# Help with a circuit

Discussion in 'General Electronics Chat' started by whatever_51, Nov 20, 2012.

1. ### whatever_51 Thread Starter New Member

Nov 20, 2012
3
0
Hey everyone I am trying to figure out this circuit here.
it acts a variable frequency supply for a quadruple mass spectrometer.
Can anyone tell me how the DC and AC flow through the circuit and how we end up having two different outputs the positive and the negative one.

2. ### SPQR Member

Nov 4, 2011
379
49
This is a very interesting circuit, and I'd like to try to look at it a bit, and comment, but definitely wait for the experts.

1. So the two capacitors C1, C2 tell me that current flow up to their far side must be AC.

2. The two capacitors C5, C6 also allow AC current to flow beyond them, thus delivering AC current to the two output terminals.

3. AC current flows through R1, R2, R3, R4 and gets to the two diodes.

4. The orientation of the diodes forces current flow clockwise around that inner section, and when you see that, you'll probably be getting rectified current between "A" (the ground) and VR1 and VR2. So now we have the "ground" for the DC.

5. This rectified current is smoothed by R2, C3 and R3, C4 just like in the RC circuit from a power supply.

6. The two VRs allow you to adjust the DC voltage between the ground "A" and each side of the DC apparatus

7. I'm just not sure about the R5, R6 - They probably have something to do with the magnitue of the DC voltage presented to the two output terminals.

Now...we wait for the experts to give us the right answer!

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3. ### whatever_51 Thread Starter New Member

Nov 20, 2012
3
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4. The orientation of the diodes forces current flow clockwise around that inner section, and when you see that, you'll probably be getting rectified current between "A" (the ground) and VR1 and VR2. So now we have the "ground" for the DC.

Can you help me understand where does the DC current come from, I mean the one passing through the R3, C4 part of the circuit?
cos as far as I understand it, this part of the circuit is connected to the negative end of the power supply?

4. ### SPQR Member

Nov 4, 2011
379
49
Whenever you see a diode, you need to think that current is mostly going to flow in one direction (obviously there are exceptions, but not in this case).

So let's take the upper limb, and assume that there is positive current flow through C1, R1, and now you see that diode D1.

It's pointing in the right direction to let current flow toward D3 and VR1, and it does.

Now follow the current down R2, R3 and look! there's another diode D2.
BUT! it's pointed in the wrong direction to allow current flow through it.

So you have a clockwise loop of current through D1.

The next 1/2 cycle of the AC current comes from the bottom, and again, is blocked by D2, and can enter D1.

This gives you a full-wave rectifier (see Wiki - http://en.wikipedia.org/wiki/Rectifier).

So in that "inner loop" you've got current flowing in little "full wave bumps", and that can be smoothed out by the multiple RC circuits in the rest of the inner circle.

Take a look at a full-wave rectifier and see how the directions that the diodes point forces current to flow in only one direction.

Then look at an RC circuit commonly seen in power supplies and note how it can take a full-wave bumpy pattern, and smooth it out to make nice DC.
http://www.vias.org/kimberlyee/ee_29_13.html

Again, the experts will be much better at explaining it.

Last edited: Nov 20, 2012
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5. ### whatever_51 Thread Starter New Member

Nov 20, 2012
3
0
Thanks SPQR very informative