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Old 8th July 2006, 06:04 PM   #1
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Default LV Filament supply quandry

i have constructed my 2nd 6.3vdc regulated filament supply(whew!) and it works fine. However the only tranny i had available was a 6-0-6v unit and after the 10000uf filter cap i get 15+ vts(before the reg..

i am looking to trim a couple of volts from before the regulator in order to decrease power dissapation from the lm350.(in the standard config as in the device data sheet, and heatsinked with a aavid sink-thermal r=3.7) i am passing 1.2 amps(2 -12b4 fil.) and the lm 350 with heatsink does get hot. with a starting voltage of @10v things seem much better!

#1) i can trim voltage before the bridge rectifier, or after? which is better and why? i have gone about it both ways and it seems to work well both ways.

#2) i have seen it advised that to drop voltage before the rectifier, that i should use 2 resistors (1 on each secondary leg) i have tried it with one and again nothing seemed amiss. if i am using the full secondary winding and current alternates(back and forth) why would using one resistor be a bad thing? please help clear my ignorance!

thanks
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Old 8th July 2006, 06:19 PM   #2
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You may have already considered this, but an alternative you may want to try is to not use a bridge rectifier, but use 2 diodes in a full wave center tap configuration. This will give about half the voltage of a bridge rectifier.

I do not know what the minimum voltage your particular regulator requires to output 6.3 volts, but you just might get enough using the full wave circuit. You may want to try it and see.
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Old 8th July 2006, 06:19 PM   #3
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Simple trick.
Ground the center tap(0) and use one diode from each 6v side.
Just to be absolutly clear: Anode of the diodes to each 6v side and
the cathode's conected together to the filter cap.
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Old 8th July 2006, 06:32 PM   #4
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hi- thanks for your replies-i have tried this and it works but i'm right on the edge of the minimum voltage needed, any drop in ac mains and i risk losing regulation.

i know it seems a waste dropping voltage with resistors but right now i am stuck with this tranny,

will it bother the trannny to attach a dropping resistor to only 1 secondary leg which feeds a bridge configuration?
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Old 8th July 2006, 06:56 PM   #5
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No I don't think so... But you know, the only thing you do is to disipate heat in another component...


Best regards,

Magnus
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Old 8th July 2006, 07:05 PM   #6
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thanks again! yes most definitley, but i thought it the lesser of 2 evils, so-to -speak. the resistor is a 25w job bolted to the metal chassis! the regulator is board mounted, with a decent heatsink, but , i thought that asking it to dissipate @ 10 watts was a little too much. i'm trying to make do with what i have, as i am laid off at the moment.
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Old 8th July 2006, 07:10 PM   #7
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I understand.

A 25w resistor bolted to chassis is a very good solution when it comes to resistor that need to disipate some watts. 10w is not asking for too much.

Best regards,

Magnus
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