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I need help analyzing this circuit.

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dar2525

New Member
I have been trying to understand this circuit, but i do not know how rectification occurs at the output or R1. C1 is capable of voltages up to 63V and C2 is capable of voltages up tp 50V. I used Circuit Maker and actual sine wave to look at signals at different nodes. Somewhere in the circuit AC is switched to DC. I just do not know where. I know C2 performs the full wave rectifcation, but where is the half wave rectification. Also, I think D1 is to prevent signal loss, but I am not sure. Please help me understand why R3, R2 and C1 is needed at the gate of the SCR. Once the gate is triggered the SCR will conduct, correct. I am doing a Co-Op. This is just portion of the overall circuit. There are 3 phase generators to this circuit. They all look like this. I figure if i understand this circuit I can understand the rest.

Please Help
 

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audioguru

Well-Known Member
Most Helpful Member
D1 shorts half the AC from the supply! Did you sketch it correctly?
Is R1 the load?

The Silicon Controlled Rectifier is supposed to do the rectification. :lol:
 

dar2525

New Member
audioguru said:
D1 shorts half the AC from the supply! Did you sketch it correctly?
Is R1 the load?

The Silicon Controlled Rectifier is supposed to do the rectification. :lol:

Yes R1 is the load. This is how the circuit actual looks.
 

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dar2525

New Member
Russlk said:
You are having trouble understanding the circuit because it is nonsense.

I apologize for not making myself clear.Thank you for replying. Here is the actual circuit I am tryting to anaylze. R7 is the load. The three sources are representing a 3 phase generator. Each source is a 120 degrees phase difference. I understand that C4 decreases the ripple for the DC output. I know the zenor is used to prevent overvoltage at the SCR gate. I am think is the diodes are used to remove the negative portion of the sinewave from each phase source. Please respond . Any help will well appreciated
 

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Russlk

New Member
As Audioguru pointed out, when ground is positive, the diodes are a short circuit on the source. That is not good, and does not do any thing for the load. When ground is negative, I have not analyzed the gate network to see when the scr is turned on, but since the gate-cathode junction will be blown by the 1n4751 zener at about 32 volts, it does not matter.
 

mechie

New Member
Mystery Circuit ?

Hello dar2525,
Now that the whole circuit is visible it starts to look like something ...
I have edited your last attachment a bit and posted it below,

Look carefully at the change I made to the connections of the 3 phase generators - no common line to the circuit's negative line.
I also dumped the earth symbol as it isn't strictly required if I am correct here.
Could this be your circuit?

I think the zener is to simply bias the thyristors' gates to show some control action, the thyristors would have individual control based on output voltage or load.
 

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  • half-controlled-bridge.gif
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dar2525

New Member
Re: Mystery Circuit ?

mechie said:
Hello dar2525,
Now that the whole circuit is visible it starts to look like something ...
I have edited your last attachment a bit and posted it below,

Look carefully at the change I made to the connections of the 3 phase generators - no common line to the circuit's negative line.
I also dumped the earth symbol as it isn't strictly required if I am correct here.
Could this be your circuit?

I think the zener is to simply bias the thyristors' gates to show some control action, the thyristors would have individual control based on output voltage or load.

Thanks so much. This exactly my circuit, the reason I added the ground was because "Circuit Maker" refuse to do Fouier analysis without it. But I did have the phase generator connected wrong, your circuit is correct. Does the zener protect the load from overvoltage or does it protect the SCR gate from overvoltage?
 

dar2525

New Member
Russlk said:
As Audioguru pointed out, when ground is positive, the diodes are a short circuit on the source. That is not good, and does not do any thing for the load. When ground is negative, I have not analyzed the gate network to see when the scr is turned on, but since the gate-cathode junction will be blown by the 1n4751 zener at about 32 volts, it does not matter.

I made a mistake there is no ground in the circuit. Mechie has the circuit drwan correctly. So the source is never shorted and when the source is negative voltage the diode start to conduct. Does the SCR conduct during this stage?
 

mechie

New Member
Re: Mystery Circuit ?

dar2525 said:
Does the zener protect the load from overvoltage or does it protect the SCR gate from overvoltage?

OOPS - edit time :oops: A zener will only conduct when the base is positive with respect to its cathode and it is forward biassed.
**broken link removed**
I meant to say :arrow: A THYRISTOR will only conduct when the base is positive with respect to its cathode and it is forward biassed.
Now the link makes sense as well :wink:


Look at the attached simplified circuit -- do you see a bridge rectifier ? (two diodes and two thyristors).
Think about a normal bridge rectifier's action, if the (now single phase)supply's upper terminal goes positive (coloured red) then the current will flow through SCR1 (ignore gating signals for a minute ...) to feed the circuit's positive rail.
Current will flow through any load to the negative rail (I am assuming conventional current flow), and return to the supply via D2.
On the next half-cycle the current would flow from the lower terminal through SCR2 and return through D1.

So far it's dead simple :?:

To turn an SCR (thyristor) on you have to bias the gate positive WRT the cathode ?
The zener (I added a zener biassing resistor, RZ to feed a current through it) will give a constant voltage, assume it is a 20v zener.
If the positive output rail is less positive than this zener voltage (20v) the thyristor will be turned on, remaining on until reverse biassed.
This output will be sort-of voltage regulated to this zener voltage but with peaks reaching the full AC peak-to-peak voltage ?

A three-phase supply is only a bit more complicated because of the way it can use any of three phasees as positive and any other as the negative (depending on which is most positive and which is most negative).

This is a Half-controlled Rectifier, a fully-controlled rectifier would use thyristors throughout, no diodes.
 

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  • half-controlled-bridge-step1.gif
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  • half-controlled-bridge-simple.gif
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