m00dawg

Multi-Tap Transformers Versus Voltage Regulators?

119 posts in this topic

Nothin wrong with stating it and saying you don't know of it's consequences... At least we know a little more than before, even if there are some blanks... Maybe someone can elaborate (fill in the blanks)?

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well when using a center tapped transformer to get two different voltages, you can't really avoid having more load on the "lower half" of the transformer compared to the upper half, that's part of the concept. I guess as long as the transformer is strong enough to still have some "headroom", that shouldn't be a problem.

When playing with values on that simulator, I noticed that when taking a lot of power from only one of the rails while the capacity is rather low, some flow back into the center tap could happen, which in turn might be bad for the capacitors. I'm not really sure how to configure the transformer part on that simulator to get realistic results (e.g. I found no way to set the transformer to other relations than 1:1). So if it works well in reality, probably everything is fine.

If you wanna be on the safe side, in my opinion the solution is either using really a lot of capacity (let's say two or three 2200uFs per rail) or maybe after all changing the concept and using two bridge rectifiers, which wouldn't be a bad idea either.

S

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Would a diode make sense after the center-tap instead? I was playing with multiple rectifiers in the simulator yesterday and it ended up looking like it was doing crazy things, particularly when using a common ground. Of course I could be doing it wrong :)

I'm going to finish making the DIN cable today and, if it is reasonably easy to do, can try powering my existing MB-SID (dubbed ShoeSID as it's in a shoebox :P) with it. Otherwise I may just wait until I'm far enough along on my MB-6852 since it would be easier to test that way I think.

Given the results I saw in the simulator, I agree that more caps might be necessary. Are perhaps a few more with higher capacitance. Good thing about building this as a "power brick" (as opposed to inside the synth case) is that there's plenty of room to add caps so I don't see that as a huge issue. 4700uF seems to be plentiful just by browsing my electronics supplier. From what I saw, the +5V rail is the one that might need more capacitance right? I think the 9V should be fairly squared away with a 2200uF or so and the standard filtering caps we've been talking about?

I suppose I could bit the bullet and opt for doing this with a single tap and regulation. I did some Googling of C64 PSU schematics and one of them just has a 7805 to drop it down. So, if that's indeed true, the C64 Optimized PSU design basically does just that. That said, another design I saw has it using a multi-tap xformer *shrug*

http://www.commodore.ca/manuals/funet/cbm/schematics/computers/c64/c64-powersupply.gif

http://www.devili.iki.fi/Computers/Commodore/C64/Service_Manual/Page_04.html

If I opted for regulating, I guess I just need to find a beefier regulator and heatsink solution. I hate wasting heat though (hence why I wanted to try the center-tap design).

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you guys may want to reconsider what happens to the current paths on the 2 halfwaves

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I wrote for 30 minutes this morning in reply to this thread.

Then I hit "delete" instead.

I can say for sure: The bipolar supply in the MBMixer puts a LOT more load on the + side than the -, and it seems to be working fine.

The analog portions load both sides evenly (or should..??)

then I added the core, a VFD display, a VMusic2, and a USB flash drive to the positive side only.

I think as long as each winding of the transformer is rated above the _total_ current requirements of all attached circuits, it should be fine.

I also think I talk too much, and I should spend a bit more time reserching this at the old alma-mater, STFU.

You all have a great weekend!

LyleHaze

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you guys may want to reconsider what happens to the current paths on the 2 halfwaves

would you mind to consider telling us what exactly you mean?

m00dawg: no need for using a simple transformer - you're on a good track now. 4700uF is surely enough, but maybe two 2200uF ones are better because you could keep the profile low. I also simulated a version with a diode on the center tap but didn't really like the result. What I meant with two rectifiers is: just use two rectifiers, same input of both connected to one end of the transformer, the other end connected to center (for 7805) and other end of transformer (7809). then connect both the negative rectifier outputs together right after rectification to get a common ground. Except for loading the transformer halves differently, this would work exactly the same way as you'd do with two separate transformers or a transformer with multiple secondaries.

