Schneider Electric VW3 A3 303 user manual Calculating the polarization, Example

9. Appendix

bCalculating the polarization

MPrinciple

You must ensure that the equivalent bus resistance is between 162 Ω and 650 Ω.

The equivalent bus resistance (Re) depends on the polarization resistance of the slaves (Rs) and the master (Rm):

1 1 1 1 …

-------= --------- + ---------+ --------- + Re Rm Rs1 Rs2

If Re is too low, reduce the number of slaves.

If Re is too high, adapt the master’s polarization (if possible) or add polarization resistors (Rp).

1	=	1	1	1	1		…
-------		-------	+ ---------	+ --------- + --------- +
Re		Rp	Rm	Rs1	Rs2
				Master
				R	G	5 V
							Rm	5 V
				Rm				Rp
					0 V
D1
120 Ω
1 nF
D0
Common
5 V								Rp
Rs1

Rs1 0 V

0 V

R

G

R

G

Slave 1 Slave n

Example 1

If the master has 470 Ω polarization and all the slaves have 4.7 kΩ polarization, a maximum of 18 slaves can be connected.

Rm = 470 Ω

Rs = 4.7 kΩ

A/Re = 1/470 + 18 x 1/4700 i.e., Re = 168 Ω

Example 2

If the bus polarization Rp is 470 Ω (installed in the master) and 2 slaves have 4.7 Ω polarization, the equivalent polarization is: 1/Re = 1/470 + 1/4700 + 1/4700

i.e., Re = 1/ (1/470 + 1/4700 + 1/4700) and therefore Re = 390 Ω

390 Ω is between 162 Ω and 650 Ω, and the schematic is correct.

For an ideal equivalent polarization (650 Ω), the master’s polarization can be adapted so that: 1/650 = 1/Rm + 1/4700 + 1/4700

i.e., Rm = 1/(1/650 - 1/4700 - 1/4700) and therefore Rm = 587 Ω

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