A slightly modified Spyder power supply

[mth5044]

Another power supply thread 

So here are two schematics I drew up, mostly based on the spyder at GEOFEX with some random changes with an LM317 and the weber transformer (8x11VAC@300mA and 1x9VAC@2A). The top schematic is repeated 6 times to use 6 of the 8 11VAC leads off the transformer. Now, the bottom schematic, I'm not 100% sure about, but I took the idea that two batteries in series pumps out ~18V, so why not two 9v DC supplies? That would take up the last 2 11VAC leads.

So a few questions:

I don't understand how to use the equation supplied on the LM317 datasheet to determine the Vout range of the regulator. The equation is Vo = 1.25V * (1 + R2/R1) + Iadj(R2). I understand that Iadj(R2) is an abbreviation for current of some sort, but I don't know what value to use there...there were some other sites that neglected that value when figuring out the voltage. So, I figure just keep R1 a good 10k, R2 at 0 (all the way down), Vo = 1.25V, which is low enough. If R2 was a 1k pot, turned all the way up, the voltage out would be 13.75V. Which I think is OK, because I found an equation on this forum posted by composition4, which shows an increase in voltage when rectifying from AC to DC, and goes something like this: 11VAC * 1.414 - 1.4 = 14.15VDC, so I'll be able to swing to just under that with the 1kpot/100k resistor combo. Is this information still all working here?

Is the second schematic going to work the way I put it together?

For the 9VAC tap, do I need to filter that or anything, or can I just throw the two leads from the transformer to the power out jack?

Is the 470uF cap at the end overkill? Is there enough filtering, or not enough?

I've been thinking about having an arduino read one of the voltages at a time and display it on 3 seven segment displays. Any thoughts on where to attach it, and any coupling that may need to be done so that when that voltage is selected for display, it won't cause sags or anything? Maybe there is no way around it, in which case I will just label the rotation of the pot with marker voltages. It's novelty anyway. Along those same lines, does anyone know where to get those knobs that fit into a trimmer like on some ZVex pedals? 

Thanks for your help. You guys are stellar. 

[petemoore]

  Heat sink if you're dropping large voltage and drawing current.
  But you may not need heatsinking, testing for heat [quick test or burn your finger] first, then if it's not smokin' hot, test voltage, disconnect.
  Adjust the resistor:
  Don't exceed the max or minimum [haven't read lately]. Use a stop resistor.
  Very soon you should be in the ballpark and can set the corrected resistance you found using the stop resistor and variable resistor [R + Potentiometer], using a fixed resistor or resistance [more than 1 resistor making the R value.

[mth5044]

Ah yes, the old touch it and see if it hurts test  I think I have a temperature gun laying around somewhere, so I'll try that out first.

As for measuring the voltage, I'm planning on leaving the variable voltage in there, so I'm not planning on finding 9v and leaving it, more setting variable voltages. Thanks for the reply!

[defaced]

Now, the bottom schematic, I'm not 100% sure about, but I took the idea that two batteries in series pumps out ~18V, so why not two 9v DC supplies?

Yes it will work, but there's a better way to do it: stack the secondaries, use a single bridge and primary filter cap.  Two less parts and same net ending. 

Check the data sheets for the regulators to see recommended high freq cap sizes (the 0.1u and 1u caps). 

For the 9VAC tap, do I need to filter that or anything, or can I just throw the two leads from the transformer to the power out jack?

No need to filter except for maybe a ferrite bead.  Loot on the PS you're replacing.  If you see a round nub thingy anywhere on the power lead that goes to the device, that's a ferrite bead. 

I've been thinking about having an arduino read one of the voltages at a time and display it on 3 seven segment displays. Any thoughts on where to attach it, and any coupling that may need to be done so that when that voltage is selected for display, it won't cause sags or anything?

It might be oversized, but I'd look at getting a small DMM and just hardwiring it in.  No point in re-inventing the wheel.  Or just put test points on the output and bring a DMM with you. 

[jasperoosthoek]

Now, the bottom schematic, I'm not 100% sure about, but I took the idea that two batteries in series pumps out ~18V, so why not two 9v DC supplies? That would take up the last 2 11VAC leads.

Why did you sum the voltage after the rectifiers? Why not just connect the AC outputs together and use a single rectifier? That way you only need one rectifier and there will be less voltage drop.

