# design problem

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Oct 12, 2011.

1. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
please can some one help me i need to be able to control the out put on R3 by changing R4
thanks

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2. ### SgtWookie Expert

Jul 17, 2007
22,182
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Please simply post the image without embedding it in a Word .doc file; you can paste a screen print into MS Paint, then crop it and save it as a .png file. .png files require no other software than a browser to read, and are very compact.

You have some very large problems with this schematic.

250V across 1k Ohms (R5) is 250mA and 50 Watts of power dissipation.

I can only guess that you are wanting to regulate the voltage across R3, but most of your circuit components would disappear in a puff of smoke if you tried building it.

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3. ### crutschow Expert

Mar 14, 2008
12,502
3,060
So what happens when you simulate the circuit? Show us your simulation results.

We can't help if we don't know the problem.

4. ### crutschow Expert

Mar 14, 2008
12,502
3,060
You missed R6 which limits the maximum circuit current to 25mA.

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
I see R6 as a 10 Ohm resistor? So, I guess that limits it to 247.5mA through the TL431... I still see smoke in his future.

It'll be less than that due to all of the parallel paths.

I simply saw R6 as being so low in value that it was basically inconsequential, but perhaps I'm not correct on that.

6. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
How can i do the feedback for this circuit ,and what the changes i have to do to make it work

7. ### SgtWookie Expert

Jul 17, 2007
22,182
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V1 is far too high of a voltage for the circuit.
Reduce it to 36v, as that is the absolute maximum for the TL431.

Datasheet: http://www.fairchildsemi.com/ds/TL/TL431A.pdf

You need to tell us more about your source voltage. Is that 250VDC batteries, rectified mains, the output from a regulated power supply?

Is R3 going to always be a fixed 20K Ohm resistive load, or will it change?

I am not certain what your plans are for this circuit.

8. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
Hi after i save my schematic as png file how can i posted to be look as you did ,i do attatch for png file or what

9. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
Please can some one fix my circuit i need it urgently

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10. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
I pasted your image into MSPaint.
Then I cropped the image to just the schematic.
Then I saved it as a type .png file.
Then I uploaded it using the "Go Advanced" and "Manage Attachments" buttons.
Then I returned to the text box, clicked the "Insert Image" icon (looks like a postcard w/mountains) and pasted in the image link.

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11. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
R3 is fixed and v1 is coming from power supply(lambda power supply)

12. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Why will you not answer my questions?

I cannot help you unless you answer them. I don't know what you really need to do. What you have now is not safe, and will not work.

Jun 23, 2008
351
1
14. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Your image post attempt did not work for some reason; it seems like you tried to post the forum as an image.

Anyway, have a look at the attached. I am not fond of the circuit. You must make certain that the potentiometer is insulated from the shaft. At least use a plastic knob, and mount the pot on an insulated bulkhead/panel/box.

R1 prevents excessive current in case the pot is turned all the way up. C1 reduces the noise from the resistors. ZD1 keeps the gate voltage within +15/-1 of the source terminal to prevent it from being destroyed. You will need to use a heat sink on the MOSFET, as power dissipation will approach 800mW when the voltage across R3 is 125v.

There is no regulation of current or voltage. You may need to use a small capacitor across R3 to keep the voltage relatively constant.

• ###### Taky HV.png
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