Doubts and questions about diy power supply build

[sonicdebris]

Hello everyone, I'd like to ask for some help with building a power supply. I've done a few calculations, sourced part of the components, read various schematics and discussions on this forum, but I have quite a few remaining doubts, I hope someone can help me spotting mistakes or confirming my calculations.

I need to power two 9V pedals that draw 14mA and 20mA respectively, I want the lines to be isolated so I decided to base it on the spyder with some modification, specifically using one dual secondary transformer and adding a pair of ceramic caps in parallel to the electrolitics ones.

For reference:
http://geofex.com/article_folders/oldspyder/oldspyder.htm
https://electronics.stackexchange.com/questions/21686/whats-the-purpose-of-two-capacitors-in-parallel

TLDR:

My planned BOM (per line, except for the transformer that is shared):

- 1 x 230V / dual 12V transformer, 1.5VA power rating
- 1 x Integrated rectifier bridge, 1.5A/125V
- 1 x AN7809 voltage regulator
- 1 x 1000uF/50V electrolytic capacitor (post-rectifier)
- 1 x 10uF/50V electrolytic capacitor (post-regulator)
- 2 x 0.1uF/50V non-polarized capacitors (in parallel with each electrolytic one)
- 1 x slow-blow fuse (in series with the primary) ~10mA

Doubts:

1) Is the transformer enough for my needs (esp. in terms of max power)?
2) Do I need a heat sink for the regulator?
3) How do I select the right non-polarized capacitor?
4) How do I select the right fuse?


Now, some details on my "homework", to give some context about my doubts.

RECTIFIER BRIDGE

I got it at a local shop, the package reads "Gl B80C1500 424M". I looked for a datasheet and the closest I could find is this one:

https://www.vishay.com/docs/88501/b40c1500g.pdf

It should be 1.5A/125V. The datasheets mentioned a 1V drop, but I couldn't understand whether it was total or per-diode. So I just measured it, and it turns out that all the ones I got have a 0.7V drop total.


TRANSFORMER

The required peak voltage at the secondary would be:
pedal_vcc + regulator_drop + rectifier_drop + capacitor_ripple = 9 + 2 + 0.7 + 0.2 = 11.9V
So the secondary should be rated for at least 8.41Vac

I would get this transformer with two 12V secondaries, and a power rating of 1.5VA:
http://www.farnell.com/datasheets/2548831.pdf

If my calculations are correct, this would mean 62.5mA max current per line, so I should be fine. Will I be, really, or am I too tight?
Is there something I am forgetting to consider?

Alternatively I could go for a 3.2VA-rated one:
http://www.farnell.com/datasheets/3160389.pdf

But it would be more difficult to fit in the enclosure I already have.


REGULATOR

I would like to ensure I don't need some special thermal dissipation mechanism.

The regulator is a AN7809, and the datasheet I found doesn't mention thermal resistance:
https://pdf1.alldatasheet.com/datasheet-pdf/view/111552/PANASONIC/AN7809.html
But looking for 7809, other datasheets come up that indicate the junction-to-case and junction-to-ambient thermal resistances, I just added those together to get 70 deg C / W.

The power dissipated in the regulator should then be:
(secondary_vpp - rectifier_drop - pedal_vcc) * pedal_current = (16.9 - 0.7 - 9) * 0.02 = 0.145W

The regulator temperature should be:
ambient_temp + (dissipated_power * thermal_res) = 27 + (0.145 * 70) = 37.17 degrees C

So, it looks like I don't need a heat sink after all.


CAPACITORS

I got some Nichicon 1000uF/50V electrolitics to put after the rectifier. They are supposed to be "audio grade" ones, with lower ESR, but I would still put a ceramic one in parallel. At the shop they gave me a couple of 0.1uF ceramics that read "BC Y5V 104Z", I am not sure they are 50V rated but I am afraid they are just 25V.

Thing is, capacitors really confuse me, there are too many types and I am not sure what to look for and how to read the specs for all of them, any advice here would be appreciated (eg, maybe there's another non-polarized type that's better suited, easier to source or whose ratings are more explicitly stated)

The 1000uV capacitor should give me a ripple of ~0.2V.

After the regulator I would put another 10uF electrolytic and the same 0.1uF ceramic.

FUSE

I will put a fuse in series with the primary. The current at the primary with both pedals connected should be ~4mA, but at the shop they only had regular (non slow-blow) fuses, rated for 100mA/250V, so I think I will have to source those online. Any advice on how to search for and pick one?

I made a google sheet with some calculations, I'll leave it here for reference:
https://docs.google.com/spreadsheets/d/13SvKBiMNzKqsHhqNrp9r4_vCmkUM7XZihO2s6yBiO6g/edit?usp=sharing

Sorry for the long post, I wanted to provide as much context as I could, and thanks if you made it till here!

