hi all
i've built 2 regulated psu using pedga's schematic.
only difference is that i am using +12/-12 vac transformer secondaries to get a regulated +15/-15vdc. also, in place of the 2.2k resistor, i had a variable resistor. higher capacitance was also used. a diode was also placed in the direction of ADJ to OUT pin of lm338t.
one of them works fine, the other(built at a later date), does not seem to work correctly.
the voltage will fluctuate from 3+v to 8+v then drop to 3+v then back again. between the cycle, it will hold steady for a short while at 3+v and also at 8+v.
after rectifier stage is ok, measured close to 19vdc. somewhere from lm338t onwards, there seem to be a problem. i've checked my connections. they seem fine.
any ideas where i might have possibly gone wrong?
i've built 2 regulated psu using pedga's schematic.
only difference is that i am using +12/-12 vac transformer secondaries to get a regulated +15/-15vdc. also, in place of the 2.2k resistor, i had a variable resistor. higher capacitance was also used. a diode was also placed in the direction of ADJ to OUT pin of lm338t.
An externally hosted image should be here but it was not working when we last tested it.
one of them works fine, the other(built at a later date), does not seem to work correctly.
the voltage will fluctuate from 3+v to 8+v then drop to 3+v then back again. between the cycle, it will hold steady for a short while at 3+v and also at 8+v.
after rectifier stage is ok, measured close to 19vdc. somewhere from lm338t onwards, there seem to be a problem. i've checked my connections. they seem fine.
any ideas where i might have possibly gone wrong?
What voltage do you measure at the adj pin for the one that has the problem? If that fluctuates, try it with the adj cap disconnected, it can be leaky (or reversed). Also, check/remove temporarily any diodes at the adj pin to make sure they are not leaking or something. Does the reg get warm at all?
Jan Didden
Jan Didden
i've taken out the variable resistors out for measurment, they seemed ok. i'm actually aiming for +/-15vdc.
the first circuit that i built worked, it is now powering my opa627/buf634. since i have 2 transformers of the same type, i wanted to build another one so that i can use one for each channel.
the first circuit that i built worked, it is now powering my opa627/buf634. since i have 2 transformers of the same type, i wanted to build another one so that i can use one for each channel.
An externally hosted image should be here but it was not working when we last tested it.
What's the voltage on the adj pin doing. Try to measure the voltage between the output pin and the adj pin of the reg. If that remains constant about 1.25V, than the reg is OK.
To measure is to know!
Did you change the divider on the rteg output, because for the 15V version you need to change it from what you showed in the schematic upstairs.
Jan Didden
To measure is to know!
Did you change the divider on the rteg output, because for the 15V version you need to change it from what you showed in the schematic upstairs.
Jan Didden
With 2*12VAC you´ll get about +-18VDC maximum depending on your transformer.
Aiming for +-15VDC isn´t a problem in most cases (I´m using the same setup with 7815,7915 and it works nicely) but the voltage-headroom is very small and regulation might get worse or don´t work properly. Don´´t know the LM338 too well though.
If you don´t need so much voltage swing set the resistors up for +-12VDC to see if it´s that.
But check the more obvious things first.
Aiming for +-15VDC isn´t a problem in most cases (I´m using the same setup with 7815,7915 and it works nicely) but the voltage-headroom is very small and regulation might get worse or don´t work properly. Don´´t know the LM338 too well though.
If you don´t need so much voltage swing set the resistors up for +-12VDC to see if it´s that.
But check the more obvious things first.
You built a motorboat!
Your system is probably oscillating. Because the capacitors are so large it doesn't really look like an oscillation, but that's probably what you have. With 4700 uf and almost no load you probably have a time constant of minutes.
Bypass capacitors is the normal fix for this.
In addition to the capacitors you have, it would be worthwhile to add a 10uf and a 0.1uf bypass capacitor to the regulator mounted as close to the regs as possible. The 10uf should be tantalum--the higher the voltage the better (since at the same uf value, the higher voltage ones have lower ESR).
Normally the 0.1 goes on the input side of the reg and the 10uf on the output side. But in your case, because the input capacitance is so huge on the input side, you may want to put a 10uf on both the input and output side.
Hope this helps.
Your system is probably oscillating. Because the capacitors are so large it doesn't really look like an oscillation, but that's probably what you have. With 4700 uf and almost no load you probably have a time constant of minutes.
Bypass capacitors is the normal fix for this.
In addition to the capacitors you have, it would be worthwhile to add a 10uf and a 0.1uf bypass capacitor to the regulator mounted as close to the regs as possible. The 10uf should be tantalum--the higher the voltage the better (since at the same uf value, the higher voltage ones have lower ESR).
Normally the 0.1 goes on the input side of the reg and the 10uf on the output side. But in your case, because the input capacitance is so huge on the input side, you may want to put a 10uf on both the input and output side.
Hope this helps.
hi guys, thanks very much for all your input.
i did not change the resistors as they gave me the output voltage that i need on the psu that is working.
+19v o input
o output
o adj
the heatsink also do not show continuity when i used a multi-meter to check them.
i'll check the voltage on the adj to out pin tonight.janneman said:
i did not change the resistors as they gave me the output voltage that i need on the psu that is working.
i'll re-check that again. but i am pretty sure that it is right. from the image above, it is like this (from top up)carlosfm said:
+19v o input
o output
o adj
they are insulated with those thermal pads.jackinnj said:
the heatsink also do not show continuity when i used a multi-meter to check them.
i have one 0.1uf cap soldered after the rectifier and one 0.1uf soldered as the last cap on the output. these are under the board.drdagor said:
took out the heatsinks last night and tried the supply again.
only getting about 3.5v or so.
voltage across adj and output pin was in the millivolts range.
i'm gonna remove everything from the lm338t onwards over the weekend and solder the components one by one, testing at each stage to see if it works.
only getting about 3.5v or so.
voltage across adj and output pin was in the millivolts range.
i'm gonna remove everything from the lm338t onwards over the weekend and solder the components one by one, testing at each stage to see if it works.
The regulator should keep a constant reference voltage of around 1.25V between OUT and ADJ.
A true voltage between ADJ and OUT in the millivolt range might sound like a damaged chip.
How did the soldering work? Did you probably overheat the LM338? Or overload it?
You shouldn't leave the chip unconnected. At least use a replaced voltage divider network, no ADJ bypass cap and no OUT bypass cap for a short measurement. With an appropriate input voltage of >15V, the regulator should really show some sensible behaviour now. Otherwise it might be damaged...
Just to make shure I understand you right: You use a network of 120R and 2.2k according to Pedja's circuit?
Check with the datasheet: On page 4 it shows the equation to calculate the resistor network for a required output voltage. Using 120R as the upper and 2k2 as the lower resistor would calculate to Pedja's required value of around 25V.
For 15V and 120R as the upper resistor, the equation leads to a value for the lower resistor of ca. 1.3k ...
If you really used the wrong values here, that would explain your problem. It would also explain the too low reference voltage between ADJ and OUT, because for Vin < (Vout+Vdrop) the regulator is fighting against itself, showing ill behaviour at the OUT pin. That makes me wonder why the other board works...
Sebastian.
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