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7815/7915 PS Problem

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acsww4

New Member
Ladies and gents,

Please let me preface this by stating that I have a novice to average electronics knowledge-base. Now, I am building a Minimoog analog synth from scratch and have slowly but surely hit every roadblock known to man when it comes it troubleshooting (it's probably good for me). So right now, I am working on the MIDI2CV interface with its own dedicated +/- 15v PS and it is based on a standard 78xx/79xx circuit. Now, the issue is that at zero load (yes it has a few LEDs that act as a load) the PS shows a nice +/-15v respective to their individual rails. However with the MIDI2CV unit connected, I get +16v on the positive rail and -2.4v on the negative rail. I am using a 36vct 0.7A transformer with the dual secondaries wired in series. That gives me +22.5v and -22.5v pre-filter cap. Mind you, the MIDI2CV board has never been powered and everything was brand new before powering up the board. Also, the orientation of all the components is correct as I triple checked. WHAT DO I NEED TO DO TO MAKE THIS WORK?!!??!!?!? All kidding aside, please help.

The MIDI2CV interface is shown here: https://midisizer.com/midi2cv-mk2/

With gratitude,

Adam
 

Musicmanager

Active Member
Hi ACS .. welcome to ETO

Do you have a schematic for the power supply, or at least a sketch of the circuit ?

S
 

Musicmanager

Active Member
OK .. ... before we go any further, I am also a novice so someone else may well chip in and help you better, however .. ..

2 questions .. ... LM7915 pin arrangement is different to LM7815 .. ..are you connected correctly ?

.. .. ..your transformer .. 36v - 0 - 36v correct ? in series ?

S
 

acsww4

New Member
The author of the layout states that the pinout is correct for any 79xx series regulator as they're interchangeable. As far as my transformer, I have wired the dual secondaries in series as I didn't need current as much as the voltage. So from peak to peak my multimeter reads about 45V. Peak to center on either end is about +/- 22.5V. My current guess is that there is a bad common somewhere in the circuit. I will have pictures up in about 3 or 4 hours when I get home. Thank you for the input as well!
 

Musicmanager

Active Member
The author of the layout states that the pinout is correct for any 79xx series regulator as they're interchangeable
Yes, that's true, but 7815 gnd is centre pin; 7915 Gnd is LH pin , looking at the front of the chip;

Outside of that ? There's never a genius around when you need one :D

S
 

AnalogKid

Well-Known Member
Most Helpful Member
95% of the time, problems with 3-terminal regulators come down to pinout. Not that that includes the tab if these are TO-220 power transistor packages. The two tabs are at different voltage potentials, and cannot be mounted on the same heatsink without using an insulation kit.

Separate from that, this doesn't quite sound like a bad ground connection yet. Check the voltages at the two regulator device output pins when the external load is applied. Also, a photo of your assembly would help.

ak
 

JonSea

Well-Known Member
Most Helpful Member
Are you using the circuit board shown in your link? If so, the pinouts should be ok.

I'm not certain I understand the transformer arrangement. Is there one center tapped winding? Two windings you've connected to form a center tap, or 4 windings?

I suspect the phase of one of the windings is reversed.
 

atferrari

Well-Known Member
Could you desolder one of them and test the other with load? Then, do the same with the other regulator.
 

dr pepper

Well-Known Member
Most Helpful Member
The circuit you showed us looks perfectly acceptable and ought to work.
So its stands to reason you have something connected wrong, omitted or shorted.
My favorite would be what music manager says you have the - reg wired up wrong.
Download the datasheet for the 7915 and compare the pinout to your circuit.
Of course as mentioned if your using a pcb then the above is unlikely.
Does the voltage on either of the smoothing caps change when you connect the load?
You did connect the trans centre tap to ground?
 

acsww4

New Member
So it turns out that my power supply is not the culprit. On the board I am supplying power to, there is a voltage reference chip (LT1021) that had shorted the DNC pins to the ground pin. That is only part of the issue. After removing that chip, there is still a short to ground on the poitive rail. My negative rail is reading a beautiful 15.000v. The positive rail is still reading about 3v. I don't even know where to start...
 

acsww4

New Member
Oh, this is after rebuilding the +/-15v PS. I have verified that the same symptoms appear with a bench power supply as well.
 

dr pepper

Well-Known Member
Most Helpful Member
Sounds like you've blown a reg.
N.C. pins are not necessarily not connected in the chip, manufacts sometimes use them in manufacturing, grounding them isnt good unless you know its ok to do so.
 

acsww4

New Member
I will check my regs today when I get home. I believe there are three on the board. There are two 317's and a 7805 on the board, so I will check all three. Thank you for the advice!
 

dr pepper

Well-Known Member
Most Helpful Member
I've just used a lm340 5.0 and a Lm117 3.3 on a project, the Lm340 is a salvaged item, both work fine.
If your using stripboard then the decoupling caps are vital.
 

acsww4

New Member
All these components are spanking new on my board. It is a MIDI to control voltage converter for a synth I am building. The components are correct as is the wiring of said components. So my guess is that I had a factory dud and it shorts to ground off the bat. Have you ever seen a voltage regulator bad from the factory?
 

dr pepper

Well-Known Member
Most Helpful Member
Yes i have, as on another thread I had issues today with suspected counterfeit parts, they work but not as expected, and not as a known good item.

When you said you had them squashed down to the board, are you saying that was the cause of the problem?, like there was an unintended contact /short, you can do that sometimes with a 78 reg, because the tag is ground, one of my very early projects I grounded the tag of a 79 reg, not knowing the tag was the o/p, oops, is this what happened to you, no shame in that.
You got it working yet then?, your solder job looks perfectly acceptable from the comp side at least.
 

acsww4

New Member
They're small to-92s. It was my first time using those and I really seem to prioritize aesthetic over functionality without even knowing it sometimes. So yes, they were about flush with the board LOL. That is what caused the short. After removing one regulator and checking the positive rail voltage at nearly 5v from about 3, a little neuron fired in my brain and wham, all three regs were shorting...
 
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