7805 voltage regulators produces 60Hz pulses on output. - Page 1
Forum section: electronics / repair

author: Tom Swift
author_rank: Contributor
timestamp: August 15, 2023, 08:36:58 pm
content: 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!

author: Andy Watson
author_rank: Super Contributor
timestamp: August 15, 2023, 09:09:03 pm
content: How exactly have you connected the variac? Does it have an isolation transformer? (They usually don't).

author: Shonky
author_rank: Frequent Contributor
timestamp: August 15, 2023, 09:26:36 pm
content: 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.

author: cruff
author_rank: Regular Contributor
timestamp: August 15, 2023, 09:38:12 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 15, 2023, 09:40:20 pm
content: Quote from: edavid on August 15, 2023, 09:25:54 pmExactlyQuoteThe 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!

author: tunk
author_rank: Super Contributor
timestamp: August 15, 2023, 10:09:53 pm
content: Don't know if it will make any difference - do you have a load on it?

author: Tom Swift
author_rank: Contributor
timestamp: August 15, 2023, 10:26:46 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: August 15, 2023, 10:55:57 pm
content: The rectifier circuit you show requires a center-tapped low-voltage transformer for a source.The ground wire is not such a center tap.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 16, 2023, 12:01:51 am
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: August 16, 2023, 01:55:40 am
content: 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.

author: aeg
author_rank: Regular Contributor
timestamp: August 17, 2023, 12:12:10 am
content: Doesn't the PET 8296 already have a power transformer inside?

author: Tom Swift
author_rank: Contributor
timestamp: August 17, 2023, 10:16:05 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2023, 10:27:23 pm
content: 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.

author: Tom Swift
author_rank: Contributor
timestamp: August 17, 2023, 10:51:58 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2023, 11:40:26 pm
content: 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 thingsCommodore 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.

author: Tom Swift
author_rank: Contributor
timestamp: August 17, 2023, 11:57:44 pm
content: It seems to be running just fine. I am just unhappy with the ripple on the line. I'll investigate more.Thanks!

author: Andy Watson
author_rank: Super Contributor
timestamp: August 18, 2023, 01:28:56 am
content: Quote from: Tom Swift on August 17, 2023, 11:57:44 pmWithout 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.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 18, 2023, 03:32:02 am
content: 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.

author: Tom Swift
author_rank: Contributor
timestamp: August 18, 2023, 04:11:32 am
content: Ah, ha! Good call, David. The input voltage ripples from 7VDC to 6.3VDC so I think we found the 200mV drop.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 18, 2023, 09:09:23 am
content: Quote from: Tom Swift on August 17, 2023, 10:16:05 pm12V 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.Quote from: Tom Swift on August 15, 2023, 10:26:46 pmYeah, any cheap 9VDC 3A adapter and it'll be fine, or just dump everything and use a 5V 3A adapter directly!

author: m k
author_rank: Super Contributor
timestamp: August 18, 2023, 09:23:34 am
content: 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.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 18, 2023, 09:30:02 am
content: 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.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 18, 2023, 10:43:50 am
content: A dc wall adapter will be much cheaper (No bulky tranformer inside).

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 18, 2023, 11:28:03 am
content: Hi Tom,Quote from: Tom Swift on August 17, 2023, 10:16:05 pmI'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

author: Tom Swift
author_rank: Contributor
timestamp: August 18, 2023, 10:10:32 pm
content: 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!

author: Shonky
author_rank: Frequent Contributor
timestamp: August 19, 2023, 12:55:00 am
content: Quote from: Tom Swift on August 18, 2023, 10:10:32 pmErr No? 12VAC centre tapped (i.e. 6V + 6V) will produce around 9V DC unregulated when rectifited. 6V RMS is about 8.5V peak.

author: macboy
author_rank: Super Contributor
timestamp: August 19, 2023, 02:55:54 pm
content: Quote from: Shonky on August 19, 2023, 12:55:00 amI agree. A 12VAC CT transformer sounds right. Too much power dissipation with a 18 VAC one.

