Greetings, all.
I've been working on a standalone bipolar power (±17V) supply to power an 8x4 summing amp I've been playing around with. I'm using LT1084s (the adjustable variety). This is a textbook design. Nothing out of the ordinary. Hammond toroidal transformer, 3 10,000uF filter caps per side, bypass cap on the adj. terminal and a 1k trimmer in series with a 470R resistor on the adj. terminal to trim voltages. Output stability tantalum cap as recommended by datasheet. This it not my first rodeo with regulated bipolar supplies, and they've generally worked as expected without a hitch. This is, however, my first go round with LT1084s.
Things have been weird from the get-go. First, math be damned, regulated output voltage with no load (well, with an indicator LED and series resistor) is WAY higher than the 1.25V(1 + R2/R1) calculation works out to be. I adjusted the fixed resistor on the adj. pin down to get me into the necessary range.
More troubling, however, is the HORRIBLE instability of the "regulated" output voltages. DC output voltage on both rails will vary between 16.84Vdc up to 17.07Vdc.
I haven't seen this with supplies I've built with 78xx and/or LM317 regulators.
Schematic attached. Note, the "3900uF" caps are 10,000uF in the actual build, the LT1084s are not "18v" out, they're the adjustable model. Finally (the initial weirdness) R35 and R36 had to be lowered to 100R each for output voltage to fall into the proper range.
Any input would be greatly helpful, particularly anyone with experience working the LT108x series high-current regulators.
UPDATE: Stranger still, if I set the output voltage to exactly 17.00V, over time it will creep up to about 17.2V, hover around there for 10-20 minutes, then creep down again. It's almost as if there is an EXTREMELY low frequency oscillation in the supply. Voltage coming off the rectifiers is stable. Only weird AFTER the filter caps.
I've been working on a standalone bipolar power (±17V) supply to power an 8x4 summing amp I've been playing around with. I'm using LT1084s (the adjustable variety). This is a textbook design. Nothing out of the ordinary. Hammond toroidal transformer, 3 10,000uF filter caps per side, bypass cap on the adj. terminal and a 1k trimmer in series with a 470R resistor on the adj. terminal to trim voltages. Output stability tantalum cap as recommended by datasheet. This it not my first rodeo with regulated bipolar supplies, and they've generally worked as expected without a hitch. This is, however, my first go round with LT1084s.
Things have been weird from the get-go. First, math be damned, regulated output voltage with no load (well, with an indicator LED and series resistor) is WAY higher than the 1.25V(1 + R2/R1) calculation works out to be. I adjusted the fixed resistor on the adj. pin down to get me into the necessary range.
More troubling, however, is the HORRIBLE instability of the "regulated" output voltages. DC output voltage on both rails will vary between 16.84Vdc up to 17.07Vdc.
I haven't seen this with supplies I've built with 78xx and/or LM317 regulators.
Schematic attached. Note, the "3900uF" caps are 10,000uF in the actual build, the LT1084s are not "18v" out, they're the adjustable model. Finally (the initial weirdness) R35 and R36 had to be lowered to 100R each for output voltage to fall into the proper range.
Any input would be greatly helpful, particularly anyone with experience working the LT108x series high-current regulators.
UPDATE: Stranger still, if I set the output voltage to exactly 17.00V, over time it will creep up to about 17.2V, hover around there for 10-20 minutes, then creep down again. It's almost as if there is an EXTREMELY low frequency oscillation in the supply. Voltage coming off the rectifiers is stable. Only weird AFTER the filter caps.
