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Series / Parallel Operation of SCR
* (Number of SCRs in string)
When η = 1 each SCR in string has 100% identical characteristic or utilization of thyristor will be maximum. But it is not possible practically.
Series Operation

Voltage across each of remaining (n – 1) SCRs = (n – 1) I_{2}R
For a string voltage V_{s}, the voltage equation for the series circuit will be
V_{s} = I_{1} R + (n –1)I_{2} R
= V_{bm} + (n – 1) R [I_{1} – (I_{bmx} – I_{bmn})]
let (I_{bmx }– I_{bmn}) = ΔIb
RI_{1} = Vbm
Parallel Operation
When current required by the load is more than the rated current of a single thyristor, SCRs are connected in parallel in string. For equal sharing of currents VI characteristics of each SCRs during forward conduction must be identical as far as possible.
For parallelconnected SCRs, voltage V_{T} across them must be equal. Figure (b) show that for a same voltage drop V_{T}, SCR_{1} shares a rated current I_{1} whereas SCR_{2} carries current I_{2} much less than rated current I_{1}. The total current carried by the unit is I_{1} + I_{2} and not the rated current 2I_{1} as required. Therefore string efficiency is given by:
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