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DIY Stompboxes => Building your own stompbox => Topic started by: jplebre on October 10, 2011, 06:35:36 PM

Title: Bipolar power supply
Post by: jplebre on October 10, 2011, 06:35:36 PM
Hey guys

need a bipolar power supply for a project and was wondering if this would work ok.
Also, not sure about the second pair of caps (the 100nF ones) not too sure about their function.

For the values and voltage that we are talking, would it be a good procedure to add bleed resistors to the caps? noticed all pro audio stuff (good stuff!) i've worked on had ones (thank you guys, I can now spot these things! :P)
(http://i1217.photobucket.com/albums/dd394/jplebre/bipolarpsu.png)
Title: Re: Bipolar power supply
Post by: amptramp on October 10, 2011, 11:43:51 PM
The 100 nF capacitors are usually ceramic or flim types that retain a low impedance into high frequencies.  You cannot count on an electrolytic to handle high frequencies and most electrolytics act like inductors above audio frequencies.

Bleeder resistors at the output are used to prevent the output voltage from remaining high once the unit has been turned off.  Some regulators are specified to be used with a reverse-biased diode connected from output to input to avoid reversing the voltage across the internal power transistor of the regulator.  This would be a better idea as it addresses a possible failure mode.  Bleeders are also used to ensure voltages do not remain at an unsafe level once the unit is switched off, but 15 volts is not usually a safety issue.  You may not be able to control what amount of capacitance is connected to the output at the load, so regulator protection via diodes may be useful.
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 11, 2011, 01:46:49 AM
I would also suggest that you use 2 1100uF caps instead of 1 2200uF, it reduces the ESR and spreads out the ripple current. this will make them last longer and preform better.
Title: Re: Bipolar power supply
Post by: jplebre on October 11, 2011, 02:03:56 AM
As simple as putting 1 diode parallel with each regulator, cathode pointing to the input? the 4001 would do as we are talking of 15v right?

@Pablo weird, I can only find 1200uF caps with higher ESR :S but it does make sense about the ripple current:
http://uk.farnell.com/panasonic/eeutp1v122/capacitor-radial-35v-1200uf/dp/1890573
http://uk.farnell.com/panasonic/eeutp1v222/capacitor-radial-35v-2200uf/dp/1890583
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 11, 2011, 02:24:00 AM
resistance in parallel reduces the total resistance. its not about the esr per cap its about how much its reduced by putting 2 caps in parallel.

0.029 ohms x 2 in parallel is 0.0145 or you could add 3 and get 0.009
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 11, 2011, 02:35:41 AM
Also I don't recommend using $2-3 caps for power supply designs, its just a wast of money.
http://uk.farnell.com/panasonic/eeufc1v122/capacitor-1200uf-35v/dp/9692320
Title: Re: Bipolar power supply
Post by: jplebre on October 11, 2011, 03:22:45 AM
noted, will keep expensive caps for audio paths :)
Title: Re: Bipolar power supply
Post by: jplebre on October 11, 2011, 03:08:10 PM
Alright, update PSU with suggestion from the folks here:

The new (cheaper) and improved design :)

Another thought on the 100nF cap. If we are worried about the higher frequencies couldn't we use an inductor? or am I missing something essential on why it shouldn't be there?

(http://i1217.photobucket.com/albums/dd394/jplebre/bipolarpsu_v12-1.png)

Edited to reflect a change: D5 had polarity reversed.