I don't understand how to use the equation supplied on the LM317 datasheet to determine the Vout range of the regulator. The equation is Vo = 1.25V * (1 + R2/R1) + Iadj(R2). I understand that Iadj(R2) is an abbreviation for current of some sort, but I don't know what value to use there...there were some other sites that neglected that value when figuring out the voltage

I think the equation should be:  Vo = 1.25V * (1 + R2/R1) + Iadj*R2
The equation from the datasheet is against the laws of physics, you cannot add a current to a voltage without some factor (resistance) relating between the two quantities. What the Iadj*R2 probably means is that there will be some current that flows out of the ADJ terminal. That current creates a voltage offset on the R2 resistance which increases the output voltage. I think you can safely ignore that part of the equation for as long as you use resistors in the order of kOhms. (not hundreds of kOhms). The regulator works by having the output at the voltage where the voltage between Vout and Vadj is 1.25.

If R2 was a 1k pot, turned all the way up, the voltage out would be 13.75V.

1.25*(1+1k/10k)=1.375V Oops ;-) I think you need a 1kpot and a 100 ohms resistor for R1.

For the 9VAC tap, do I need to filter that or anything, or can I just throw the two leads from the transformer to the power out jack?

What do you plan to use it for. Which pedals need 9VAC?

s the 470uF cap at the end overkill? Is there enough filtering, or not enough?

It probably is overkill. It will mean that the regulator has to work a bit harder. But it is still within the recommended limits stated in the datasheets (1uF to 1000uF). Just go for it.

[sjaltenb]

Is the 470uF cap at the end overkill? Is there enough filtering, or not enough?

I have not experimented much, but I did the same thing for my board with a 14 output transformer. The output was 12Vac and I put it through 2200uf and .1uf before the regulator and a 470uf, 100uf and .1uf after. I was looking for the cleanest possible power. Probably overkill, and I did not experiment with different designs to see which caps mattered. But I can assure you, it is DEAD quiet, especially with the Muffs, it takes away all the fuzz and hiss when the pedal is engaged.

It might make the system work a little harder, but you are probably only going to use it for one or two pedals since you are doing a Spyder design, right? So its prob only going to pull a few Ma anyway.

Good luck. Looks good otherwise! When did you order the Weber tranny? Im guessing you got the one with the three primary leads, as it doesnt look like the have the 120vac version anymore.

[mth5044]

Now, the bottom schematic, I'm not 100% sure about, but I took the idea that two batteries in series pumps out ~18V, so why not two 9v DC supplies?

Yes it will work, but there's a better way to do it: stack the secondaries, use a single bridge and primary filter cap.  Two less parts and same net ending. 

Check the data sheets for the regulators to see recommended high freq cap sizes (the 0.1u and 1u caps). 

For the 9VAC tap, do I need to filter that or anything, or can I just throw the two leads from the transformer to the power out jack?

No need to filter except for maybe a ferrite bead.  Loot on the PS you're replacing.  If you see a round nub thingy anywhere on the power lead that goes to the device, that's a ferrite bead. 

I've been thinking about having an arduino read one of the voltages at a time and display it on 3 seven segment displays. Any thoughts on where to attach it, and any coupling that may need to be done so that when that voltage is selected for display, it won't cause sags or anything?

It might be oversized, but I'd look at getting a small DMM and just hardwiring it in.  No point in re-inventing the wheel.  Or just put test points on the output and bring a DMM with you. 

I thought you might be able to stack the secondaries, but I'm completely unfamiliar with AC, so I thought I'd play it safe for this thread and just stack the DC. I will ammend that on the schematic, thanks!

What do you mean by high freq cap size? All I can see on the datasheet is that they need to be 0.1 and 1u, so I figured that if I just got a high enough voltage rating they would be ok? Datasheet for LM317: http://www.fairchildsemi.com/ds/LM/LM317.pdf

I'm 'replacing' a 1Spot, however I'd like to keep it for a back up, so I'd rather not amputate. It's a good power supply, just looking for another project I guess. I'd imagine they are a beast to open. I think they are some kind of 'switching' powersupply, so I don't know if they would have the same bead thing in it. But if I don't need anything in the way, that's great!

You are right about the meter. If there was a need to have it, putting two access points in would be the best way to do it.

Thanks a lot for the help!

[mth5044]

Now, the bottom schematic, I'm not 100% sure about, but I took the idea that two batteries in series pumps out ~18V, so why not two 9v DC supplies? That would take up the last 2 11VAC leads.

