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I'm planning on running a Buffalo32 with two Placid bipolar driving each side of the analog output stage. I will use 10VA Amvecos, will these be sufficient? I have used 7VAs before for this, but with the regular TP bipolar supply. Also, will this configuration be content with 1" heat-sinks on the Placid?
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I am interested in this as well.
According to the manual, Buffalo32 draws 68mA from VA+ and 43mA from VA-. I would think that if you set the current lower on the Placids than the standard 200mA (say 100mA VA+ and 75mA VA-) 10VA should be fine.
I'm not sure how to calculate the heat production though. It is not as immediately obvious as it is for linear regulators, considering you have both the CCS and shunt transistors.
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Calculating the dissipation is not too hard. If you know approximately how much excess current the shunt element will need to shunt, you just calculate the drop accross the shunt transistor. That will be the output voltage. For the CCS the voltage drop is the rectified DC voltage minus the output voltage. So lets say your doing 15VAC trafos and 15V output. That will be something like 22V(rectified) - 15VDC so 7VDC. So if your CCS is setup for 250ma you figure 250ma at 7V drop. So lets further say that your target device draws 200ma. Then your shunt element will have to shunt 50ma (250ma - 200ma) in static conditions. So you calculate the dissipation as 50ma with a 15V drop. I hope that helps! Cheers! Russ Edited by user Tuesday, October 13, 2009 3:39:16 PM(UTC)
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Rank: Advanced Member
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Ah, the CCS burns off the excess voltage. I sort of understood how the shunt part works, but this helps me understand the concept a lot better.
So with respect to heat production, it doesn't seem that much higher than a linear regulator - provided you don't set the CCS way higher than you need it.
Which brings me to another question...... I know you have said that 3V headroom is fine for voltage, but what would you recommend as a good safety margin to set for current draw? A certain percentage above peak, x mA higher?
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My rule of thumb for now is just to set the shunt current to at least 50% over the peak delta, if I know it. So if the static current draw is 200ma, and the highest peak will be 220ma. Then setting at least 230ma would be good, and 250ma a bit better because you have some more headroom for current draw.
Cheers!
Russ
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