rating according to the numbers of dc coils you have .
As i under stand each coil is 110 Vdc/20 W this means that each coil is 0.18 A(20/110) so your recifier sizs should be
0.18 *no. of coils*1.3= ....A
1.3 :this is a factor of sefety

Do I need to worry about back emf? Or will the diodes in the rectifier cope with this?

Click to expand...

Don't worry about the back emf because you energize the DC coils through a 110 Vac relay(Interface Relay) not directly from the PLC
your 110 Vac Relay is energized from the PLC if you are useing Transistor output Module you should use "Free wheeling diode" in parallel with your 110 Vac relay coil to protect your output from back emf.Most of PlC manufactures provide this diodes internally in thier transistor output module so make sure that it exist in your PLC and connect it.
best luck

you should use "Free wheeling diode" in parallel with your 110 Vac relay coil to protect your output

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You NEVER put DIODE in parallel with AC Relay Coil!!!
I would work for positive half-period but once the
polarity changes (this is AC!!!) you would have short
circuit through the diode. The PLC output would not be
protected, it would probably be damaged.

      Rectifier Bridge  

         B250C800

         _______

        |       |

AC -----| ~   + |---------+         

        |       |         |

        |       |      ___|___ 

        |       |      | \   |   DC Coil

        |       |      |__\__|

        |       |         |

AC -----| ~   - |---------|

        |_______| 

Since you are from Europe, you would probably have an easy
access to bridges such as B250C800 (250V, 800mA).
They are very small and cheap so you can use higher rated parts
such as B400C1000 (400V, 1000mA). They are nicer for wiring
over four diodes since they only have four neately labeled
terminals ( AC, AC, + , -).
Don't vory about kick back from the DC coil if you are using
bridge - you already have diodes accross the coil.

panic
sorrt for the mistake.