author: Tom Swift
author_rank: Contributor
timestamp: August 19, 2023, 09:33:39 pm
content: Far from me to question your feedback. However, I probe the expected 9VDC unregulated pin and it reads 6.4VDC on the meter. I probe the output of one side of the transformer and it reads 6.8VAC. I don't see how you're going to get ~9VDC out of 6.4VDC rectified out of 6.8VAC besides sacrificing a few chickens.UPDATE: I put one side of the transformer output and it's EDIT: Vmax is 9.6V. It's RMS being ~6.8VAC. Around 6.8VDC with a small amount of ripple enters the input of each of the 7805 voltage regulators. Barely enough to drive it. Am I losing power before then? Everything on power supply is new and I tested the large cap before installing it.Someone asked so I attached how I wired it up.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 19, 2023, 10:37:22 pm
content: Something is wrong if you have 9.6V peak before the rectifier diode and only 6.8 "VDC" on the filter cap. You're saying theres ~3Vpp of ripple? 6.8VAC rms does not equal 6.8VDC filtered.Got a scope trace of before and after a rectifier diode?How did you test the filter cap? If it's stuffed you will get excessive ripple and problems like this.

author: Tom Swift
author_rank: Contributor
timestamp: August 19, 2023, 10:44:58 pm
content: Hey, Shonky. Thanks for the reply. I checked it using a DEREE LCR Meter DE-5000.From the transformer, it's 9.6Vpp (6.8VACrms). After the rectifier diode and the filter cap it's down to 6.8VDC with a small amount of ripple. This enters the voltage regulator. All measurements were on the scope using center tap as ground.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 19, 2023, 10:51:04 pm
content: So you're saying there's a 2.8V drop on the rectifier then?Also peak is not the same as peak to peak. 9.6Vpp is not 6.8Vrms.Scope trace?

author: macboy
author_rank: Super Contributor
timestamp: August 19, 2023, 10:58:55 pm
content: Those numbers literally do not add up. If you have 9.6v peak (not peak to peak) on the AC side, you should have about 9.0 VDC (with some ripple) at the DC side. If not, perhaps the rectifier diodes are faulty. Little else could explain the too high drop across them. Measurement error is also possible but it seems you are doing ok. You have lost 3 volts somewhere and need to find it.

author: Tom Swift
author_rank: Contributor
timestamp: August 19, 2023, 11:04:38 pm
content: My bad. I hit Vmax and not Vpp. Checking again, it's 19Vpp and 10Vmax. I'm double checking the circuit now.UPDATE: I double checked everything and even replaced the two rectifying diodes and and the large filter cap and I get the same thing.

author: floobydust
author_rank: Super Contributor
timestamp: August 20, 2023, 02:32:09 am
content: Do you really have a 12V 3A transformer? Because this is far above wallwart size and at 36VA it would not sag like your circuit is supposedly doing at 1-2A load.The transformer secondary waveform is a badly distorted sine-wave, flat topped so a true-RMS multimeter must be used but even then it won't tell you anything useful.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 20, 2023, 02:34:25 am
content: What do the waveforms look like?

author: floobydust
author_rank: Super Contributor
timestamp: August 20, 2023, 02:59:26 am
content: Quote from: Shonky on August 20, 2023, 02:34:25 amAll waveforms you'd find are textbook ideal transformer ones and perhaps OP can use his scope to looksee, as others have asked him to do.The trick is the rectifier diode's conduction angle is very narrow maybe 5-10° and very high current and this is what makes it a very difficult circuit in reality.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 20, 2023, 03:01:43 am
content: Quote from: floobydust on August 20, 2023, 02:59:26 amThey should be textbook, but we've never seen them despite asking.

author: macboy
author_rank: Super Contributor
timestamp: August 20, 2023, 04:34:59 pm
content: Quote from: fmashockie on August 20, 2023, 04:26:28 pmTrue.QuoteNonsense. Care to explain how or why?

author: fmashockie
author_rank: Frequent Contributor
timestamp: August 20, 2023, 04:41:08 pm
content: Sorry I deleted my comment after realizing that is incorrect. If it is 12V center tapped, RMS should be 8.5V. Which should be enough for that 7805 to regulate 5V just fine.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2023, 04:49:52 pm
content: Quote from: fmashockie on August 20, 2023, 04:41:08 pmWhen driving a non-LDO regulator such as the 7805, thepeakvoltage from rectifying 6 V rms (from a 12 V rms CT secondary,not8.5 V rms) is 8.5 V, which should suffice for the dropout voltage of the 7805, but you need to calculate the minimum voltage of that rectified waveform (which depends on the diode drop, load current, and filter capacitance) to make sure the voltage applied to the 7805 does not drop below the dropout (roughly 7 V for a 7805) on the minima.(Guess if I learned this the easy way or the hard way?)

