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avr300  
#1 Posted : Sunday, July 4, 2010 7:12:49 AM(UTC)
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Placid BP. The positive side of the BP dropped on me today. It has been running since yesterday without problem.

I can't get over 24mV measured over R2. Unconnected it still gives 13V.

It's running unboxed, but the heat sinks still gets pretty hot.
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avr300  
#2 Posted : Sunday, July 4, 2010 7:42:38 AM(UTC)
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Solved!

Crappy VR2 pot. Now they (VR1 and VR2) have been substituted with proper resistors.
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avr300  
#3 Posted : Sunday, July 4, 2010 9:13:14 AM(UTC)
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Russ, I'm I missing something here, or is there an error in the BP schematics, the 2.1.2 ?

The current througt the CCS looks like going through VR2 ??

In 2.1.1 the placement of VR2 understandable.
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avr300  
#4 Posted : Sunday, July 4, 2010 9:39:17 AM(UTC)
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Ohh, just saw http://www.twistedpearau....aspx?g=posts&t=1131

Sorry,

But still it doesn't make sense to me. What's the purpose sending the whole current through those tiny pot's ? I think that's what killed my VR1+2.

I noticed elsewhere, that 500mA is no problem for the Placid. How would VR1+2 end up with .5A through?
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Russ White  
#5 Posted : Monday, July 5, 2010 8:25:44 PM(UTC)
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The pots never come close to their power rating. It is not the current that matters so much as the power dissipated.

There is a bias resistor in series to prevent that even getting too high.

The reason we did it this way is to allow a fixed voltage ref for the CCS.

Of course, if you really want to you could replace the pots with fixed Rs if you really must get tons of current through there, or if you know what current it is your after. A fixed R is actually better in this case.

Edited by user Monday, July 5, 2010 8:32:25 PM(UTC)  | Reason: Not specified
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Russ White  
#6 Posted : Monday, July 5, 2010 8:31:20 PM(UTC)
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Also the pots should not fail unless they were adjusted past their stops.
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avr300  
#7 Posted : Monday, July 5, 2010 11:17:51 PM(UTC)
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Actually I really think it did here. It has not been past its stop.

Anyway, replacing it with a 8.2R resistor gives a nicely 145mA draw - so the pot was/it cracked.
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sureshm  
#8 Posted : Tuesday, July 6, 2010 12:42:42 AM(UTC)
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Russ White wrote:
The reason we did it this way is to allow a fixed voltage ref for the CCS.


Russ, any reason why this approach is taken only for the Bipolar Placid and not the single power supply?
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Russ White  
#9 Posted : Tuesday, July 6, 2010 3:35:23 PM(UTC)
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Yes, because we found thatthe approach used for placid did not mirror well, because of differences in VGS.

In any case AVR300 is on the right track. Once you know the current you want, you can quite easily just short the pot and install a fixed resistor at R1/R2.
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avr300  
#10 Posted : Wednesday, July 7, 2010 2:19:52 AM(UTC)
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Just for the record - if anyone else wants to set it up for the IVYII, ie. aprox. 150mA - I replaced the pots with 8.2R. The R1 (and R2) 1R still is in place. 10R in all could be ok, but I have not tested it.

And again, Russ and Brian, thx. for making this BII/IVYII/Placid combo available - it sounds absolute fantastic, even compared to BI.

I'm really looking forward to the Legato.
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