# SCR controlled with UJT

Thread Starter

#### justtrying

Joined Mar 9, 2011
429
I desperately need some help with figuring out how to solve these things. I can attempt to solve for turn on voltage for UJT (it is probably wrong though), but I do not have a clue of how to even approach the SCR part.

I do know the principles of operation, but have no clue how to apply them, i.e. in theory having the capacitor will delay the time when the SCR turns on thus further delaying the angle. But I do not now how to calculate it. Is charge time for capacitor calculated and then simply added on or what...

Also, if anyone wants to shed light on valley voltage calculation (10% of Vbb is not good enough)...

I know it's a lot but we had a whole of 2 hours to cover all thyristors...

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#### jimkeith

Joined Oct 26, 2011
540
Really need to know exactly what you are solving for...

Lets just discuss in a practical sense some of the details:

The SCR phase control turns on late in the cycle and the line voltage appears across the load--the SCR remains conducting until the applied voltage drops to zero or reverses polarity.

Once the UJT fires or triggers, the voltage across C is applied to the SCR gate--the gate may be roughly modeled by putting two diodes in series with about a 100Ω series resistor--the resulting gate pulse is quite short--RC is approx 0.1uf * 50Ω = approx 5us

The valley voltage is a non-issue as it is a parameter used in relaxation oscillators--that is not the case here as after the initial pulse, no significant voltage occurs across the UJT circit, thus it is finished for the remainder of the cycle.

The time it takes for C to charge to the trigger voltage is added to the beginning of the cycle--or at least from the point where the applied voltage exceeds the zener voltage--if the time exceeds ½cycle, the UJT never fires.

See thumbnail for typ waveform--hope this is helpful

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Thread Starter

#### justtrying

Joined Mar 9, 2011
429
Yes that would be helpful, I need to find the conduction angle. I know what to do when it is just SCR (using the graphs), but I do not understand how to account for the delay created by the UJT control.

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Thread Starter

#### justtrying

Joined Mar 9, 2011
429
having a discussion with myself here... applying the capacitor charging formula I get t charge of 2.07 ms which converts to a delay angle of 44.7 deg.

I have tried to appliy the same principle to a lab question and I get completely wrong answer compared to the experimental results. The only conclusion I can draw is that my calculations are wrong.

#### nany2204

Joined Nov 12, 2011
1
I desperately need some help with figuring out how to solve these things.
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