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Old 17th May 2011, 08:22 PM   #1
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Default Output voltage Alwsr super reg

Hi,

I have a question about the output voltage of the alwsr.

I build the alwsr super reg 5v as the manual discribes.
The preregulator gives a nice 2,5v and the led turns on.
Input voltage is about 12v.

But now when i measure at the output it´s give not 5v but 6,7v unloaded.
Is this normal or is it because there´s no load on it?


The resistors R8,R9 for the output voltage are 1k.
Do i something wrong?

Magura
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Old 18th May 2011, 11:30 AM   #2
agdr is offline agdr  United States
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Where do you have the Vsense and 0Vsense lines connected? With no load Vsense should be connected to the output (OP1 or OP2) and OVsense connected to ground (SRground).
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Old 18th May 2011, 05:58 PM   #3
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The Vsense is connected with a wire link to OP2 and a wire link between 0vsense and srground.

Maybe wrong resistor value of R5? 249r

Last edited by Magura456; 18th May 2011 at 06:01 PM.
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Old 18th May 2011, 06:42 PM   #4
agdr is offline agdr  United States
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That is OK for R5, that is the value his schematic shows.

What voltage do you get on the cathode of VR1 (the connection between VR1 and R5), with respect to ground (SRground or 0Vground), and what part number are you using for VR1?

What voltage do you get between pin 3 of the IC (the connection between R7 and the IC) and ground?

And what voltage do you get across resistor R9?

Last edited by agdr; 18th May 2011 at 06:51 PM.
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Old 19th May 2011, 03:38 AM   #5
agdr is offline agdr  United States
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Actually... for the ALWSR 5V output version he lists the LM336-2.5 in the bill of materials for VR1, but then shows VR1 as a TL431 on the schematic for the 5V version. Either would work, but R5 would be different.

R5 = 249R would be OK for the TL431 as shown, but for a LM336-2.5 R5 would be better off as 1k. The TL431 goes up to 100mA but the LM336-2.5 maxes out at 10mA.

Also note the TL431 is a 3 terminal device, with the reference pin tied to cathode in this case, as shown (I'm looking at the version 9a ALWSR info). But the LM336-2.5 would only be used as a 2 terminal device, with the adj pin unused.

Last edited by agdr; 19th May 2011 at 03:55 AM.
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Old 19th May 2011, 03:30 PM   #6
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Quote:
Originally Posted by agdr View Post
That is OK for R5, that is the value his schematic shows.

What voltage do you get on the cathode of VR1 (the connection between VR1 and R5), with respect to ground (SRground or 0Vground), and what part number are you using for VR1?

What voltage do you get between pin 3 of the IC (the connection between R7 and the IC) and ground?

And what voltage do you get across resistor R9?

The voltage on the cathode of VR1 is 1,08V.
I use a TL431 ac lpg Farnell 146-8785

Between pin 3 and R7 is also 1,08V

The voltage across R9 is 2,76V

Thanks for your help btw
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Old 19th May 2011, 04:22 PM   #7
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Quote:
Originally Posted by agdr View Post
Also note the TL431 is a 3 terminal device, with the reference pin tied to cathode in this case, as shown (I'm looking at the version 9a ALWSR info). But the LM336-2.5 would only be used as a 2 terminal device, with the adj pin unused.
I didn't tied the reference pin to cathode actualy....
I'll try it tonight.
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Old 19th May 2011, 05:10 PM   #8
agdr is offline agdr  United States
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Quote:
Originally Posted by Magura456 View Post
I didn't tied the reference pin to cathode actualy....
I'll try it tonight.
Good - that is necessary. It is a special configuration of the TL431 that makes the output equal to the internal 2.5V reference, as in figure 1 on the datasheet (Vref = 2.5V):

http://focus.ti.com/general/docs/lit...1&fileType=pdf

You should get 2.5V across the TL431 with the change, and also on pin 3 of the op amp.

If it were me I would use 1K for R5 even with the TL431. The 249R AW is using results in 10mA through the TL431, which is OK but seems larger than really needed. 1K would give you 2.5mA which is more than enough and still above the 1mA minimum for the part.

AW is trying to make the impedances to ground look the same, looking back from the op amp inputs, which is good. On the inverting (-) pin you have 500R, which is the 1k R8 and R9 in parallel, with the output being considered nearly an ac short to ground via the output cap. On the non-inverting input you have the 499R in series which does the same thing, since the TL431 (or LM336-2.5) looks like nearly an ac short to ground too. So the value of R5 doesn't affect the input impedance since it is in parallel with the TL431 for ac purposes.

Last edited by agdr; 19th May 2011 at 05:15 PM.
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Old 19th May 2011, 06:09 PM   #9
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So i tied pin 1 ref. to pin 3 of the tl431. Pin 3 to ground is 0V.
Output voltage is now 7,6V...

I measure pin 3 to R7 2V.

The voltage over R9 is now 6,7V instead of 2,76V
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Old 19th May 2011, 10:45 PM   #10
agdr is offline agdr  United States
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What voltage do you get between ground and the other side of R5 (not connected to the TL431)?

It sounds like your TL431 might be bad if it's pin 3 to ground is 0V. If there is 7,6 volts on the other side of R5, then the TL431 should be producing 2,5V if it is working. If you are using a circuit board it would be a good idea to check solder connections around R5 and the TL431 for solder bridges, and double check with that datasheet link that the TL431 is in the right way.

Last edited by agdr; 19th May 2011 at 10:55 PM.
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