# Thyristor as switch

#### Momentory

Joined Sep 4, 2022
67
I have two connected circuit

• 12v dc battery
• 12v dc motor
• Thyristor ( input current max gate current 0.05 ampere) - (1.3v max gate voltage)
• Resistor 39 ohm
• 1n4007 diode
• 1.5v dc battery

The thyristor will be the switch for this circuit

My question is very basic but i want to understand how it works

In the circuit that has the thyristor

What is the voltage of this circuit ( A ,B), Is it 1.5v or 12v since the circuit connected to both circuit?

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#### Ya’akov

Joined Jan 27, 2019
6,575
What do you mean by “voltage of this circuit”?

There is no special attribute of a circuit called “voltage”. Components are identified by ”voltage” to indicate nominal supply voltage, nominal output voltage, or nominal operating voltage.

I don’t know what your question means because a circuit doesn’t have a voltage, it has many voltages wich can be measured at different points. Could you clarify?

#### Momentory

Joined Sep 4, 2022
67
What do you mean by “voltage of this circuit”?

There is no special attribute of a circuit called “voltage”. Components are identified by ”voltage” to indicate nominal supply voltage, nominal output voltage, or nominal operating voltage.

I don’t know what your question means because a circuit doesn’t have a voltage, it has many voltages wich can be measured at different points. Could you clarify?
Sorry for this
I edited the post

A is the gate of thyristor
B is connection point between both wires

#### Alec_t

Joined Sep 17, 2013
13,153
Have you checked with the datasheet that the cathode and anode connections are the right way round for your particular thyristor? The centre pin of a thyristor is usually the anode.
A is the gate of thyristor
That's confusing. 'A' conventionally denotes the anode.
No A or B in the pic?

#### Momentory

Joined Sep 4, 2022
67
Have you checked with the datasheet that the cathode and anode connections are the right way round for your particular thyristor? The centre pin of a thyristor is usually the anode.

That's confusing. 'A' conventionally denotes the anode.
No A or B in the pic?
"A" is not referring to the anode of the thyristor

It is just a word to point the points i want to measure the voltage from

A and B are points not referring to thyristor anode or cathode or gate

Battery negative wire goes to andode (middle pin) and wire from cathode goes to the motor

#### Ya’akov

Joined Jan 27, 2019
6,575
Sorry for this
I edited the post

A is the gate of thyristor
B is connection point between both wires
If I understand you correctly, the answer is 0V. There is no potential between those two points.

#### Momentory

Joined Sep 4, 2022
67
If I understand you correctly, the answer is 0V. There is no potential between those two points.
There is a potential

The wire goes from the thyristor gate (a) to the cathode wire of the thyristor and there is a battery of 1.5v between

#### BobTPH

Joined Jun 5, 2013
5,773
Your circuit is incorrect. The cathode (pin 1) should be connected to both battery negatives, and the motor negative should be connected to the anode(pin 2.)

#### Alec_t

Joined Sep 17, 2013
13,153
Battery negative wire goes to andode (middle pin) and wire from cathode goes to the motor
That's the wrong way round. The anode must always be more positive than the cathode.

Joined Jan 15, 2015
6,979
Below is a basic schematic of a SCR / Thyristor circuit.

SW1 is a N/C push button switch, SW2 is a N/O pushbutton switch. Momentary press on SW2 will start the motor and momentary press on SW1 will stop the motor. The anode, gate and cathode of the SCR are labeled. The resistors form a voltage divider for the gate voltage. Now using this example what exactly do you want to know.

Ron

#### Momentory

Joined Sep 4, 2022
67
Below is a basic schematic of a SCR / Thyristor circuit.

View attachment 276280

SW1 is a N/C push button switch, SW2 is a N/O pushbutton switch. Momentary press on SW2 will start the motor and momentary press on SW1 will stop the motor. The anode, gate and cathode of the SCR are labeled. The resistors form a voltage divider for the gate voltage. Now using this example what exactly do you want to know.

Ron
I need the thyristor to has its own battery, but i do not understand why my circuit is wrong

#### BobTPH

Joined Jun 5, 2013
5,773
Your circuit is wrong because the thyristor anode and cathode are reversed. Do you understand that it only conducts in one direction?

#### MrChips

Joined Oct 2, 2009
27,137
Thyristor does not turn off DC unless current is zero.

Edit: Let me rephrase that. Thyristor does not switch off if current is not zero.

Joined Jan 15, 2015
6,979
OK your circuit reflects a TO220 case for your thyristor. Normally less a data sheet to look at it would look like this.

Now just note the pinout for Cathode, Anode and Gate. This is what everyone is getting at. To gate it ON the gate needs to be positive with respect to the cathode. Yes, you can use a separate power source for the gate as long as they share the same common.

Ron

#### crutschow

Joined Mar 14, 2008
30,803
I need the thyristor to has its own battery
Why, when it can be triggered from the motor battery?

Joined Jul 18, 2013
25,990
but i do not understand why my circuit is wrong
As per @Reloadron & @MrChips , you do realize once conducting, the SCR does not turn off when the gate power is removed, as long as the supply is present?

#### Momentory

Joined Sep 4, 2022
67
Why, when it can be triggered from the motor battery?
I am just need to understand it connect with separated circuit

#### Momentory

Joined Sep 4, 2022
67
As per @Reloadron & @MrChips , you do realize once conducting, the SCR does not turn off when the gate power is removed, as long as the supply is present?
Yes, i understand this so the toggle button will off so the circuit power will off as well the thyristor

#### Momentory

Joined Sep 4, 2022
67

Is this right?

#### Ian0

Joined Aug 7, 2020
6,312
I don't think so, but if you drew a proper circuit diagram it would be far easier to tell.