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

Design Concepts
1

ok, i’m going to use a transformer with two 115VAC inputs, and two 6VAC outputs. as i have 240VAC here in the UK, i assume i need to commect the two ‘inner’ input pins and connect the 250V across the two outer pins, correct?

then, to get both 6VAC and 12VAC from the trafo, i need to short the ‘inner’ pins of the output pins and use the two outer pins for 12VAC and use the shorted connection and one of the outer pins for 6VAC. is that correct?

also to rectify the AC, would i then need two rectifiers, or would one do, as in the PSU designs of NorthernLightX and Altitude?

many thanks for any help given.

2

ok, here’s the schematic, please excuse the poor eagle skills!

anyone see any problems with that design? i’m going to leave the caps and regulators in place on the SID boards. cheers.

3

I would add some caps after the regulators (10 uF electrolytic and 100 nf)

I definitely like the dual secondary idea though and for the normal MBHP stuff that does not need the  -12, this is the way to go

I would look up how the ground is referenced though since i’m not sure how that applies in a half wave/ bridged mix

might want to read this:

http://sound.westhost.com/power-supplies.htm

4

thanks for the tip about the extra caps. this design seems to work well. fitted all the bits onto stripboard. took up 11tracks x 21 holes: quite compact! getting a stable 15V and 5V, though i’ve yet to try it on the 4 cores and SIDs. will keep you posted.

@Altitude, can you not get hold of a 4 output 24V transformer, each with 6V? that way you could bridge a few, and make 12V, 5V and -12V quite easily. feeding the 7805 with ~7V (that’s around 7V not 7VAC!) makes it run cool, which is a load off my mind! cheers for all the input!

5

This design of rectifier is not good at all. The fact is that the half of the bridge is parallelized by joining the both ground. This induces an incorrect signal and generate a lot of buzz.

The best design is too put the 7805 after the 7815. But this requires a big voltage drop in the the 7805. Don’t forget to mount a heat sink.

Ludo

6

i’m sorry, i don’t understand what you mean by ‘half the bridge is parallelised’  ???

i read 12VAC across the two outer pins on the trafo, and 6VAC from the the middle to either outer pin. that was my logic: to get 6VAC and 12VAC from one trafo, so that the 7805 wouldn’t have to do as much work. is there a way to do it with that trafo?

7

If you link the ground, you can see that a part of both rectifiers are redundant. This results in a bad rectification that injects a lot of noise.

ludo

8

link the ground? it’s AC, there is no ground, correct? or do you mean the centre two output pins on the transformer? still confused…

9

Ground is 0V in the DC part of the circuit. I think you will link them.

ludo

10

i see, no i’m keeping them separate, or is that not a good idea?

11

Normally all the 0V connections would be joined,

and they probably HAVE to be if they are working

for the same project.

Connecting them in your drawing would bring bad things.

12

ok, i’m completely new to all this eletronical malarkey, so please treat me as a newbie, because that’s what i am! why would the grounds HAVE to be connected? and why, if they were to be connected, would that bring bad things?

i appreciate the input, thank you.

13

OK, now I’ve switched over to my better browser.

I’ll do my best to answer your questions, and I

welcome anyone to jump in and correct me.

In most DC electric systems, including the kind of

circuits we play with here, all voltages are measured

against a common reference, often called Zero Volts

or even ground (whether or not it’s really grounded).

so +5 Volts means 5 volts above 0V, and -12 volts would

be 12 volts below.  IF both have the same 0V reference,

then you’d expect to see about 17 volts between them.

(The difference between +5 and -12 is 17)

However, if these two voltages are “isolated”(not sharing

any common reference), then there could be ANY difference

beween them. even 2000 volts would easily be possible.

Obviously this could be hazardous to IC’s that have an

upper voltage limit.

Now back to your drawing.

There are two separate secondaries on the transformer.

In your drawing, they are joined (between S1 and S2)

This means that they are not isolated. They are connected in

Series. This is fine.. so far.

Now we add a bridge to create DC, powered from the

sum of the secondaries, S1+S2. Cool. Higher voltage.

We add another bridge, but it’s only using S2. That works,

but the 0V connection we have created is now higher than

the 0V of the first supply (by the voltage in S1)

Now out in the final application, all the 0V lines will be

connected together, as the “universal common return”

if you’ll forgive my description.

Now, because the 0V connections have been joined, BOTH

sides of the S1 primary are just one diode away from the

same potential. in short, you’ve shorted your secondary.

Suddenly current consumption goes up, lots of heat gets

generated, and the beautiful supply goes into theatrical

meltdown mode.

My explanation is a compromise, due to me trying to be

non-technical, or possibly to me being completely wrong.

If some reader is laughing at how stupidly I’ve described

the problem, PLEASE jump right in and do a better job.

Also, I offer this to help, not to criticize. I’m new here, and

if I’m not helping then I’ll just shut the *&^% up.

Good Luck,

LyleHaze

14

thanks for explaining that! your reasoning stands, i just measured just under 5V between the two grounds.  :cry:

now what? can i not do it this way with this transformer?

15

Actually,

to be more accurate, I don’t know that you’d have a meltdown.

Perhaps I was being dramatic there.

I am sure that it would generate a lot of unexpected noise in the

power supply. (noise being stuff you don’t want mixed in with

the power you expected)

If you are building an audio circuit, you’ll want the cleanest, purest

power supply you can get. I’m building one now with

-8V, -5V, +5V, +8V for the analog stuff, and another +5 digital to

power a core board and VFD.

Even though I want to keep the digital +5 apart from the analog

supplies, they all MUST have a common ground, or it just won’t work.

How much current do you need on each supply?

Good Luck,

LyleHaze

16

in NorthernLightX’s psu:

the centre pin of his dual 12V transformer is used as ground. is this what i need to do with the first pin of s1 to give me 12VAC and 6VAC?

i need enough to power 4xCores and 4xSIDs and 1xLED Backlight and Control Surface C! in recent tests, Altitude measure this lot pulling around 500mA, i would like around 1000mA if possible.

thank you for your help.

17

Ouch.

I just reviewed your earlier posts, we may have another problem..

You connect the secondaries in series, get 12VAC.

You rectify that to DC, you get (12 * 1.4) VDC - (2 * Diode vF)

There is a small “forward voltage drop” across any diode in conduction.

Even with “perfect” diodes you only get 16.8 volts (no load) available.

What’s the minimum input voltage of your 7815?

You are dangerously close to starving that regulator.

Or maybe I just screwed up the math, I do software better than hardware.

LyleHaze

18

yes i think you’re right, maybe i should chuck a 7812 in there, and bridge the regulators on the sid boards?

what do you reckon on using the first transformer pin as ground then using a rectifier to give 12VAC and 6VAC above that? does that work?

19

If you’re only doing positive voltages, I would do it like this:

Keep S1 and S2 connected in series, like before.

Use a single bridge rectifier, from the bottom of S1 to the top of S2 (12 VAC)

Ground the negative side of the bridge rectifier (0V for all voltages)

Big capacitor from bridge V+ to 0V (smoothing the DC)

Use positive regulators from there, with the option of a dropping resistor if the heat is too great.

Use heatsinks. They’re cheap. positive regulators may be attached directly to a metal case (0V)

Also, you may use multiple 7805’s to spread the heat load.

Sorry, no means for artwork today, I’m too lazy.

What’s next?

LyleHaze

20

thanks for your suggestion. would something like this work?: