# UJT Triggering of an SCR

#### copala3

Joined May 31, 2019
11
Hi.I have no idea how to identify the materials in this circuit.How do I determine the materials and values to be used here? How can you help me ?

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#### AlbertHall

Joined Jun 4, 2014
9,276
If the Supply is a DC voltage then, once the SCR is triggered, it will stay conducting until the current falls below the holding current. This will be only a very small fraction of the SCR rated maximum current. What do you want the circuit to do?

#### copala3

Joined May 31, 2019
11
I will determine the values of the materials in the circuit and observe the output signal with the oscilloscope.But I don't know which resistance capacitors should be used for which values

#### copala3

Joined May 31, 2019
11

#### peterdeco

Joined Oct 8, 2019
95
It's been a long time since I worked with SCR's but, does the circuit even need a UJT? Pushbutton to SCR gate to turn it on?

#### Alec_t

Joined Sep 17, 2013
10,902
Is this an academic exercise or do you want to actually build the circuit? Be advised that UJTs are pretty much obsolete and may be difficult to source at a reasonable price. There are other, simpler ways of firing an SCR with some delay, if that is what you want to do.
Note, this circuit can turn a LED on but won't make it flash repeatedly.

#### MrAl

Joined Jun 17, 2014
7,126
Hi,

It looks like some are still active. Price ranges from about $1.30 USD to around$3.00 USD.

#### MrAl

Joined Jun 17, 2014
7,126
Hi.I have no idea how to identify the materials in this circuit.How do I determine the materials and values to be used here? How can you help me ?
Hi

R1 and C1 are selected to provide the delay you need, given the part number of the (programmable) unijunction transistor you are using. That would be based on the Vcc*(1-e^(-t/RC)) formula used for RC charging.

R1 has to be low enough to force enough current into the gate of the SCR to turn it on reliably.
R2 helps to set the threshold voltage.
R3 just has to discharge the cap when the circuit resets.
R4 just has to pull the gate low once the turn on pulse ends, and keep it low.
C2 has to be high enough to push enough current into the gate of the SCR for a long enough time period to reliably turn the SCR on.

But do you know how to use the RC charging formula above?
Do you know what unijunction you are going to use?
Do you know what SCR you are going to use?

Have you designed anything else like this in the past?

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#### SLK001

Joined Nov 29, 2011
1,543
You can make a UJT out of a NPN and a PNP transistor.

The circuit looks like a simple timer to turn on a load after the switch is thrown.

#### Dodgydave

Joined Jun 22, 2012
8,809
You don't need C2, C1 charges up to a value of approx 10% of the supply, to turn on the unijunction transistors Emitter , and it conducts connecting the bases B1, B2 together firing the thyristor..

#### SLK001

Joined Nov 29, 2011
1,543
You don't need C2, C1 charges up to a value of approx 10% of the supply, to turn on the unijunction transistors Emitter , and it conducts connecting the bases B1, B2 together firing the thyristor..
You DO need C2. It will give a positive pulse to the SCR, ensuring reliable triggering. It also isolates R3 from pulldown R4, so the SCR doesn't randomly trigger.

#### Dodgydave

Joined Jun 22, 2012
8,809
You DO need C2. It will give a positive pulse to the SCR, ensuring reliable triggering. It also isolates R3 from pulldown R4, so the SCR doesn't randomly trigger.
You don't need R4 either..or C2 !!

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