S

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How is that different from what I had first though? Can you perhaps provide a screenshot of the circuit app? I think I may be missing something here, but when I hooked it up both sides were getting the same voltage (instead of one getting say 14V and the other 7V).

I was thinking that, instead of using a full wave rectifier, I could use two half waves (so basically 3 diodes each top - two point forward and one points back. I'd need more capacitance, but, at least in the simulator, this seems to work.

As for the caps, a 2200uF/25V cap is 13mm in diameter and 26mm high. A 4700uF/40V is 18mm in diameter and 35mm high. Not a huge jump I wouldn't think, but that would save space on the board. There's plenty of space but unless people want to bulk order the printed boards, I'd have to go with someone like BatchPCB, which chargers $2.50 per sq. in. So that can add up :)

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How is that different from what I had first though? Can you perhaps provide a screenshot of the circuit app? I think I may be missing something here, but when I hooked it up both sides were getting the same voltage (instead of one getting say 14V and the other 7V).

uhm yes sorry, just forget what I said ;) I think the solution that you already built, with some added capacitance, is still not a bad idea :)

S

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would you mind to consider telling us what exactly you mean?

...

i meant if you think the load is not balanced, you should map out the paths the current takes for the 2 halfwaves (when x1-1 potential is higher than x1-3 and when x1-3 is higher than x1-1) and see for yourself.

i thought it was a rather clever design when i realized this, if only because i would not have thought of it...

going with 2 bridges would almost require a dual secondary, but these are more common (in equal v) nowdayz than a real center tap, maybe because centertaps prefer a balanced load and most designs use more on the + side.  btw, for those that don't already know, the commodore c64 tx is a dual 9v ...

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If I understood you correctly, the thing I noticed in the sim was that you could starve one of the sides out. If I placed an simulated LED without a resistor, say, on the 9V side, eventually the 5V side wouldn't get any power. Doing that, though, meant that the 9V was drawing amps worth of current according to the sim.

That said, it's basically assured that the +5V is going to draw more current than the +9V (at least with a full control surface, etc.). Both should be well under the 3A rating that my xformer has though and shouldn't starve out either half.

This is getting a bit beyond my understanding though, to be fair.

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So I'm kinda going back and forth between having my power brick generate both 9VDC and 5VDC and having it generate 9VAC and 5VDC. If I did the latter, it would be, theoretically a drop-in replacement for the C64 PSU. I didn't want to do this initially because my thought was that I could do more filtering in the PSU itself and also manage heat better this why.

While playing around with this in the simulator, though, it got me to thinking. Why not use two rectifiers? Specifically, what if I only use the negative from one of the rectifiers (say, the one hooked up to the center-tap) and used that as my GND for everything after the rectifiers? I tried that in the simulator and everything seemed to work out fairly well. And if I built a C64-compatible PSU, I still need to provide a common GND pin right?

Does this mess up anything with ground potentials and the stuff we were talking about earlier in this thread?

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Ok so I'm still mulling over whether or not to do a C64-compatible implementation (I'm leaning on doing it the way we've been talking - the only thing that makes me hesitate is if I opt to go with a standard Pac-Tec case and backplate for my MB-6582), but thought I would share my current multi-tap design after we've talked about all this. I know. I went way overboard on the caps :) Not so much in number, but in the capacitance of the electrolytics. I figured more can't hurt, particularly since we've been talking about increasing the capacitance on the +5. So I basically tried to find common caps that fit on a 2 sq. in. board, while still having 2 electrolytics before the regulator. The caps are getting to be a bit tall at this point, so this solution might not make a whole lot of sense in a low profile design (though they aren't THAT tall).