author: fmashockie
author_rank: Frequent Contributor
timestamp: August 20, 2023, 04:56:02 pm
content: Thank you for that clarification Tim. Sorry for getting those mixed up.

author: aeg
author_rank: Regular Contributor
timestamp: August 21, 2023, 01:57:26 am
content: I think the nominal loaded voltage of the original transformer might be 16VCT. It looks like it has an open-circuit output of 18VCT, based on pictures from this eBay listing:eBay auction: #354673905073Here are the pictures ripped from the above listing:For context, the winding on the left goes to J8 on the mainboard, the higher voltage winding on the right goes to the monitor:Context photo taken fromhttps://www.nightfallcrew.com/17/01/2013/commodore-cbm-8296/

author: floobydust
author_rank: Super Contributor
timestamp: August 21, 2023, 03:11:44 am
content: In that pic note the Commodore 324748-01 transformer primary has 220V/240V taps.In that pic note the transformer secondary Molex connector is burnt, brown indicating a high resistance connection.I've been through these 5V power supply hassles on an Altair 8800, Cosmac ELF and Sinclair ZX-81, nothing has changed lol.It's attention to details when troubleshooting that matters, not so much a redesign.Either you have too much voltage for the regulators and they run too hot, or not enough and you get reboots/crashes occurring with any mains sag.

author: Tom Swift
author_rank: Contributor
timestamp: August 21, 2023, 06:49:12 am
content: Excellent detective work, aeg. I attached a picture of both the output of the 12V transformer I have and the output of an actual 8296 transformer. The 12V is missing that little 'oompf' that I need. The actual transformer is to about 46Vpp whereas the 12V transformer is about 40Vpp.It's unlikely I'll be able to find a transformer with these specs, (however, knowing on here I'm sure someone will pull up one after reading this or to tell me to make one). I am getting a 18V 9-0-9 transformer soon. I could use the variac to power it down to get the appropriate output voltage, (or power up my 12 but that's probably not a good idea).

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 24, 2023, 07:05:31 pm
content: The minimum unregulated DC for a 7805 type standard regulator is 8.5 V, so a 6 V transformer, whether ni–phase centre–tapped or full–wave bridge rectified is inadequate!You would need a minimum of 12–0–12 V (bi–phase) or 12.5 V (bridge) feeding either 2 x 1N4001 or a single W02 (you can use a single W02 with a centre–tap transformer if you cut the negative lead off), with an electrolytic smoothing capacitor of 2200 μ 25 V – if you use a "bi–phase" centre–tap transformer connect both the negative supply output lead, the negative end of the 2200 μ 25 V electrolytic, and the centre tap of the transformer to the same earth point so there's no common return lead to drop ripple voltage across!This will give about 16 V unregulated before the 7805, which should be enough for the regulator i.c. to stabilise it correctly!Chris Williams

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2023, 07:28:49 pm
content: Yeah but that's a finger burner, the heatsink will roast. Around 8-9VDC input to the regulator is cool running. W02 dies a hot death as well, the ripple current is way over its measily 1.5A/50A surge spec. Note the 1N5402 3A/200A parts used in the original here.The problem people are ignoring is a transformer's rated output voltage is for sinusoidal full load. A capacitive-input filter has very high peak currents that cause simple VAC rating to go out the window and not work as expected. There is a lot of sine-wave distortion when you look with a scope.Important is the size (VA rating) being bigger and older parts have better regulation than modern parts that use minimum copper and steel for low cost.OP hasn't said if he's breaking the piggy bank and using a 3A (36VA) part or something smaller which I suspect he is.Commodore went to a switching power supply in later builds, I would consider that if cost is the concern here.

author: Tom Swift
author_rank: Contributor
timestamp: October 17, 2023, 11:23:29 pm
content: This story has a happy ending. I got a 3A 12-0-12 transformer, winded down the voltage to the actual machine's transformer voltage using the variac. The board powered up perfectly and is rock solid. Thanks everyone for helping me sort this out.