Why did you sum the voltage after the rectifiers? Why not just connect the AC outputs together and use a single rectifier? That way you only need one rectifier and there will be less voltage drop.

I don't understand how to use the equation supplied on the LM317 datasheet to determine the Vout range of the regulator. The equation is Vo = 1.25V * (1 + R2/R1) + Iadj(R2). I understand that Iadj(R2) is an abbreviation for current of some sort, but I don't know what value to use there...there were some other sites that neglected that value when figuring out the voltage

I think the equation should be:  Vo = 1.25V * (1 + R2/R1) + Iadj*R2
The equation from the datasheet is against the laws of physics, you cannot add a current to a voltage without some factor (resistance) relating between the two quantities. What the Iadj*R2 probably means is that there will be some current that flows out of the ADJ terminal. That current creates a voltage offset on the R2 resistance which increases the output voltage. I think you can safely ignore that part of the equation for as long as you use resistors in the order of kOhms. (not hundreds of kOhms). The regulator works by having the output at the voltage where the voltage between Vout and Vadj is 1.25.

If R2 was a 1k pot, turned all the way up, the voltage out would be 13.75V.

1.25*(1+1k/10k)=1.375V Oops ;-) I think you need a 1kpot and a 100 ohms resistor for R1.

For the 9VAC tap, do I need to filter that or anything, or can I just throw the two leads from the transformer to the power out jack?

What do you plan to use it for. Which pedals need 9VAC?

s the 470uF cap at the end overkill? Is there enough filtering, or not enough?

It probably is overkill. It will mean that the regulator has to work a bit harder. But it is still within the recommended limits stated in the datasheets (1uF to 1000uF). Just go for it.

Yup, the secondaries will be stacked instead of stacking after rectification. Thanks!

Thanks for the explanation of the equation. It's been... 2 years since Physics 2 

Yes, you are right about the pot and resistor values! I was thinking to hard 

I don't have any pedals that need 9VAC at the moment, but I figured I'd give it a jack since it's there. Properly labeled so that it doesn't accidentally get plugged into something else 

[mth5044]

Is the 470uF cap at the end overkill? Is there enough filtering, or not enough?

I have not experimented much, but I did the same thing for my board with a 14 output transformer. The output was 12Vac and I put it through 2200uf and .1uf before the regulator and a 470uf, 100uf and .1uf after. I was looking for the cleanest possible power. Probably overkill, and I did not experiment with different designs to see which caps mattered. But I can assure you, it is DEAD quiet, especially with the Muffs, it takes away all the fuzz and hiss when the pedal is engaged.

It might make the system work a little harder, but you are probably only going to use it for one or two pedals since you are doing a Spyder design, right? So its prob only going to pull a few Ma anyway.

Good luck. Looks good otherwise! When did you order the Weber tranny? Im guessing you got the one with the three primary leads, as it doesnt look like the have the 120vac version anymore.

Alright, thanks! You are right, for now, I won't need to daisy chain any of these outputs. But, I've had big pedalboards before, no reason it can't grow again 

I didn't order the weber transformer yet. I like to get the design done before parts get ordered. Yeah, they only have the three primary transformer in stock. It's a buck cheaper and still has the 120VAC in, so I guess it doesn't really matter. Just clip the other leads back and call it a day.

What transformer were you using for your supply that had 14 outs @12VAC? Sounds like it could be a contender if the price and mA is right.

Thanks everybody!

[JKowalski]

http://www.futurlec.com/Panel_Meters.shtml

They make pre-made panel meters for voltage and current display. Pretty much plug n' play.

But I would suggest an analog meter, since they look really neat compared to digital displays and they are very simple to set up as long as you know what you are doing. Read about them - selecting the right d'arsonval meter will enable you to get accurate readings without loading the voltage with any noticeable current.

[sjaltenb]

Is the 470uF cap at the end overkill? Is there enough filtering, or not enough?

I have not experimented much, but I did the same thing for my board with a 14 output transformer. The output was 12Vac and I put it through 2200uf and .1uf before the regulator and a 470uf, 100uf and .1uf after. I was looking for the cleanest possible power. Probably overkill, and I did not experiment with different designs to see which caps mattered. But I can assure you, it is DEAD quiet, especially with the Muffs, it takes away all the fuzz and hiss when the pedal is engaged.