Feel free to use, comment on, suggest flame, etc. the design if you like :) Seems like, at this point, the design is more a result of our collective communal intelligence than just my own :)

4286_sch_png3e87fcea16fc9d196178418d675f

4288_brd_png8999fcb6f049b7d3bd8054eea0cb

4286_sch_png3e87fcea16fc9d196178418d675f

4288_brd_png8999fcb6f049b7d3bd8054eea0cb

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If you didn't intend some unexpected connector pinout, I'll have to annoy you again ;) I.e. if the input connector should reflect the "natural" order of transformer outputs, you'll need to change the schematic in order to have the outer both pins connected to the rectifier while the center tap is used as + for the 7805.

other than that, the cirquit looks good. I'd probably keep it like that. Cloning the C64 PSU would mean you need a different transformer (2x9V without! center tap), plus you wouldn't be able to keep all the heat outside the midibox. The original C64 plus "PSU_optimized" is not the only way to happiness, it's just a clever way of making use of the C64 PSU which many people get for free while buying used C64s.

Re the layout - looks good :). I'd maybe change the Isolate value of the gnd plane to get more clearance around the pads (lower probability for manufacturing errors) and try to get the thermals a bit wider (the little traces connecting gnd pads to the plane). It's always better to have a good connection to gnd to achieve a silent supply. If I remember correctly, this is done in Eagle by changing the "trace" width of the gnd polygon.

S

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Oooh heat is a very good point. With no regulators in the synth, the only thing that right might eat up heat is going to be the SIDs it would seem. Well, that basically sells it for me! :)

As for the pin-outs, I did that to make the board layout simpler since the top pins both go to the rectifier. I'm using Molex connectors there and was likely going to label the bottom pin CT or something along those lines to make it clear that it's not hooked up how one might initially think it would be.

On the note of the Molex connectors, I picked those because they can push more than 1A. Seems like the standard SIL headers can't quite push that much from what I found (or, more specifically, the wires that fit into the crimp terminals for the female connectors).

As for the GND, I didn't even know you could change the parameters of the GND plane! I played around with that and have attached an update. I was getting some stray airwires so I manually ran the GND traces and then applied the plane after that. Seems to have done the trick.

As always, thanks for the input!

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So not much is new but I thought I'd let everyone know that I got the necessary parts in the mail! I haven't sent the board to BatchPCB yet (I'm trying to get a few boards ordered at the same time so save a bit on the processing fees) but I plan to soon. The 6800uF caps were a bit bigger than I thought they would be, but still small enough to fit in a smaller enclosure (2U should be fine, I think).

I can post up the full parts list if anyone is curious, although all my parts were ordered from Allied Electronics (I don't have the part #'s for Mouser). I'll likely bust out a Wiki page after all is said and done too (assuming all this works :P)

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Hmm...well I finished soldering all the parts to my MB-6582 mainboard and started stuffing chips. Seems like my design my have a bit of work still. I'm still using a protoboard as the printed board isn't in from BatchPCB yet. Trouble is, with only the 1 PIC, Banksticks, and the LCD, I pull around 4.8V. If I add a SID to the mix, the voltage drops to 4.3V or so which is not enough to make the PIC happy.

I was originally still using 2200uF and 47uF caps so I swapped out the 2200uF for the bigass 6800uF. This seemed to help as I was able to run the testtone app with the PIC, Banksticks, and SID stuffed (no LCD). Sounds like there maybe some AC stuff leaking through as well since you can definitely hear a hum from the output of the SID. Using the LCD still draws too much juice it seems as, with it, the PIC never boots up fully (it just spews CCs like it's stuck in a start cycle).

Anyone have any thoughts to what's going on? I would have thought the voltage regulator would have cut out if the voltage dropped too low?

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I would have thought the voltage regulator would have cut out if the voltage dropped too low?

it should definitely have dropped out at 4.3V. Something is weird....

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Agreed. I've traced the board a few times now and can't find out why it's exhibiting this behavior. I'm going to check voltages at each point probably today. I don't want to do that for too long though since I don't want to damage my PIC or especially my SIDs. Trying to see what I have around the house that can generate a fair amount of load on the +5V rail.