[iainpunk]

all the math and reasoning seems to be owkay 

the only complaint is that you put the same value cap before and after the regulator, in some circumstances, the regulator can go in to some odd resonance and kill itself with heat, i have had this happen to me in school and its a hard one to debug when you aren't aware of this problem even existing, i recommend 15uf and 0.15uf.

cheers, Iain

[antonis]

First of all, Welcome.. 

Of course, you don't expect to "solve" the whole power supply >thing< in a single thread, do you..?? 

[PRR]

> adding a pair of ceramic caps in parallel

Why? If all your friends got tattoos on their bippies, would you do it too?

The primary fuse protects the line cord. 100mA 125V is suitable. That PT is too small to be protected by any fuse. It might not light your cigar even dead-shorted.

[sonicdebris]

> adding a pair of ceramic caps in parallel
Why? If all your friends got tattoos on their bippies, would you do it too?

Ahah! Fair enough! It's just that the reasoning about them helping to counterbalance the electrolytic's ESR at high freqs seemed to make sense, and I come from a crappy little d'addario supply that makes my bass distortion literally "whistle" at around 10k so maybe I was a bit overzealous. I can do without, I am not great at soldering anyway so those 4 missing capacitors will reduce the amount of sweating

The primary fuse protects the line cord. 100mA 125V is suitable. That PT is too small to be protected by any fuse. It might not light your cigar even dead-shorted.

Good to know, I didn't really want to "protect" the transformer or anything after it, just make sure no part in the circuit can catch fire because of malfuntioning at any point.

the only complaint is that you put the same value cap before and after the regulator

I was referring to the additional ceramic capacitors to put in parallel to the electrolytic ones, which are indeed different, so I guess I am ok there.

Of course, you don't expect to "solve" the whole power supply >thing< in a single thread, do you..?? 

Well, I was indeed thinking of opening a separate thread about simulating the circuit

[antonis]

"Audio grade"electros are mainly designated for signal path interpolation (in series with signal)..
Of course, you can use them for power supply ripple regulation but it shoul be an overkill, from cost point of view..

[amptramp]

If I was going to build a single-output power supply for a pedal, I would use a doorbell transformer.  They have one distinction from other transformers: they have enough leakage inductance that even if you short the outputs together they don't draw enough current to do any damage.  For multi outputs, multiple transformers.  It's not as if doorbell transformers are expensive.

[ljglmail]

Audio grade"electros are mainly designated for signal path interpolation (in series with signal)..They have a very high degree of linearity and are used in the audio industry for channel interconnects , speaker drivers, microphone capsules and cable transmission.

Toshiba 2SJ106 is the most popular of this family of "electros". It's much more linear than 1SJ106 and has no problem driving a cable running through 60 feet of air.This is a well known fact among professional audio engineers who like to use this type of tube on their projects.What I like about Toshiba 2sj106 is that they work great as line driver tubes, not so much as line receivers.So if you need to drive a long cable with your phono preamp or run speaker cables through your basement, you can't go wrong with these tubes.Just try to keep them away from a strong magnetic field.

And yes, they will make your sound more detailed and cleaner,provided your source is clean in the first place.Again,it's better to do it right the first time rather than try to fix it afterwards!

[FiveseveN]

Blink twice if you're not a bot.

[Mark Hammer]

The OP may be overthinking this.  Given the stated current draw of the two pedal, I gather they are both analog, in which case I don't personally see any need for isolating the power provided to them, other than to assure that neither "robs" the other of available current.

[idy]

So much cheaper and safer and easier (and likely almost certainly better) is to buy a one spot. Or two if you are an isolation fanatic.

Interesting topic and all, and your shopping list of questions are good, but seriously, if you are "not great at soldering" trying to design, engineer and construct a piece of line voltage gear would not be a recommended starting place. You will be "getting your feet wet" while sticking your finger into a power outlet!

No offense intended, and welcome to the forum!

[iainpunk]

So much cheaper and safer and easier (and likely almost certainly better) is to buy a one spot. Or two if you are an isolation fanatic.

Interesting topic and all, and your shopping list of questions are good, but seriously, if you are "not great at soldering" trying to design, engineer and construct a piece of line voltage gear would not be a recommended starting place. You will be "getting your feet wet" while sticking your finger into a power outlet!

No offense intended, and welcome to the forum!

OP hasn't been active since 2020 and 'ljglmail' is most probably a bot, atleast it reads like something a bot took off of wikipedia and ran through google translate 5 or 6 times on a different language

cheers

[GibsonGM]

I think I've argued with ljgl before   Pretty well-versed for a bot.  What will they think of next?  Could be on coffee break from the Nigerian prince job...