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LM317: Voltage Output falls when device is loaded

Robert Campbell
Prodigy 30 points
Part Number: LM317
Other Parts Discussed in Thread: LM337

I have built two prototypes of a dual rail +-15V PSU using LM317 and LM337 devices.  The circuit is attached.  Both boards are suffering the same problem.

When the output rails are very lightly loaded (power indicator LEDs only, so around 15ma per rail) then the output voltages are correct at +-15V.

However when I raise the load above 100mA then the +15V output rail falls to +6.96V.  It starts to fall after about a second and takes around 10secs to fall to that level, where after it remains at that level.

You will see the circuit contains a 'mutual shutdown' element which is designed to protect 5532 audio ICs from being fed from only one rail, should the other rail fail.

With that mutual shutdown circuit in place (and the output loaded above 100mA), I get the following voltage readings on the two devices:

LM317

input  +6.96V

output +5.36V

adjust +4.92V

LM337

input -39.9V

output -6.08V

adjust -4.82V

I have also attached an AC coupled scope image of the +15V rail output rail.

Note the huge imbalance between the input voltages to the regs (under light output load these are the same at around 23V into each device).

I have tried removing the mutual shutdown circuit and found that it is the +15V rail which falls under load.  The -15V rail remains at the correct output voltage, so it seems I have a problem with the LM317 circuit or pcb layout.  I have built two of these boards so it is unlikely to be two faulty LM317s.

Your help to resolve this issue would be appreciated.

Regards

Robert

PSU MUTUAL SHUTDOWN CIRCUIT - Schematic.pdf

  • 0
    TI__Mastermind 29350 points

    Hi Robert,

    It seems to me that the linear regulators are not the source of the problem here.  They appear to be operating based on their input voltage.  The LM317 has much less input voltage, for some reason, under load so its output is drooping accordingly.  The LM317 is a nearly 50 year old device and requires more headroom to regulate than modern LDO's, which is why you see a nearly 1.5V difference from Vin to Vout at even this light load condition.

    Since the LM317 is following Vin, you will want to troubleshoot what is going on with D12 and / or TF3.  The issue happens after a few seconds so it may be thermally related.  You can try removing the linear regulator circuitry and simply load down D12 with an electronic load to determine if the voltage droops as you have already seen.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 30 points

    Hi Stephen

    Your advice was spot on!  I found that the output centre tap of the transformer was not connected to the 0V bus on the pcb.  A single 'dot' missing on the schematic which got carried over to the pcb layout.  I added a wire link and all is well.

    Thanks for your help

    Regards

    Robert