# Phase controlled rectifiers with SCR

#### freak101

Joined Aug 7, 2017
37
Hi,

This is a single phase half wave circuit with RL load and a battery along with various waveforms.

Ig is the gate pulse.
Now how these waveforms will change if I connect a freewheeling diode ?

It is assumed that the diode is ideal.

#### ericgibbs

Joined Jan 29, 2010
8,861
hi 101,
Is this really a Homework question.?
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#### freak101

Joined Aug 7, 2017
37
Umm yes.

#### ericgibbs

Joined Jan 29, 2010
8,861
hi,
OK.
As it is a Homework question, you should post your understanding of what you think will happen,?

I don't know if you have used LTSpice as a simulator, but it is a great tool for checking circuitry.

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#### freak101

Joined Aug 7, 2017
37
Oh yes so here it goes...
At wt=pi we have zero input voltage but non zero current through SCR (due to inductor). As a result, SCR is shorted in the interval wt=pi to beta.
In this interval, the diode is also shorted. So we should get zero output voltage but we have a battery in there too so I am not sure if I am correct (dont forget the inductor too).
When SCR is open circuit, the inductor is going to maintain the current while discharging itself. So the net output voltage is going to be sum of battery and drop across resistor ?
I tried simulating the first circuit in multisim. I spotted some issues there as I had no "downward" spike in the output voltage.
Thanx.

#### freak101

Joined Aug 7, 2017
37
BTW, I am just curious why you thought that it isnt a homework question ?
Thanx.

#### ericgibbs

Joined Jan 29, 2010
8,861
hi 101,
If the TS does not state its Homework, we have to make an assessment on whether its Homework or any other Topic based on the opening post.
The limited information you gave did not give a clear indication, that is why I asked.

Some members enjoy and concentrate on answering posts in their area's of interest, so we try to place Threads in a Forum where the TS will get the best response to his/her query, it's that simple.

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#### ericgibbs

Joined Jan 29, 2010
8,861
hi 101,
Using Multisim did you set the SCR firing pulse as shown in the diagram.?
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#### freak101

Joined Aug 7, 2017
37

The sine wave is 230 Vrms @ 60Hz while pulse is of height 10V. The firing angle is around 42 degree.

#### ericgibbs

Joined Jan 29, 2010
8,861
hi,
I can try the circuit in LTSpice, could you post the other component values, so that we both use the same circuit.?
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#### freak101

Joined Aug 7, 2017
37
this is the circuit

and this is the pulse setting

Thank you so much.

#### ericgibbs

Joined Jan 29, 2010
8,861
hi,
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#### ericgibbs

Joined Jan 29, 2010
8,861
hi 101,
This what I see on LTS, with and without the diode.

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#### freak101

Joined Aug 7, 2017
37
I got the same output with the diode connected but not with the other one.
Please explain how that downward spike disappeared when diode was connected. I think I was right in one of my earlier post.
Also can you post the output current current wave ? That should seal the deal.
BTW, the input is 325Vrms right ?

#### ericgibbs

Joined Jan 29, 2010
8,861
hi,
The clamp diode becomes forward biassed when Vout tends to go Negative with respect to 0V.
I used 230Vac RMS * 1.414 =325Vpk

Note: I used 50Hz as the source, it makes the timing easier, the plots are close enough for 60Hz to show the effect you are querying.

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