Mr modnaR's attempt at a MBSID psu - help needed (probably)

[lylehaze]

Go back to the 12V bridge.

Use multiple 7805's

2, maybe even 3 of them

#1 for first two cores

#2 for the last 2 cores

#3 for the LCD display.

Then a 7812 for your SID chips.

All on a common ground.

It's not the most "elegant" solution,

but it'll work well.

Hey, 7805s are cheap.

LyleHaze

[lylehaze]

Or,

Use 2 transformers..

Even the same one you have now (got a second one?)

Connect one transformer with series secondaries for 12VAC @1A

Connect the second transformers secondaries in parallel for 6VAC @ 2A

You have choices.

Good Luck,

LyleHaze

[Mr modnaR]

see, now i don't get why that would work, and using the 6 and 12 from my existing transformer would not work?? is it because the two secondaries are joined? if so, could i get a 4 output transformer with 4 6VAC secondaries, and bridge two of them to make 12, and have two 6VACs? or is it the fact that the primary is common that is causing the problem?

cheers.

[lylehaze]

OK, now you're "getting there". Good Job!

It has nothing to do with the primaries.

It has everything to do with the secondaries!

Yes, if you can find a transformer with 4 6V@1A secondaries, it will do it all.

Actually, 3 would be enough if you only need 6V @1A

Each secondary is "isolated" from all the others, until the designer starts

connecting them. (like your series connection between S1 and S2)

Connect 2 in series, and you add the voltages (6V@1A + 6V @1A = 12V@1A)

Connect 2 in parallel (assuming equal voltage) and you sum their current capacity.

(6V@1A + 6V@1A = 6V@2A)

Connect 2 in parallel if the voltage is unequal, and you have a problem.

Connect out of phase with each other, and you have a problem.

That's why the phase is marked with a dot.

How can I help refine your understanding?

Ask questions!

LyleHaze

[Mr modnaR]

cheers, yeah that helps clear things up a lot, thank you. now to find a three or four output transformer!

[Mr modnaR]

going back to this flawed design: you say that the 7805 will recieve negative voltage, which is bad. is it not possible to stick a diode in front of it, or will that still mean noise?

[lylehaze]

I have never seen a bridge connected in the manner you have chosen.

I cannot predict with certainty what you'll get. I can, however, see

a few problems that you won't appreciate.

For starters, you have decided again to ground at the transformer.

This is usually done for bipolar supplies. (bipolar means both positive and

negative voltages are needed)

Now let's walk through this: If the bottom of S1 is our reference 0V,

then the junction between S1 and S2 will be positive half the time, and

negative half the time. (Alternating current) Since you only need the

positive voltages, you will have to lose half of your potential power at

this point, or pass it on to your regulators and toast them.

The top of S2 will be twice the voltage of the top of S1. But it will also

be positive half the time, and negative the other half. So again, same problem

as with the other junction, just twice the voltage.

So, without even looking at your "unusual" bridge configuration, you have

set yourself for your "best possible" setup being half-wave rectified, which

is a waste of transformer capacity, and very difficult to smooth out.

Now I'll qualify my comments. I'm not an electrical engineer, though I do have

a good amount of experience in small circuit design. I do not have the expertise

of some others on this list. I'd like to think I have some experience in explaining

technical topics in an accessible way, but my word is NOT the final nameity.

If you really want to build that circuit, even though I've tried to explain the

problems it will have, please go right ahead. There is no better way to learn

than by doing. Just make sure your power supply is properly fused.

Good Luck,

LyleHaze

[Mr modnaR]

no, you explain it so well, thank you. i understand now! (fingers crossed)

[Mr modnaR]

ok got a slightly different design:

i'm going on the premise that the optimised psu design that TK suggests uses the 5VDC as a ground for the 9VDC, hence making 14VDC. is that possible?

cheers

[lylehaze]

That circuit will work, but it adds to the load of the 7805, which you're already

expecting to supply to 4 cores and an LCD display.

Would be better if your second transformer could do it's voltage without

piggybacking above the 5 volt regulator.

Getting better..

LyleHaze

[Mr modnaR]

yeah i thought something would be wrong... i was just trying the last possible thing to prevent me from having to buy anything else! i have a 7809, but all my 7812s have been soldered to the sid boards, and the pins have been chopped, which means the heatsinks i have won't fit! grrr, my parts ordering finger's about to get a workout! thanks once again man, much appreciated!

[lylehaze]

A few spares are a good thing.

Have Fun,

LyleHaze

[Altitude]

ST and TI both make 2 amp 7805's in a TO-220 package. Thats what I'm using and you should check them out since normally they only go to one amp in the medium size package and after crunching the numbers, I can see getting to over 1 amp with a C control surface

[Mr modnaR]

ST and TI both make 2 amp 7805's in a TO-220 package. Thats what I'm using and you should check them out since normally they only go to one amp in the medium size package and after crunching the numbers, I can see getting to over 1 amp with a C control surface

is that just the 5V rail, or the whole lot?

[Altitude]

just the 5V rail.

[Mr modnaR]

ok cool. am i right in thinking that 4 SID boards won't pull more than 400mA? would a 600mA rated trafo be ok in that case?

cheers

[Altitude]

ok cool. am i right in thinking that 4 SID boards won't pull more than 400mA? would a 600mA rated trafo be ok in that case?

Based on what I measured, yes

[Mr modnaR]

ok, thanks, much appreciated. how's your PSU going, by the way?

[Altitude]

  I'm in the midst of wrapping up my 9090 so all my time is committed to that but i have plans to work on it this weekend since I have all the parts I need for the MOSFET based one.  All my MBHP stuff is on the slow development path since I have so much left to do as far as the C surface goes and the mixer

[c0nsumer]

  I'm in the midst of wrapping up my 9090 so all my time is committed to that but i have plans to work on it this weekend since I have all the parts I need for the MOSFET based one.  All my MBHP stuff is on the slow development path since I have so much left to do as far as the C surface goes and the mixer

By the way, how's your 9090 going? Got any photos of it? I started stuffing the boards last weekend but took a break for some brewing and to finish the MIDIbox SID-NUXX.

What are you doing for an enclosure?

-Steve

[Altitude]

9090 is stuffed and i'm just waiting for pots from the UK to arrive (no one stateside sources that i could find).  I bought some 2u cases from a place called enclosures and cases in MN. They are all aluminum with extruded sides and are pretty nice (and cheap, 65$ each). I'm in the process of finding some 909ish knobs so i am holding off sending the panels for machining but I should be firing up for the first time in the next couple of weeks 

[Therezin]

ST and TI both make 2 amp 7805's in a TO-220 package. Thats what I'm using and you should check them out since normally they only go to one amp in the medium size package and after crunching the numbers, I can see getting to over 1 amp with a C control surface

@ Mr. modnaR:

Maplin sell these in the UK, I used one on my optimised PSU. They've got them in 5, 9, 12 or 15V for 77p.

Part numbers:

L78S09CV - UJ55K

L78S05CV - UJ54K

[Mr modnaR]

cool thank you.

[Mr modnaR]

found this transformer. PCB mount 9vac and 14vac outputs. also -14v too!. can anyone foresee any problems with this one?

cheers.

[Mr modnaR]

ok, forget the above post.  ::)

new idea: 6VAC input to [rectifier and 7805] and [1:2 ratio transformer].

1:2 trafo to [rectifier and 7812].

would this work?