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Power Supply Troubleshooting

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captainate

Member
Hey folks,

Been working on a DIY Project and built the power supply for it today. It's a bass DI based on the A-Designs REDDI from the GroupDIY forum. Things are a little slow over there, and this issue isn't specific to the project, so I thought I'd ask the question here. I'm getting 288V from the B+ output, and 5.3V from the DC Heater supply. These figures come from measurements without load, but I'm assuming the voltage drop for B+ under load is nothing close to 100V. LM317 resistor values are textbook for a 6.3v output. The 470uF capacitor shouldn't be dropping voltage since it is in parallel, correct? I can just replace R2 with a trimmer to compensate but am curious for more educated minds to help me figure out what's going on. Supply voltage for HI AC is 200 as written on transformer, but closer to 211 measured.

Power Supply.jpeg
 

Les Jones

Well-Known Member
Most Helpful Member
If you are feeding 211 volts AC into the bridge rectifier then the DC output with no load wil be close to 211 x root 2 = 211 x 1.414 = 298 volts. This is because the capacitors will charge up to close to the peak value of the sine wave. The source resistance at the output will be about 6 K ohms. Are you sure that it has not been designed for the mains supply voltage of about 120 volts in thr USA ? 120 volts AC into the bridge rectifier would give about 120 x 1.414 = 169 volts. With the resistor values of with the resistor values shown You should get an output of 1.25((1500+390)/390) = 1.25 x 1890/390 = 6.06 volts You need to take into account the tolerance of the resistor values and the tolerance of the 1.25 volt reference voltage of the LM317. Also check that you have at least 9 volts DC at the input to the LM317 as it needs at least the input to output differential to work correctly.

Les.
 
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captainate

Member
I'm not particularly sure of anything, but many other people have built this project and everyone's using a 200v transformer. I have asked the same question on the other forum with no reply so far. I'll just put a trimmer at R2 and call it good for the DC supply. But without changing the power transformer, is there anything I can do to get the B+ output down to 170?
 

alec_t

Well-Known Member
Most Helpful Member
Minimum rated load current for the LM317 can be as much as 10mA. The bias resistors provide a load of only ~3mA, so that might be affecting the output voltage.
I'm assuming the voltage drop for B+ under load is nothing close to 100V.
What is the expected load current for the 170V B+ supply? With the 6k resistor chain it would take only ~17mA to drop 100V.
 

AnalogKid

Well-Known Member
Most Helpful Member
Check the LM317 datasheet. It recommends a value for R1. Start with that and recalculate your value for R2.

Also, you are very close to an overvoltage on the 10000 uF bulk filter cap. For better long-term reliability, capacitors should be rated for at least twice the peak voltage they will see in a circuit

What is a DI?

ak
 

audioguru

Well-Known Member
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The 8VAC might be 9VAC with no load. Its peak is 12.7V and the rectifier bridge drops it to about 11.5V with no load. Then the 10000uF capacitor will blow up since its voltage rating is too low at 10V. Use a capacitor rated at 16V or 25V instead.

The datasheet for the LM317 shows resistor values (R1 is 240 ohms) for the more expensive LM117. R1 should be 120 ohms (1.25V/10mA= 125 ohms) for a cheaper LM317 so that the output voltage does not rise when there is no load. Then R2 should be 485 ohms for an output of 6.3V. Use a 1k trimpot set close to halfway.
 

spec

Well-Known Member
Most Helpful Member
Hy Captainate,

Where are you from: care to put it next to 'Location' in your user page so that it displays in the window on the left of your posts.

In the circuit below, make R1, 120 Ohms, R2, 470 Ohms and fit C1, a 100nF (0.1 uF) disk ceramic capacitor (not surface mount) as shown. Make sure that the layout of the three components is compact and that the leads on the components are as short as possible. This will reduce the chance of the LM317 oscillating. If you always make R1 120 Ohms when using an LM317 or LM337 you will ensure that the minimum current of 10ma is always drawn from the LM317/337 (as Alec says in post #4).

(crossed posts AK and AG:))

spec
2016_08_17_!ss1_ETO_LM317_APPLICATION_CCT.png
 
Last edited:

captainate

Member
Check the LM317 datasheet. It recommends a value for R1. Start with that and recalculate your value for R2.

Also, you are very close to an overvoltage on the 10000 uF bulk filter cap. For better long-term reliability, capacitors should be rated for at least twice the peak voltage they will see in a circuit

What is a DI?

ak
I did suspect this cap was underrated and got a 16v. I'll get a 25v for the next build (I'm making three). A "DI" (direct input) is an audio device that accepts instrument inputs and converts their signal into the correct signal for a mixing console.
 

captainate

Member
Hy Captainate,

Where are you from: care to put it next to 'Location' in your user page so that it displays in the window on the left of your posts.

In the circuit below, make R1, 120 Ohms, R2, 470 Ohms and fit C1, a 100nF (0.1 uF) disk ceramic capacitor (not surface mount) as shown. Make sure that the layout of the three components is compact and that the leads on the components are as short as possible. This will reduce the chance of the LM317 oscillating. If you always make R1 120 Ohms when using an LM317 or LM337 you will ensure that the minimum current of 10ma is always drawn from the LM317/337 (as Alec says in post #4).
Thanks for the help! (Scrooge too!) I'm from Eugene OR, living in Los Angeles now.
 
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