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One problem down, two new ones crop up -- prob w/ LM317/337 regulators

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Praxis:
So, I'm working on a classic simple bench-top split power supply, using this schematic: http://www.aaroncake.net/circuits/supply3.asp

Well, one problem is that I'm getting about 26 volts out at either rail, when the transformer should give +/- 18v.  I've got a 680 ohm resistor across the outputs as a load, and I'm measuring the output across the resistor. 

Another more serious problem is that if I crank the pots down the bloody sense resistors incinerate themselves.  Vout only gets to about 23 or 24v when this happens.  I'm using 220 ohm 1/4W resistors.

I've checked and re-checked the connections and problems.  Any ideas why this is happening?

IanB:
What are the voltages on C1 and C2?

Did you take care to connect pins 1, 2 & 3 correctly on the regulators?

Did you take care not to connect the regulator heat sinks to any electrical points they should not be connected to?

Did you wire R2 and R1 to the right points in the circuit?

What is the voltage across R2 (& R3) when they are not smoking?

Are you sure you connected R2 and R3 to the right points in the circuit?

alm:

--- Quote from: Praxis on August 14, 2011, 09:40:38 pm ---Well, one problem is that I'm getting about 26 volts out at either rail, when the transformer should give +/- 18v.  I've got a 680 ohm resistor across the outputs as a load, and I'm measuring the output across the resistor. 

--- End quote ---
18Vrms AC after full-bridge rectification will give sqrt(2)*18V DC, because the diodes + caps work as peak detectors. The DC voltage is close to the peak AC voltage, which in the case of a sine is sqrt(2) times larger than the RMS voltage that's usually quoted. Subtract about 2V for the drop across the two diodes. You can probably find some illustrated examples on a site like Wikipedia. Note that max current decreases with even more than sqrt(2), because of the uneven loading of the transformer (caps only charge during the peaks). About 0.6 is usually used.


--- Quote from: Praxis on August 14, 2011, 09:40:38 pm ---Another more serious problem is that if I crank the pots down the bloody sense resistors incinerate themselves.  Vout only gets to about 23 or 24v when this happens.  I'm using 220 ohm 1/4W resistors.

--- End quote ---
V^2/R = 2.6W for 24V/220ohm, no wonder that they commit suicide. The 24V output voltage is wrong, however, it should be about 1.25V if R1/R4=0 ohm. Are you sure that the wiring is correct, i.e. no short between the regulators via the heat sink (the case of the LM317 is Vout I believe, but the case of the LM337 is Vin)? Does Vout drop if you decrease R1 at all, i.e. does it work until it goes to zero and then blow up, or is Vout a constant ~24VDC?

ejeffrey:
Stupid question, but are you sure that you have the 317 on the positive side and the 337 on the negative?  Other than that, what they said.  Check for shorts between the body and the heatsink, make sure the sense resistors are wired right.

Make sure the regulators are the right way around: make sure you check the diagram in the data sheet to see if it is a bottom view or a top view!

Cranking the pots "down" should make the output voltage lower not higher.

Zero999:
AronCake is full of poorly designed circuits - this is one of the better ones, at least it works. It just needs a bit of tweaking for optimal performance.

26V sounds about right for an 18V transformer, Google for RMS.

R2 and R3 should be 120R otherwise the output voltage may be too high when unloaded but if you do that you should change the pots to 2.5k.

If you plug the resistor values into the formula given on the LM317 datasheet you get 27.3V but you won't be able to get the voltage as high as this. The maximum output voltage of this circuit when unloaded will be around 24V but there won't be any voltage regulation. The maximum output voltage when fully loaded will be around 15V. This means you'll have a lot of dead band i.e. when it's fully loaded 55% of the pot's adjustment won't make any difference to the output voltage and there'll be no regulation.

I'd recommend increasing the capacitor values to 3300uF, using 1k for the pots and 82R for R2 and R3; this will give a maximum output voltage of 16.5V with minimal deadband at full load. The pots should be rated to at least 250mW.

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