# control characteristics of ac and dc operation of a single phase thyristor

#### PEARL4EVER

Joined Mar 6, 2017
4
1. can power be delivered to the dc load if one diode in the bridge rectifier fails?
2. what are the possible ways of modifying the circuit to deliver a certain amount of load current say 13A
3 how do one calculate the maximum gate current that can be delivered by the trigger module?

#### MrAl

Joined Jun 17, 2014
7,849
Hi,

You should show your schematic and any other necessary items like part values.
You also have to specify what failure mode is applicable to the bad diode: failed short or failed open.

#### PEARL4EVER

Joined Mar 6, 2017
4
thank you here is the circuit pertaining to the question I asked.

#### MrAl

Joined Jun 17, 2014
7,849
Hi,

You think you can post a circuit that is just a little more out of focus than that one? ha ha

Seriously it would be good if you could post a more clear picture of your circuit. That one is too small for the text font being used so it's hard to read the values and bad for reading the schematic too really.
Try posting a better picture.

You can calculate the max current by either shorting the transistor or assuming a 0.2v CE drop and subtracting the SCR gate voltage, then calculate the current based on the available voltage and resistance in the current path. We can look at this better but a better drawing would be good first.

You did not specify if the diode failure was short or open. Do you want to look at both possibilities then?

#### PEARL4EVER

Joined Mar 6, 2017
4

#### MrAl

Joined Jun 17, 2014
7,849
Hi,

That's a better rendition of the drawing, but did that get uploaded like 10 times or something?

Anyway, i think we should take one question at a time. Let's start with #1, and a diode that failed open.

Do you have a basic idea how this circuit works when it is working normally, as to how it triggers the SCR and how the load(s) get current?

Can you still trace a DC path through the load(s) and SCR even if it is just a pulsing DC?

Also, can you figure out the peak voltage of the capacitor, and whether or not it can still trigger the gate well enough to turn the SCR on?

Can you show any work at all that you've done or thought about?

#### PEARL4EVER

Joined Mar 6, 2017
4
i have done a lot like adjusting my variable resistor value to get some specified firing angles, and a lot of research work on the circuit and also trying to replace my resistive load with reactive load to see different effect. to be sincere my electrical knowledge is limited and am really struggling to get things done. please I desperately need this help. thank you

#### MrAl

Joined Jun 17, 2014
7,849
Hi,

Ok, well let's start simple. Do you know how to calculate the voltage at the top of C1 calling the bottom of C1 ground (0v) with the SCR turned 'off' ?