It might make the system work a little harder, but you are probably only going to use it for one or two pedals since you are doing a Spyder design, right? So its prob only going to pull a few Ma anyway.

Good luck. Looks good otherwise! When did you order the Weber tranny? Im guessing you got the one with the three primary leads, as it doesnt look like the have the 120vac version anymore.

Alright, thanks! You are right, for now, I won't need to daisy chain any of these outputs. But, I've had big pedalboards before, no reason it can't grow again 

I didn't order the weber transformer yet. I like to get the design done before parts get ordered. Yeah, they only have the three primary transformer in stock. It's a buck cheaper and still has the 120VAC in, so I guess it doesn't really matter. Just clip the other leads back and call it a day.

What transformer were you using for your supply that had 14 outs @12VAC? Sounds like it could be a contender if the price and mA is right.

Thanks everybody!

I actually had it custom wound, a guy on another forum had his guy make it, shipped all the way from Europe! It cost a pretty penny but has 15 total secondaries 

[mth5044]

http://www.futurlec.com/Panel_Meters.shtml

They make pre-made panel meters for voltage and current display. Pretty much plug n' play.

But I would suggest an analog meter, since they look really neat compared to digital displays and they are very simple to set up as long as you know what you are doing. Read about them - selecting the right d'arsonval meter will enable you to get accurate readings without loading the voltage with any noticeable current.

Woah, they come premade  The analog voltage one would be cool, but if the power supply is mounted on a board, it might be hard to see if light is not adaquate. Maybe modding the enclosure for a bulb would be possible. Or adding a digital one below it or something. I agree though, analog ones do look awesome, I had never seen them as inexpensive as in the link you provided. I will google that word (d'arsonval) and see what I can learn  Thanks!

I actually had it custom wound, a guy on another forum had his guy make it, shipped all the way from Europe! It cost a pretty penny but has 15 total secondaries 

Yikes, sounds expensive. I think I'll stick with the weber  Thanks!

[mth5044]

I was looking at the pot and resistor ratio for the LM317, trying to optimize the resistor for the best value. I came up with 95.3 ohms, which is a common one at mouser, with a 1% tolerance. Then I remember pots had a tolerance too. It is almost impossible to get a pot with greater than 5% tolerance at a decent price and in a usable format. With a pot that has the normal 20% tolerance at 1k, the max voltage could be anywhere between 17v and 11.7v  

Would the best way to get this to work accurately with a 20% pot be to use a 1k pot/trimmer, then put another one in parallel with it to set it's resistance limits?

EDIT: I guess that wouldn't work if the pot was too small, if it was only 800 ohms, it's minimum resistance with that tolerance, changing a parallel resistor would not make it any higher. So, maybe start out with a 2k adjustable pot, then a 5k set and forget pot, there should be enough resistance room to make the adjustable pot act like the 1k pot I need.

/thinking out loud

[mth5044]

Or maybe do what RG did and make the voltage adjusters on the LM317 set and put a pot wired backwards to adjust the voltage 

[mth5044]

Sorry to keep bumping this thread, but I keep coming up with questions to which I can't find answers.

Can I use a plastic enclosure for this? The transformer is about 2.3" high, a little to big for our common metal enclosures. However, radioshack has a good 3" high abs plastic enclosure. Do I need to use a three prong cord still if the plastic enclosure isn't going to be grounded? And since the enclosure is plastic, can I use those nice smaller dc jacks? 

[jasperoosthoek]

Wall warts get away with plastic enclosures and a two prong chord. So I think the answer is yes. It is also safer against electrocution... I had to put a piece of plastic in my metal enclosure for the unlikely scenario that the solder joint of the power chord breaks inside the box. Also I had to place all the power sockets in a thick piece of PVC. I would personally prefer plastic for the power supply and metal for the effects.   

[JKowalski]

I actually had it custom wound, a guy on another forum had his guy make it, shipped all the way from Europe! It cost a pretty penny but has 15 total secondaries  

Yikes, sounds expensive. I think I'll stick with the weber   Thanks!

It's pretty easy to rewind transfomers yourself. I have done it once for a 8 output supply using a cheap $1.99 20VA transformer from inside a wall wart. As long as you leave the primary alone and do everything sensibly you shouldn't have much trouble or danger involved.

I may end up making a picture tutorial on rewinding a transformer for these supplies, since I have a tone of random transf. in a box, half of them I have no idea what to do with.