35 replies to this topic

[m00dawg]

Ah I didn't realize you were using 2200uF caps for every rail. Have you thought of sharing those amongst the rails instead and using bigger caps or putting some in series?

[fussylizard]

Not sure I follow on how to share them across rails. Can you elaborate?

Using 2200uF caps before/after the vregs on each rail is clearly overkill. I did it to keep the build simple. My earlier design had various 1000uF 35V caps, 2200uF/16V caps, 2200uF 25V caps, etc. I thought if others used the design it might be confusing, so I traded a little board space to use the largest caps needed on everything for consistency.

I've also thought about tweaking the layout to keep all the diodes in the same orientation to avoid those mistakes as well...

[m00dawg]

The large electrolytic caps are generally to smooth out the AC voltage after it has been rectified. So you can likely just use the larger caps after the rectifier and share it amongst all the regulators. See my above deign for a bit what I'm talking about that.

To put it another way, you likely will want to use smaller caps around each of your regulators (check their spec sheets for the sizes) but probably don't need to have a big electrolytic cap before each regulator.

If you wanted to go above and beyond, you could use larger electrolytics and more of them but share them amongst all the regulators. I'm having trouble describing it without a picture I guess Some designs I have seen used 4x 1000uF capacitors to help smooth out the AC current amongst them. Supposedly that was better than using one big 4000uF cap though I've never put that to the test (I don't have an oscope to get a good reading for that).

Err I hope that helps

[Shuriken]

100uf after should be enough to keep it smooth even on longer lines.

[fussylizard]

Oh, I see on the series thing. I think the reason it is better is that the smaller caps can react faster, so you just use multiple to get the total capacitance you need.

@shuriken - 100uF is enough? I really should put a scope on it.

@all - At this point I think I'm going to go with my current version just to move on. Will let ya'll know how it turns out...Thx for all the input!

[middleman]

Hi fussylizard, in a classic linear design, the large capacitor between the rectifier and the regulator is used as a 'reservoir' to smooth out the rectified AC and to supply the regulator with top up DC, when large load spikes are experienced.

Its also a good idea to put a low value resistor in series with the recified and smoothed DC and the regulator. This makes up a passive R/C low pass filter, removing any residual ripple, making the job of regulation easier for the IC. The resistor is usually chosen to match up with the capacitor so as to make the cuttof less than 50 Hz. I forget the math for this, but it should be out there. The other thing to keep in mind is that components are not perfect, and a large electrolytic capacitor will have some resistance. For this reason, it can help to put a smaller capacitor in parallel with the reservoir capacitor.

[fussylizard]

@middleman - Interesting details on linear PSU design, thanks!

[fussylizard]

Hey all,

Just a quick recap on where things are. I got the board made (BatchPCB) and it tests out fine. I went ahead and built it with a standard 7805 regulator which gets *very* hot (not surprisingly!) but I wanted to test it out. I'm going to order a switch-mode 7805 drop-in replacement to handle that problem. They are about $15 a pop, but have very high switching frequencies so I should be OK even if I drive an AOUT_NG and SSM filter (which is the plan!).

Two pics to share-

Here's the populated board:
 IMG_1105-mbseq-PSU_working.JPG   102.38K   44 downloads

Here is my PSU board mounted in my MB-SEQ. I'm using the same case that Altitude used, though I'm quite a ways from being 100% complete on it. You can also see in that pic a prototype of Altitude's 4xIIC board, as well as my painting of the base (in-progress...first attempt was a train wreck...)
 IMG_1113-mbseq-PSU_in_case.JPG   111.42K   38 downloads

Will let you know how the switching regulator works out...

Regards,
C

[fussylizard]

Oh BTW you can see in the pic that I mounted the caps horizontally to save vertical height in the case. That's why all the caps are along the edge like they are. Worked out pretty well.

[rosch]

You can also see in that pic a prototype of Altitude's 4xIIC board,

interesting interesting!!
that would be my next step, to design a 4x IIC out board and have Mike make it (and to be honest, have him check it and ask a bunch of not so clever questions too )
but... if there's gonna be some ...availability.... i might be able to not kickstart my brain

[m00dawg]

Oh BTW you can see in the pic that I mounted the caps horizontally to save vertical height in the case. That's why all the caps are along the edge like they are. Worked out pretty well.

Curious as to why you have so many caps? 3 for positive/gnd and 3 for gnd/negative or something?

As far as the heat, you don't appear to have a on those regulators and that should make a *BIG* difference. Be careful not to connect your heatsink solution to both the positive and negative regulators though. For the positive regs, you should be able to even use the metal chassis as a sort of heatsink.

EDIT: Aha, I think I figured out what you're doing with the caps. It looks like you are using a multi-out transformer? If so, got a model number? I've been looking for something like that for a while.

Edited by m00dawg, 12 September 2011 - 14:10.

[fussylizard]

interesting interesting!!
that would be my next step, to design a 4x IIC out board and have Mike make it (and to be honest, have him check it and ask a bunch of not so clever questions too )
but... if there's gonna be some ...availability.... i might be able to not kickstart my brain

Here's the thread about the 4xIIC board: http://midibox.org/f...ic-pcb-for-seq/

I'm not sure what Altitude's plans are for it. I was already ordering some stuff from BatchPCB so I ordered a prototype 4xIIC board for Altitude at the same time. Maybe drop him a PM?

[fussylizard]

Curious as to why you have so many caps? 3 for positive/gnd and 3 for gnd/negative or something?

As far as the heat, you don't appear to have a on those regulators and that should make a *BIG* difference. Be careful not to connect your heatsink solution to both the positive and negative regulators though. For the positive regs, you should be able to even use the metal chassis as a sort of heatsink.

EDIT: Aha, I think I figured out what you're doing with the caps. It looks like you are using a multi-out transformer? If so, got a model number? I've been looking for something like that for a while.

I'm using a center-tapped toroidial transformer, Digikey p/n TE62063-ND. I can't remember if that was the exact one from the MB-808 or not...I think it uses the 18V version, this is the 15V version. As I mentioned earlier in the thread, I basically ripped off the MB-808 PSU circuit and tweaked it for ease of construction. Hence the capacitor overkill...just wanted to limit the different types to buy.

I may put heat sinks on the +/- 12V regs...I'll just have to see how they fare heat-wise. The switching reg for the 5V side should be OK.

[fussylizard]

Here's the 5V switcher BTW: DigiKey p/n PT78ST105V-ND

It switches at 650 kHz, so should be OK for audio purposes. Altitude has used this and had good results, so I plan to give it a go. Will report back my findings...

[m00dawg]

I'm using a center-tapped toroidial transformer, Digikey p/n TE62063-ND. I can't remember if that was the exact one from the MB-808 or not...I think it uses the 18V version, this is the 15V version. As I mentioned earlier in the thread, I basically ripped off the MB-808 PSU circuit and tweaked it for ease of construction. Hence the capacitor overkill...just wanted to limit the different types to buy.

I may put heat sinks on the +/- 12V regs...I'll just have to see how they fare heat-wise. The switching reg for the 5V side should be OK.

I can't say if it will be enough, but heatsinks will make a substantial difference. It doesn't take much for regulators to get super hot without one which can be understandable since all the extra heat is only being radiated out a relative small metal tab compared to the size of even a small TO-220 heatsink.

I was able to reign in heat from my first MB-6582 PSU by using just power resistors and heatsinks on everything and things were actually pretty stable.

[Lamouette]

Here is some news about this psu ?