7805 voltage regulators produces 60Hz pulses on output.

[Tom Swift]

Hello.  I need help... and I have a question about this issue.   

I included the schematic to the circuit that produces 5VDC from a 12VAC source (disregard the notes on it).  I'm using a variac to provide the VAC.  I have a feeling that might be my mistake.

You can see there is 60Hz spiking in the 5VDC output of the voltage regulators.  You'd think the regulators would squash that.  The variac supply and ground is on the same power line as the oscilloscope.  I was thinking ground loop?  Inductance?  I don't know.  I replaced every cap on the board and am still getting this odd spiking in the supply.  For what it's worth, I feed 5VDC to the board (at the regulator outputs) and it's fine.  Any feedback would be great.

Thanks!

[Andy Watson]

How exactly have you connected the variac? Does it have an isolation transformer? (They usually don't).

[Shonky]

You should see 120Hz if the transformer centre tap is working and both rectifier diodes are working.

Put the scope on the 15000uF cap before regulator as well.

I doubt the variac connection has any effect however if it's set too low you may have some issues and not have a high enough input to the 7805s.

Possibly the diode across the 7805 is breaking down. If reversed, the output would just follow the input minus a diode drop.

Edit: err if you are generating the 12VAC directly from the variac and not connecting the centre tap then yes it will only half wave rectify. Will do some wierd stuff if connected with no ground.

[cruff]

Are you actually getting the voltage you need out of the transformer and diodes? If it is below the minimum regulation voltage for your 7805 variants then you'd see pulses.

[Andy Watson]

But they don't have center taps either,

Exactly

which makes me think there's a stepdown transformer after the variac.

The OP didn't mention a step-down transformer. If there is no center-tapped transformer that would explain the 60Hz signal (rather than the expected 120Hz). It would also potentially create some funky ground-loop currents!

[tunk]

Don't know if it will make any difference - do you have a load on it?

[Tom Swift]

Thanks all for the replies.

I'm using a 10A Tenma variac with wall socket output with ground.  I attached a picture of it.

I connect the hot and neutral lines to pins 1 & 3 on the board and ground to pin 2.

I'm using the board as the load.

I'm getting the feeling I should just use a 12VAC wall wart at this point.  Variac seemed like a good idea at the time.

Oh, I should also mention that the oscope reading is on the Vcc and GND of a chip on the board.

[TimFox]

The rectifier circuit you show requires a center-tapped low-voltage transformer for a source.
The ground wire is not such a center tap.

[Shonky]

Ground you have used is a protective earthand shouldn't carry current normally.

A 12VAC plug pack will have the same problems if connected to the two AC inputs.

You could connect the 12VAC across one AC input and *ground* ie not the other AC input. It will only have wave rectify but should work.

Is there a reason to not use the proper power supply?

[floobydust]

OP you're not going from variac directly into the power supply where the 12VAC transformer output would normally be... right?
You have to have isolation from mains especially with a chinese variac or bad things can happen. It's not safe that way.

[aeg]

Doesn't the PET 8296 already have a power transformer inside?

[Tom Swift]

Thank you all so much for the replies.

TimFox was right and I should of known better.

I got a 120VAC to 12VAC 3A center-tapped transformer.  The output of the 7805's are MUCH smoother however not perfect.  It's still a little wobbly at 120Hz and am wondering if I just need a larger filter cap than the one suggested on the schematic.  Or larger ones on the input or output of the regulators?  I forget which ones would smooth it out more.

aeg, you nailed it!  Unfortunately, I do not have the case or the transformer inside.

[floobydust]

What is the load on the PSU when you are getting "wobble"?
I would make sure there is some minimum load on the voltage regs, the circuit must have single point grounding i.e. C1 GND goes right to the center tap, and the rest of the circuit takes GND from C1(-), or else the 7805 will amplify ripple. You can also have a PE ground loop between the scope and whatever this PSU connects to. Line trigger the scope to better see what is going on.

[Tom Swift]

heh... I should of said 'ripple' rather than 'wobble'.  The power supply in the schematic above is physically part of the full circuit board, (to cut costs?), to offer the load.

