I have some thyristor packs that I want to use to rectify my ac power supply to dc for a homemade welder. Thyristor is Semikron semipack 2 SSKT 162/16 E. The data sheet says the gate voltage should be max of 2v and max current of 150 ma. I have re-purposed two microwave oven transformers for a stick welder. I have these thyristor packs on hand (courtesy of my job) and would like to use them to rectify the ac output to dc. I can't seem to understand what i need to do as far as the gate trigger. I have three of them and I would like to use two for a 220v power supply for my homemade welder project.

• Peak Repetitive Off-State Voltage, Vdrm: 1.6kV
• On State RMS Current IT(rms): 250A
• Operating Temperature Max: 125°C
• Peak Non Rep Surge Current Itsm 50Hz: 5.4kA
• Gate Trigger Voltage Max Vgt: 2V
• SCR Module Type: Series Connected - SCRs
• Gate Trigger Current Max, Igt: 150mA
• Holding Current Max Ih: 400mA
• Product Range: -
• Current It av: 160A

Source.


Source.

 

You only need two thyristors for a full wave bridge and two ordinary power diodes, or four thyristors and pulse transformers across the G/K terminals.

Here is one for a motor speed control,.. the bottom drawing should be for 230Vac and R1 should be 33K.

http://www.seekic.com/circuit_diagram/Control_Circuit/HIGH_TORQUE_MOTOR_SPEED_CONTROL.html

 

Or you could use a normal full bridge pack and a Thyristor after it for just one control instead of the two.
Max.

 

I have never used thyristors before. I have to admit I'm still foggy on how there properly used. These come off a working magnet system on a crane and used to pick up huge slabs. With just the thyristors themselves, can they be used as diodes to rectify AC to DC? I assumed with the gate triggered it would act as a diode.

 

They can be, but a bit of a waste, usually an equivalent bridge would be much cheaper.
Max.

 

Thyristors are like two transistors back to back, they only conduct One way when the Gate is made positive to the Cathode, then it stays conducting until the voltage is removed from the Anode, so on AC the voltage changes direction every 20mS, and will stop conducting automatically.


https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/thyristors/

 

With the packs I have the gate needs to be no more than 2 volts to trigger. If fed with 120 like in the circuit of the motor starter , could I use resistors and a capacitor if the proper voltage was there. The diodes I assume are to keep the power positive going to the gate.

 

I know it's a waste to use them instead of a normal bridge, but I'm really just trying to learn while experimenting with devices that I have on hand.

 

To keep it turned on you only need a small resistor from anode to gate.
Max.

 

What value do I need if I'm using 120 volts input?

 

You shouldn't need more that a couple of hundred ohms.
Max.

 

Just to get this straight in my head. The resistor is lowering current into the gate right? What about the voltage into the gate?

 

The gate current will be sufficient with the applied resistor, this is all the gate needs.
Max.

 

Thanks so much for all the help. I appreciate everyone being patient with me.

 

I've Used large SCR modules for high powered rectifiers many times. I usually just put a 1/4 watt 50 - 100 ohm resistor across the gate and Anode and called it good.

As soon as the gate to cathode voltage goes over the triggering voltage, 1- 2 volts, the main conducting regions for he Anode To Cathode conduction paths fire up and essentially bypass that Gate to Anode current flow.

If something goes wrong and a gate overcurrent event happens the 1/4 watt resistor tends to burn up before the gate gives up.

 

Sounds good. I will try the 1/4 watt 50-100 ohm resistor.