DIY Power Supply help

[Guitarboy2828]

I have 4 of these transformers lying at home: http://www.mantech.co.za/ProductInfo.aspx?Item=50M0403
and I found 2 of these: http://za.rs-online.com/web/p/pcb-transformers/0504397/

So, that's 12 isolated outputs. It's not going to be small... or light.. BUT, I'll mount it under a slanted DIY pedalboard (much like the one Alan built and then sold).

I'm using a super simple layout. Transformer -> Bridge Rectifier -> Cap -> Regulator -> Cap ->



What I'm trying to figure out is.. What is the best terminal to use at the end of it? I've thought of using RCA connectors (I think Coiks do that?). I've also thought of using the normal 2.1mm sockets. But, just seems like a waste of money to buy sockets and then have to make up 12 patch cables? with male plugs on each end.

So, I wondered about using something like this: http://www.mantech.co.za/ProductInfo.aspx?Item=64M0071

I would use 3 of them. Then make a cable with the Male plug with my leads with 2.1mm jacks.

Would this work? Anyone got cooler, nearter, simpler, cheaper ideas?

Lastly, are those transformers fine? I know they're big, but other than the size and weight, they should work okay?

[R.G.]

I'll look at it in more detail in a bit, but your schematic is missing a few important parts. There should be a diode with anode to input and cathode to output on each regulator.  These are reverse biased in normal operation, and do nothing. But on a sudden loss of input voltage or a sudden reversal of input - like if a rectifier fails - they keep the regulator from dying.

[Guitarboy2828]

R.G. Thank you for your help.

Is the diode across the regulator in this picture what you're referring to?



Are the cap value's in the schematic I posted earlier the correct values?

Thanks again for your help!

[R.G.]

Is the diode across the regulator in this picture what you're referring to?

Yes.

Are the cap value's in the schematic I posted earlier the correct values?

In most power supply applications, there isn't a correct value, only an "enough" value.

Determining "enough" in your case is problematic. If you use 7809s, the chip won't current limit until it's putting out about 1-1.5A. The one transformer spec I looked at had the winding current at about 225ma, and that's only good for about 100-125ma of DC out of it because of the peculiarities of rectification and filtering.  So the regulator will let the transformer overheat if it's shorted. Changing to 78L09 regulators or LM317L regulators would be good, as these limit in the 100-150ma range. The transformers may self limit - some of them do. But the voltage may also sag and let the regulator saturate. Ripple hum will get through if that happens, and you'll hear it.

That being said, the caps you have after the rectifiers are minimal. "Enough" would be better at 470uF or more.  The 100uF after the regulators are plenty much enough. You could do with 22uF easily enough.

[Guitarboy2828]

Thanks again for all your help!

If I use a 78L09 on the bigger transformers that put out 225ma, will it limit the mAs? or will I fry the 78L09 or will it still run smoothly? So, would it be best to use 78L09s on all channels?

[merlinb]

or will I fry the 78L09 or will it still run smoothly? So, would it be best to use 78L09s on all channels?

You can't fry a 78xx. They have internal short circuit and thermal shut down, so they're almost indestricutible. Just put some fuses on the transformer and you're fine.

[Guitarboy2828]

Great, thanks so much merlinb!

[Seljer]

Heres a rough rundown you're thermal/power situation

Your transformer is for 12V AC rated at 3VA
3VA / 12V  = 0,25 amperes
The other one is rated for 5.5VA
5,5VA / 12V = 0.458 amperes divided between 2 seperate outputs so 0.229 amperes for each of them, so about the same

12V AC after rectification becomes  16.9 volts DC, after the drop in the rectifier diodes you're left with lets say 15.6 volts DC minus some ripple voltage

You're regulating 15.6 volts down to 9V, so thats a 6.6V voltage drop that the regulator has to take care of by wasting away the excess voltage as heat.


The larger TO220 packed 7809 regulator is rated for 1 ampere of current but thats only if you heatsink it appropriately!
Without a heatsink the first datasheet I could find specifies "Thermal Resistance, Junction-Air (TO-220) : 65 °C/W" and a maximum operating temperature for the insides as 125°C.
Take an ambient environment temperature of 30°C, from there to the maximum 125°C you have a 95°C rise in temperature. 95°C/ (65 °C/W) =1.46 watts of dissipated heat inside the 7809 would heat it up to it's thermal limits.
1.45 watts / 6.6 volts drop over the regulator = 0.21amperes
So without a heatsink, it roughly matches what your transformers are capable of, you'd get about a regulated 9V @ 200mA on each transformer secondary.  But in that situation they'd be heating up quite a bit!

For the smaller TO92 packaged 78L09 regulator the the datasheet says "Thermal Resistance, Junction-Air TO92 : 150 °C/W" and a maximum junction temperature of 150°C.
150°C max - 30 °C environment = 120°C temperature difference
120°C / ( 150 °C/W) = max 0.8 watts of dissipated heat inside
0.8watts / 6.6 volts = 121mA
So in this case, the regulators internal current limiting would probably kick in before its thermal limiting. You only get 100mA per output though. So you could easily run 2 78L09 regulators on each transformer secondary. Only 100mA per each output though.