Thanks
J
Title: Re: Bipolar power supply
Post by: amptramp on October 11, 2011, 04:46:26 PM
Reverse D5.  You want it to conduct only when the output is more negative than the rectified output.
Title: Re: Bipolar power supply
Post by: DavenPaget on October 11, 2011, 05:03:59 PM
I believe you want a schematic that is verified ?
(http://www.bristolwatch.com/ele/img/78xx_bipolar.gif)
I have already tried this with Rubycon 2200uf Normal range i guess , but it's okay with the ripple and ESR
@Pablo1234
2x Bad caps can cause cascade failure .
Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .
Forgot your basics ?
Title: Re: Bipolar power supply
Post by: jplebre on October 11, 2011, 05:25:36 PM
I've seen those extra diodes there as well. Are they extra protection against polarity reversal?
Wouldn't they make the Vreg diodes redundant as well?
Title: Re: Bipolar power supply
Post by: DavenPaget on October 11, 2011, 05:35:07 PM
I've seen those extra diodes there as well. Are they extra protection against polarity reversal?
Wouldn't they make the Vreg diodes redundant as well?
I bet they're there for a feedback path i guess ?
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 11, 2011, 07:00:26 PM
Quote
Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .
yes after the Regulator, the ripple current is what makes Capacitors fail an having 2 is a standard. Rule of thumb is 2 caps in parallel are always better than one big cap. On a side note useing 2 capacitors for filtering is also useful. Polyester and polypropylene have inverse Thermal Coefficients, it makes them thermally stable.

also the Regulator isn't the best at Ripple Rejection(14) f = 120Hz, VI = 18.5V to 28.5V 54.0 70.0 dB

and the diods are useful
Operation With a Load Common to a Voltage of Opposite Polarity
+VI mA78xx +VO −VO 1N4001 or Equivalent Reverse-Bias Protection VI mA78xx +VO mA7800 SERIES

Figure 5. Regulated Dual Supply
In many cases, a regulator powers a load that is not connected to ground but, instead, is connected to a voltage
source of opposite polarity (e.g., operational amplifiers, level-shifting circuits, etc.). In these cases, a clamp diode
should be connected to the regulator output as shown in Figure 6. This protects the regulator from output polarity
reversals during startup and short-circuit operation.

Figure 6. Output Polarity-Reversal-Protection Circuit
Occasionally, the input voltage to the regulator can collapse faster than the output voltage. This can occur, for
example, when the input supply is crowbarred during an output overvoltage condition. If the output voltage is
greater than approximately 7 V, the emitter-base junction of the series-pass element (internal or external) could
break down and be damaged. To prevent this, a diode shunt can be used as shown in Figure 7.
Title: Re: Bipolar power supply
Post by: jplebre on October 12, 2011, 03:58:38 AM
Alright added that to the schemo and layout as well

(http://i1217.photobucket.com/albums/dd394/jplebre/bypolar_psu_v14.png)

As for the layout, should I not worry about ground pour?
the calculators gave me a trace width smaller than the one I used so I should be fine there.
(http://i1217.photobucket.com/albums/dd394/jplebre/layout_v14.png)
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 12, 2011, 03:45:18 PM
I always use ground pours on my power supply's, its not really required but it don't hurt either.

your 7915t isn't the correct pin-out, check the data sheet.
Title: Re: Bipolar power supply
Post by: DavenPaget on October 12, 2011, 03:53:47 PM
Quote
Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .
yes after the Regulator, the ripple current is what makes Capacitors fail an having 2 is a standard. Rule of thumb is 2 caps in parallel are always better than one big cap. On a side note useing 2 capacitors for filtering is also useful. Polyester and polypropylene have inverse Thermal Coefficients, it makes them thermally stable.

also the Regulator isn't the best at Ripple Rejection(14) f = 120Hz, VI = 18.5V to 28.5V 54.0 70.0 dB

and the diods are useful
Operation With a Load Common to a Voltage of Opposite Polarity
+VI mA78xx +VO −VO 1N4001 or Equivalent Reverse-Bias Protection VI mA78xx +VO mA7800 SERIES

Figure 5. Regulated Dual Supply
In many cases, a regulator powers a load that is not connected to ground but, instead, is connected to a voltage
source of opposite polarity (e.g., operational amplifiers, level-shifting circuits, etc.). In these cases, a clamp diode
should be connected to the regulator output as shown in Figure 6. This protects the regulator from output polarity
reversals during startup and short-circuit operation.

Figure 6. Output Polarity-Reversal-Protection Circuit
Occasionally, the input voltage to the regulator can collapse faster than the output voltage. This can occur, for
example, when the input supply is crowbarred during an output overvoltage condition. If the output voltage is
greater than approximately 7 V, the emitter-base junction of the series-pass element (internal or external) could
break down and be damaged. To prevent this, a diode shunt can be used as shown in Figure 7.

