Re: Mystery Circuit ?
dar2525 said:
Does the zener protect the load from overvoltage or does it protect the SCR gate from overvoltage?
OOPS - edit time A zener will only conduct when the base is positive with respect to its cathode and it is forward biassed.
**broken link removed**
I meant to say :arrow: A THYRISTOR will only conduct when the base is positive with respect to its cathode and it is forward biassed.
Now the link makes sense as well :wink:
Look at the attached simplified circuit -- do you see a bridge rectifier ? (two diodes and two thyristors).
Think about a normal bridge rectifier's action, if the (now single phase)supply's upper terminal goes positive (coloured red) then the current will flow through SCR1 (ignore gating signals for a minute ...) to feed the circuit's positive rail.
Current will flow through any load to the negative rail (I am assuming conventional current flow), and return to the supply via D2.
On the next half-cycle the current would flow from the lower terminal through SCR2 and return through D1.
So far it's dead simple :?:
To turn an SCR (thyristor) on you have to bias the gate positive WRT the cathode ?
The zener (I added a zener biassing resistor, RZ to feed a current through it) will give a constant voltage, assume it is a 20v zener.
If the positive output rail is less positive than this zener voltage (20v) the thyristor will be turned on, remaining on until reverse biassed.
This output will be sort-of voltage regulated to this zener voltage but with peaks reaching the full AC peak-to-peak voltage ?
A three-phase supply is only a bit more complicated because of the way it can use any of three phasees as positive and any other as the negative (depending on which is most positive and which is most negative).
This is a
Half-controlled Rectifier, a fully-controlled rectifier would use thyristors throughout, no diodes.