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Old 18th May 2004, 12:22 AM   #1
mfr is offline mfr  Canada
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Default Voltage regulator OP249 question

Hello,

I have these regulators as part of a circuit: Schematics.gif

I have a problem: The measured output is -3.74/+3.74v instead of -5/+5v. I cahnged the 5.6k resistor for a 2.4k, the output changed to 4 volts. I changed the 2.4k resistors at the output of the opamps to 1.2k with no change in output voltage. Measured voltage drop on zeners at 3.74v (same as output).

Should I increase the Zener diodes value or decrease the 5.6k resistor in order to achieve the desired -5/+5v output?

About the 2sk246 FET: is the Source and Drain reversible? I wonder if the schematics reversed the two. Whould it make a difference?

Thanks,

Mihai
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Old 18th May 2004, 05:52 AM   #2
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Hi,
You reference the zener to the other supply ouput. Shouldn't it be referenced to ground?
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Old 18th May 2004, 06:42 AM   #3
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No the schematic seems ok.

The zener must be faulty if the voltage drop isn't more than 4v when you have the 2.4k resistor.

The current through the resistor and zener should be ~1.6mA which should be enough.

Change the zener, or check its datasheet to see at which current it has 5v.

Edit: The fet might be wrong but i am not sure.
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Old 18th May 2004, 07:11 AM   #4
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Quote:
Originally posted by hjelm
No the schematic seems ok.

The zener must be faulty if the voltage drop isn't more than 4v when you have the 2.4k resistor.

The current through the resistor and zener should be ~1.6mA which should be enough.

Change the zener, or check its datasheet to see at which current it has 5v.

Edit: The fet might be wrong but i am not sure.
The schematic is not OK. This schematic does nothing to regulate against output voltage fluctuations. The zeners should be referenced to gnd. Where does this come from?

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Old 18th May 2004, 07:36 AM   #5
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I think also the design is a bit "twisted".

Jan, it will probably work but the "twisted" thing is that the negative voltage will have influence to the positive.

First you must calculate the voltages you should have and of course check all parts, the inputs of the opamps, are the correctly connected?

Notice also that a 5.1 V(?) zener have tolerances and is also not so "sharp". 5.6 kohms = only 1 mA, which is rather little. I had chosen 2-5 mA at least.

If you are unsure about the zeners, jsut test them only with a resistor and the zener. Then you can see what can expect in voltage.

Mihai, check also if the voltage between the inputs of the opams are zero (less than 1-4 mV) even when you load the regulator. If yes, it seems to work. The regulator may also oscillate. Have you checked the output with an oscilloscope?

How does you pcb look like? You may have problems there?
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Old 18th May 2004, 09:56 AM   #6
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Thank you all for the suggestions.

The design is from china, probably a copy of something else?

The pcb is ok, no errors there. I have 4 of these working, one which is using 15v Zeners. It's output is at 14v (acceptable). The other 3 are 5v and all behave the same way. The chinese schematics mention using a op275 as and option instead of the op249. I don't think it will change much.

I'll try increasing the current on the zeners as you suggested.
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Old 18th May 2004, 10:10 AM   #7
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How would you give current to the zeners if you reference them to ground? You would have to tap it somewhere before the transistors. You would also need to put the sensing on the vout?

It does have a cap across the zener but admittedly it is not that big. Guesstimating you have at least 45dB damping from the other rail ripple.

Yes i think the regulator is very load sensitive but its PSRR should be high. Maby it is intended for class A use.
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Old 18th May 2004, 10:22 AM   #8
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The use for this regulator is powering 2 DAC IC's. A 100-150mA load (total). Maybe there is some advantage to keep it symetric, thus the reference across the other output. I wonder if for some reason, there is not enough v out for the zeners to work?
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Old 18th May 2004, 12:05 PM   #9
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Quote:
Originally posted by hjelm
How would you give current to the zeners if you reference them to ground? You would have to tap it somewhere before the transistors. You would also need to put the sensing on the vout?

It does have a cap across the zener but admittedly it is not that big. Guesstimating you have at least 45dB damping from the other rail ripple.

Yes i think the regulator is very load sensitive but its PSRR should be high. Maby it is intended for class A use.

You want to regulate the output voltage referenced to ground. So you develop a reference voltage to ground for the + input of the error amp. You do this by a resistor from + (or -) to a zener to ground, and feed the zener voltage to the + input. Then you feed the sample of the output voltage to the - input. The error amp will do whatever is necessary to make its inputs equal. By adjusting the division of the output voltage before feeding it to the - input you set the output voltage. The reference at the + input should be as constant and stable as possibel, after all it is a 'reference'. So, you may want to include an RC filter between the zener and the + input. But that's details.

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Old 18th May 2004, 12:18 PM   #10
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But it is referenced to ground but the other way around. Is there a big problem with it? Either it ripples with the Supply or it ripples with the output. You could argue that the output is more stable than the supply? I can see the pont in having the supply react symmetrically if you use a differential architecture of the DAC but this circuit doesn't react symmetrically.
Not arguing here really but i just thought there might be a point to it.

Nobody has answered if the FET's are turned the right way around. Are they there as a CCS?
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