Otherwise, I may just make a simple board that just regulates 9VAC down to 5V and 9V using separate regulators (avoiding the center-tap). When my boards come in from BatchPCB, I'll give that a go to see if it's a design flaw or just bad soldering on my part.

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Got a spare Vreg?

The load from the SIDs and PIC is so low, you can test it with those pulled, and it won't make much difference.

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Sure do! I was a bit confused at your last sentence, but I assume you're suggesting I should go ahead and swap it out? That's not a bad idea, actually. Dunno why I didn't think of doing that before swapping out the caps.

You think I should pull the chips though? Without the chips installed, I get predictable voltages (with the LEDs from the power board itself consuming a bit of power). I was thinking of using a spare 40 or 80mm PC fan to see if that draws enough load to give me the same results as when I have the chips stuffed.

Might do that this afternoon while the super bowl is on. You know, all that junk between the commercials :)

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hahah, I'm boned - I have a system that can play the (HD) channel that superbowl is on, that has no sound; and one that plays all the other (SD) channels, with surround sound. Can't win! If you know of a place to stream it please let me know!

Anyway ontopic.. Yeh try a fresh vreg. The second sentence I meant, yeh pull the chips and see how it goes. The load from a SId and a PIc, even fully used with sounds playing and all, is very low indeed. A couple of LEDs would probably match it ;) If it runs fine without the SID chips and then dies with the SIDs stuffed.... You might have a pair of dead sids there mate. Are they wilba-sourced? Did you handle them with proper antistatic precautions?

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Online? Nah, the NFL is particularly bad about those sorts of things. Might try ESPN *shrug*

As for the topic at hand, I don't think the SIDs are bad (they are from Wilba's orders). They were used in my previous MB-SID creation without issue. The testtone application worked too (on the new synth) so they at least somewhat work. I'll try the regulator swap first and see where I get. If that doesn't work, I'll drop the SIDs back into the old one and see if they are kaput.

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Well, I didn't have a chance to swap out the regulator yet - we're going to a friend's house to watch the SuperBowl so I won't be able to do any soldering. I did try a few things though and think that the problem might be on the MB-6582 mainboard.

I basically hooked up a fan to the 9V and 5V right from the power board (instead of through the MB-6582). With this load in place, I was getting 5.04V and 9.02V from each of the rails. Hooking the mainboard up, and adding the fan to the 5V rail, I was getting 4.5V. So there's .5V that seems to be going somewhere magical :) I'm thinking resistance is playing a role, but am not quite sure how.

I did notice some janky solder joints on the power board, so I'm going to fix these up when I replace the regulator and see where that gets me. Bugs me that the fan hooked up directly to my powerboard didn't drop the voltage at all. I'll have to re-run my tests, but I thought that I was getting 4.8V with nothing stuff on the MB-6852 board.

*scratches head*

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You're on the right track. Enjoy the game!

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Well, I replaced the 7805, reflowed a few questionable solder joints on the power board and tried the test again. I found out a little more but the end result is similar - the more load being consumed, the lower the voltage output.

To do this test, I started with some LEDs and various fans to put load on both the 5V and 9V supplies. One small 40mm fan was connected to the 5V rail with the LED. I the connected my 120mm monster to the 9V. Measing the 5V rail gave me 4.8V. I then added the LCD and the voltage drop went down to 4.5V.

This was all without any chips stuff this time, and I tried this both using the MB-6582 board and just tapping off the power board (although I wasn't able to hook up quite as many devices - the voltage drop was noted here too). Oddly enough, the 9V supply shows a drop too, but it's much smaller - only .01V with load. I tried it a few times though and it dropped every-time which makes me think it's more than just % error on my multi-meter.

So, long story short, I think the power board is still the culprit, although I don't know why. My feeling is that it has something to do with the shared ground, but that's sort of just a guess. I have enough spare parts that I may try building a simpler board as I mentioned previously to see if it might be the stuff I'm doing with the center-tap. I'll also try it again once my printed board arrives.

In the interim, though, any thoughts on this somewhat peculiar issue?

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