[Guitarboy2828]

Heres a rough rundown you're thermal/power situation

Your transformer is for 12V AC rated at 3VA
3VA / 12V  = 0,25 amperes
The other one is rated for 5.5VA
5,5VA / 12V = 0.458 amperes divided between 2 seperate outputs so 0.229 amperes for each of them, so about the same

12V AC after rectification becomes  16.9 volts DC, after the drop in the rectifier diodes you're left with lets say 15.6 volts DC minus some ripple voltage

You're regulating 15.6 volts down to 9V, so thats a 6.6V voltage drop that the regulator has to take care of by wasting away the excess voltage as heat.


The larger TO220 packed 7809 regulator is rated for 1 ampere of current but thats only if you heatsink it appropriately!
Without a heatsink the first datasheet I could find specifies "Thermal Resistance, Junction-Air (TO-220) : 65 °C/W" and a maximum operating temperature for the insides as 125°C.
Take an ambient environment temperature of 30°C, from there to the maximum 125°C you have a 95°C rise in temperature. 95°C/ (65 °C/W) =1.46 watts of dissipated heat inside the 7809 would heat it up to it's thermal limits.
1.45 watts / 6.6 volts drop over the regulator = 0.21amperes
So without a heatsink, it roughly matches what your transformers are capable of, you'd get about a regulated 9V @ 200mA on each transformer secondary.  But in that situation they'd be heating up quite a bit!

For the smaller TO92 packaged 78L09 regulator the the datasheet says "Thermal Resistance, Junction-Air TO92 : 150 °C/W" and a maximum junction temperature of 150°C.
150°C max - 30 °C environment = 120°C temperature difference
120°C / ( 150 °C/W) = max 0.8 watts of dissipated heat inside
0.8watts / 6.6 volts = 121mA
So in this case, the regulators internal current limiting would probably kick in before its thermal limiting. You only get 100mA per output though. So you could easily run 2 78L09 regulators on each transformer secondary. Only 100mA per each output though.

Wow, thanks for that detailed explanation. I THINK I understand what you're saying. A couple questions to try understand a little more.

If I put a heatsink on the 7809, it'll obviously allow for a higher thermal temperature, which will mean I could run it higher than 200mA. Which, from what I can understand about R.G's earlier post, that is bad because it'll overheat my transformer, correct? So, that isn't an ideal solution?

I'm running a bunch of DIY stompboxes: Tonepad delay, fuzz factory, OCD, etc. These pedals from what I can understand don't run on much more than a few mA. So, I should be fine with the 78L09?

But out of curiosity and learnings sake. What would be the most ideal method to get the full 200mA from the transformers?

[Seljer]

Yes, you don't want to draw more than those 200 to 250mA, because that'll overheat the transformer (and possibly create a fiery ball of death  ) as RG mentioned. There are other phenomenon that I glanced over. I believe I should have de-rated the DC current because of the rectification/filtering. These are very back-of-the-envelope calculations, there are many other things going on. RG's estimate of 100 to 150mA of usable clean regulated DC is valid.

The only pedals that draw more than 100mA are the fancy modern digital chorus/flanger/delay/modelling things, so yeah, if you're only going to be running one pedal per isolated output (or even just a couple daisy-chained) you'd be fine even with 78L09 regulators. In many fuzz circuits the biggest consumer of current is indicator LED  

Using the 7809 without a heatsink should in theory give you 200mA per output before thermal shutoff (or if the protection circuitry inside fails...escape of the magic smoke   ... but I've yet to kill one in this manner, they're pretty robust, but they *really* don't like overvoltages or higher voltages on the output than the input, that external diode is a good idea!). Lower temperatures improve the lifetime and reliability of components so it might be advisable to heatsink them or at least screw them down to a copper pad on the PCB so they're not running at over +100°C in these conditions. But then you have to implement some other from of protection mechanism for the transformer (say a 250mA fuse on each transformer secondary)...

[antonis]

So you could easily run 2 78L09 regulators on each transformer secondary. Only 100mA per each output though.

With balance resistors, to avoid the instance of one of them handle the bulk load..

[R.G.]

With balance resistors, to avoid the instance of one of them handle the bulk load..

It would make more sense, and I believe the poster meant, to use one output socket per 78L09 regulator. So you would have two 9V 100ma outputs that shared a ground.

I used to be a bigger proponent of all-isolated ground power supplies. I have found through experience that most of the time it's not necessary, even though it can be a problem solver for hard-case pedals, ones with funny ground requirements, or that put a lot of noise on ground.  I personally know of pro, touring guitarists with pedalboards containing 30-40 pedals that use one and only one common power supply for all of it.

[antonis]

I believe the poster meant, to use one output socket per 78L09 regulator. So you would have two 9V 100ma outputs that shared a ground.

My bad..

I thought that he proposed two regulators in parallel, sharing the load...
(one output, 9V 200mA total..)