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I just built a gainclone using the LM1785 and a 225v Plitron transformer. the auto-protection circuit has been shutting down the opamps. When I measure the the VDC at the rectifiers I can watch it slowly creep up until it hits about 23 volts and then the opamps shut off.I have no idea how to fix this. Any help is greatly appreciated.
Mike B
Follow Ups:
200va@+/-25vdc.the lm1875 can draw 4amps at peak.
50vdc x 4amps = 200voltamps.
more va is better than less.
what voltage(s) are you feeding the chip ?
how is your psu configured ?
if you are doing +/- then it should be from 18vdc to 25vdc for a total of 36vdc or 50vdc.
if you go higher than +/-25vdc or 50vdc then you are getting so close to the upper soar limit --safe operating area-- that the chip starts wanting to protect itself by limiting the current draw.
Hi Snkby,
I couldn't agree more, but within defined limits. If the Toroid is a 16 V output, then fine, 200, 300, 400+VA will do no harm. However, if they have a higher output toroid, then it will require critical loading to drop the Vout to ~25V (i.e the core must run close to saturation), most likely a 20V toroid has been acquired, in which case serious overrun will occur. Not an ideal solution - usually using a dropper resistor in parallel to the device, and will allow significant PSU sag on transients, without krell sized cap banks to prop the voltage up...Hence my suggestion to get PSUDii, as this will show the Vout after rectification for a given Vout at the toroid - enabling a better informed decision to be made on toroid Vout choice to be within the SOAR of the chip itself. Then the VA issue is almost irrelevant - as you say, ideally bigger is better.
Thought I'd clarify what I'd said below..
Have fun :-)
Owen
Sounds like an insufficiently loaded Toroid to me - the voltage will creep up to the max 1.4*rms if the toroid is oversized (i.e the opamp is drawing insufficient current). You need to calculate the total disipation of the gainclone to re-evaluate the sizing of your toroid (add a 1 ohm series resistor between the +ve powersupply and the +ve pin. Measure the voltage across it, and lo you have the current, from there on P=IV, so you can calculate the VA ating of the toroid required).I'd also get a copy of Duncan Amps PSU designer. It will explain alot..
Have fun
Owen
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