Expert: cleggsan - 12/17/2006

Question
most of gutor and UPS use thyristor,scr.but i do not understand the operation of thyristor actually.so it might be difficult to understand what would happen if thyristor fails?


Convertronic

Universal Bypass Switch

UNB/SBS

Type:UNB5,0-108

Art Nr: 600-050-711A0B

Ser Nr: 805 26 02283

Input  Output

Ue:230V Ua:230V

Ie:35A  Ia:25A

Convertronic

Switched sine wave Inverter UNV

Inverter

Type:UNV 108-3.3F

Art Nr:500-033-711.A0B

Ser Nr:8 0526 02282

Input:  Output:

Ue: 108V Ua:230V

Ie:26.7A Ia:14.4

Fe:DC  fa:50Hz



Fe:50 Hz fa:50Hz


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The text above is a follow-up to ...

-----Question-----
Dear all

1.Most of the UPS use this thirystor bridge rectifier that hac something we call firing pulse.What is firing pulse.Why cant we use normal diode rectifier?

2.Could you describe about static switch int he inverter/UPS.They have two sets of antiparallel diode.How they operates and change the mode from inverter mode to static switch  bypass mode?
-----Answer-----
You did not send me the make and model you are talking about.  Not all ups circuits are the same.  There are many ways to accomplish the same thing.

1.  But, generally, the reason for using a controlled voltage power supply is to keep from overcharging AND to switch it to full charge when the cells have been discharged after a power falure or interruption.  The voltage condition of the battery pack in the ups is monitored and when the unit is fully charged it sends information back to the charging circuit to reduce the current and voltage level to standby.  It can be done by firing a thyristor or any type of controlled current device.  SCRs and UJTs are used, also.

2.  To answer this I must see the schematic that you are talking about.  But, again, in general the static mode is when the cells are fully charged and it is merely idling in wait for a need to fire up and supply current during a failure.  The circuit implementation that you describe is probably a voltage offset indicator to give the proper bias to the thyristor for its dc balance and firing pulse.

Cleggsan
PS: Overcharging wet cell packs is a very dangerous thing, so protections and fail safe power supply feeds must be carefully designed and tested for their operation.  

Answer
Oh, here is the deal on a thyristor:

http://en.wikipedia.org/wiki/Thyristor

Thyristors, scrs and other types of diodes are switches that when they receive a pulse or positive bias at their gate or trigger input they switch from a high resistance to a low resistance.  Some must be forward biased constantly to turn on; others switch on and stay on until they receive a reverse bias pulse which turns them off.

You can learn about these devices from wikipedia and howstuffworks.com and other such sites.

If a thyristor fails it will no longer switch when it receives the pulse.  OR, it may be shorted down, in which case it is always ON.  So, you must learn the failure mode it is in before you can assess what it will do in the circuit.

Cleggsan