It is at a solid peak at 5VDC compared to what it originally had where it spiked.  However, it dips slightly at the end with peak to peak ripple of 200mV at 120Hz.  Am I being picky or is that acceptable for this supply?  I guessing not.  I would love it to be a rock solid 5VDC.

Again, thanks all.  I appreciate your time.

[floobydust]

I would say it's a problem to see 200mV ripple on the output of a 7805.
Many things can cause this. You aren't telling us enough to troubleshoot for you. Scope more things

Commodore PET 8296, does it have one or two 7805's in parallel?

I would first suspect the PSU is simply overloaded perhaps from more DRAM etc.
Filter cap C1 might be dried out and low value.
7805's can struggle if their heatsink compound is old and the IC is too hot.
Hopefully the IC's were not damaged by running this with no center tap.

[Tom Swift]

It seems to be running just fine.  I am just unhappy with the ripple on the line.  I'll investigate more.

Thanks!

[Andy Watson]

....  I'll investigate more.

Without changing anything else, put your oscilloscope probe-tip  on the ground-pin of the 7805. I suspect you will find that there are significant voltages being imposed on the ground wiring due to ground-loops and the charging of the capacitor.

[DavidAlfa]

Check the input, the cap voltage shouldn't drop below the regulator output + ~1.5V, so for 5V it should stay above 6.5V.
If this is the case, then probably the cap is bad or bad connection is made (The said center tap).
You shouldn't be seeing this kind of ac waveform by any means.

[Tom Swift]

Ah, ha!  Good call, David.  The input voltage ripples from 7VDC to 6.3VDC so I think we found the 200mV drop.

[DavidAlfa]

I got a 120VAC to 12VAC 3A center-tapped transformer.

12V 3A should give way more than 7V! The 7805 can't provide 3A, so it's very unlikely the transformer is being overloaded.
Are you connecting the center tap to gnd?


You should see 100/120Hz ripple, not 50/60, otherwise one of the diodes might be KO.

Edit: Oh! Is it 12+12VAC? Or 12VAC between ends?
This would explain the issue, as it could be 6+6VAC.
Measure VAC between ends of the tranformer, and also between each end and the center tap.

I'm getting the feeling I should just use a 12VAC wall wart at this point.  Variac seemed like a good idea at the time.

Yeah, any cheap 9VDC 3A adapter and it'll be fine, or just dump everything and use a 5V 3A adapter directly!

[m k]

12VAC center tapped transformer should have that 6+6 text somewhere.
Marked voltage is also nominal, close to what is a loaded value.
Unloaded value is what the formula says.

Your 6.3V means 7V minus rectifier diode, 7V is a coincidence.
What is unloaded output of the regulator?

Transformers can take some overvoltage without a problem.
Variacs can also many times supply some overvoltage.

Draw your original variac connection.
Line is clear and so is Neutral, but how Ground is connected.

[Shonky]

An AC 12V wall wart will only work if connected to one of the 12V AC lines and Ground (the centre tap connection). And then that may be a bit high. Same for a 9V DC supply. 7.5V DC would be better.

The original centre tap connection must be used as otherwise one of the rectifier diodes will block return current.

[DavidAlfa]

A dc wall adapter will be much cheaper (No bulky tranformer inside).

[Roehrenonkel]

Hi Tom,

......
It's still a little wobbly at 120Hz and am wondering if I just need a larger filter cap than the one suggested on the schematic. 

I'd sugest you try a SMALLER Cap. 15000uF is way too much for 2Amperes (two 7805 right?).
The current-peaks are hudge. Rule-of-thumb is 1mF per 1Ampere.

Good luck

[Tom Swift]

Holy mackerel.  I believe we've been dupped.  Take a look back at the schematic.  Yeah, it says 12V at the inputs but look past the large cap.  It's expecting 9VDC unregulated.  So taking in to account for transformer overvoltage, I'm thinking we need a 120VAC to 18VAC 3A center-tapped transformer.  Idiot!