MB6582 PSU trouble shooting

Testing/Troubleshooting
1

I am building a PSU for my MB6582 and right now I have problems with the 5Vdc part.

I have built a standard PSU circuit transformer -> bridge rectifier -> smoothing capacitors -> regulator -> more capacitors

The components used now are:

Transformer: Center tap 2x6V 25VA transformer that I intent to run in parallel which gives 6V@4A according to data sheet.

Bridge rectifier: B40R, should handle 40V 2A continuously according to data sheet

Smoothing caps before regulator: 4700uF 25V + 330nF

Regulator: 78S05, should handle 2A according to data sheet

Smoothing after regulator: 100uF + 100nF

I have tried this out using three transformer configurations:

  1. Both taps in paralel, 6V@4A

  2. One tap only, 6V@2A

  3. Both taps in series, 12V@2A

I have measured voltages with the MB6582 turned on and turned off at three positions:

  • At transformer secondary (ac)

  • At rectifier output (dc)

  • At regulator output (dc)

The voltages are listed here with a denoting synth turned off and b denoting synth turned on:

trafo out rectifier out regulator out

1a) 6.82Vac 8.08Vdc 5.06Vdc

1b) 6.62Vac 6.55Vdc 4.55Vdc

2a) 6.83Vac 8.08Vdc 5.06Vdc

2b) 6.56Vac 6.35Vdc 4.45Vdc

3a) 13.6Vac 17.5Vdc 5.07Vdc

3b) 12.9Vac 14.3Vdc 4.94Vdc

The synth comes alive when using the taps in series only and off course, the regulator gets really hot quickly.

Does anybody have ideas why the voltages drop so significantly and is it possible to get this thing to supply the MB6582 using the parallel 6V configuration?

Thanks

2

Well, for one thing a single tap of a 6VAC trafo will not be enough to drive a 7805 vReg.

The general formula for DC out from full wave rectified circuit is: VAC out of trafo - 1.4 (the voltage drop across 2 diodes) x 1.414 (peak voltage from VAC RMS value). For a 6VAC trafo, that is not enough ,you need over 2V difference between the input and output voltage of a vReg. You are only seeing 8VAC out because there is no load..

Here is a good explanation:

http://www.electronics-tutorials.com/basics/power-supply.htm

3

Yup Altitude is right, 6V wont cut it. Usually a 9V transformer is used to feed a 7805.

4

Yup Altitude is right, 6V wont cut it. Usually a 9V transformer is used to feed a 7805.

Not quite.

6VAC is pushing it, admittedly. 9VAC is already a bit too much. 7VAC is perfect.

The general formula for DC out from full wave rectified circuit is: VAC out of trafo - 1.4 (the voltage drop across 2 diodes) x 1.414 (peak voltage from VAC RMS value).

Here is a good explanation:

http://www.electronics-tutorials.com/basics/power-supply.htm

That good explanation says Vdc = Vac * sqrt(2) - 1.4V, and not as you quoted Vdc = (Vac - 1.4V) * sqrt(2).

Hence for 6Vac you’d get 7.084Vdc :wink:

5

Don’t forget to add a good heat sink to the 7805 if going with 9VAC

6

I used a 6V transformer for another project, but only a core module was driven by the 5V there. It works very well. I will look into the link after dinner… Anyway, when using the 12V configuration of the transformer, the voltage drop is significant too. Why is that?

7

How is this wired up? Those losses do look a little high..

8

How is this wired up? Those losses do look a little high..

Exactly like this

http://www.avishowtech.com/mbhp/ps/

except the 2200uF is 4700uF and the regulator is a 78S05.

It is made on stripboard but the ground is not on a straight line but a little bit spread out because of the physical size of the big caps plus + and - 12V that are not connected when I try this out. Is there anything I should keep in mind about the grounding except avoid ground loops when doing this? Are the traces of the stripboard big enough to carry the current?

About the ±12V. It is made using the Center Tap Transformer design here http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu sharing the ground with the 5V part. May that cause the problem in some way?

9

I used a 6V transformer for another project, but only a core module was driven by the 5V there. It works very well. I will look into the link after dinner… Anyway, when using the 12V configuration of the transformer, the voltage drop is significant too. Why is that?

I have a multiple voltage configuration transformer. I started out with 6V as well. Worked fine on just the core. But when you start adding components, the voltage drops. My banksticks kept being initialized. I couldn’t figure out why. Then i started measuring and they where only getting 3,5V. So i upped the voltage a bit.

The reason the voltage drop is significant when using 12V AC, is probably the thermal protection kicking in. The 7805 has to dispensate all the excess power as heat.

Exactly like this

http://www.avishowtech.com/mbhp/ps/

except the 2200uF is 4700uF and the regulator is a 78S05.

It is made on stripboard but the ground is not on a straight line but a little bit spread out because of the physical size of the big caps plus + and - 12V that are not connected when I try this out. Is there anything I should keep in mind about the grounding except avoid ground loops when doing this? Are the traces of the stripboard big enough to carry the current?

About the ±12V. It is made using the Center Tap Transformer design here http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu sharing the ground with the 5V part. May that cause the problem in some way?

I would recommend adding two diodes to that design. Like in this drawing. It might not be a bad idea to add a diode to your 5V supply as well.

10

I rebuilt the 5V part dead simple and got a little bit higher voltage, but not high enough. And I tested the supply under different loads and conclude that it works up to about .5A. I have to get a higher voltage transformer.

11

What do you think about a 7.5Vac 2A transformer? I don’t see a point in wasting more juice to heat than necessary…

12

7.5VAC sounds good for a 7805. That’ll be roughly 9.2VDC before the rectifier.

13

7.5VAC sounds good for a 7805. That’ll be roughly 9.2VDC before the rectifier.

And 2A should be enough for the 5V part I am sure.

I think this one look grate!

https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=56-200-67&toc=18841

14

7.5VAC sounds good for a 7805. That’ll be roughly 9.2VDC before the rectifier.

What about 12V? Is 12Vac (12*1.4141-1.4=15.5…) enough or will 15Vac be necessary?

15

What about 12V? Is 12Vac (12*1.4141-1.4=15.5…) enough or will 15Vac be necessary?

As nILS said before you need 2V extra for the 7812 todo its work, so 12Vac transformer will do just fine.

16

Finally built and tested this with a 7.5V trafo. 15VA. It works nicely, but the regulator gets pretty hot. I use a big heat sink from an ATX PSU.