Yeah i went to the datasheet and it's a clamp diode and a output protection diode and i thought it was a feedback loop diode ==
Title: Re: Bipolar power supply
Post by: jplebre on October 12, 2011, 04:37:09 PM
I always use ground pours on my power supply's, its not really required but it don't hurt either.

your 7915t isn't the correct pin-out, check the data sheet.

Damn! I thought that the TO220 would have the same pinout regardless!
Well spotted thank you you avoided a hell of a smoke and headscratching
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 12, 2011, 04:52:13 PM
ya that's always a pain with transistors, knowing the pin-out from one chip to another can get confusing sometimes.

Also when I make my power supply's I always do dual sided boards, that way the regulators have thermal pads right from the copper, in the case of the 7915 the pad is the input voltage not ground. this also gives you a larger ground plain, verry helpful when doing ground plain wiring instead of star grounding.
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 12, 2011, 05:26:44 PM
(http://i1106.photobucket.com/albums/h363/Rick_Haughton/ps.jpg)

this is a dual layer mockup of what you should end up with.
Title: Re: Bipolar power supply
Post by: egasimus on October 12, 2011, 05:30:32 PM
Minor thread hijack: What do you think of this: http://www.musicfromouterspace.com/analogsynth/WALLWARTSUPPLY/WALLWARTSUPPLY.php
It's, basically, a bipolar PSU without a center-tapped transformer. Is it any good?
Title: Re: Bipolar power supply
Post by: jplebre on October 12, 2011, 06:16:06 PM
Wouldn't the screws possibly short? I bought a couple of clip on heathsinks, ideas of which is preferable?
I tried a double sided board once (for actual circuit traces) and it went terribly but since it's only for the heatsinks I don't need to be exact and it's actually a good idea!

@egasimus looks like it has more filtering (more caps) It also has a resister that fixes an issue with the 79xx not conducting when no load is present.

Beginning to think I might add one myself.

Any ideas on this one?

Also, about the filter caps - when enough filtering is enough?

Cheers
J
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 12, 2011, 07:58:46 PM
Diminishing returns on filtering would be a decent answer. their will be a point where adding capacitors adds more cost and reduces the ripple in smaller degrees, it has a lot to do with current draw also. I typically use 2 4400uF Electrolytic caps and 1uF Ceramic for each side. I also do a 5V rail on mine though so you may not want as much capacitance.

Quote
Wouldn't the screws possibly short? I bought a couple of clip on heathsinks, ideas of which is preferable?

if you look at my drawing I have the negative voltage as the pad/copper back to mount the regulator to isolated from ground, its on both top and bottom.

top traces - component side
(http://i1106.photobucket.com/albums/h363/Rick_Haughton/pstop.jpg)

Bottom traces

(http://i1106.photobucket.com/albums/h363/Rick_Haughton/psbottom.jpg)

Title: Re: Bipolar power supply
Post by: DavenPaget on October 13, 2011, 03:41:02 AM
Diminishing returns on filtering would be a decent answer. their will be a point where adding capacitors adds more cost and reduces the ripple in smaller degrees, it has a lot to do with current draw also. I typically use 2 4400uF Electrolytic caps and 1uF Ceramic for each side. I also do a 5V rail on mine though so you may not want as much capacitance.

Quote
Wouldn't the screws possibly short? I bought a couple of clip on heathsinks, ideas of which is preferable?

if you look at my drawing I have the negative voltage as the pad/copper back to mount the regulator to isolated from ground, its on both top and bottom.

top traces - component side
(http://i1106.photobucket.com/albums/h363/Rick_Haughton/pstop.jpg)

Bottom traces

(http://i1106.photobucket.com/albums/h363/Rick_Haughton/psbottom.jpg)


I was so stupid to think 1 LM78XX can do more then 1A , with my overkill heatsinking , but i hope it can , since it will most likely stay at  25C .
Then i forgot i used 1N4007 diodes . :\
Oh well , if i need more amperage i would need to connect another "module" .
1A is pretty much enough ( 1A per rail , my xformer (50VA) and bridge rect does 4A ) .
Title: Re: Bipolar power supply
Post by: jplebre on October 13, 2011, 08:40:44 AM
@Pablo I think the data sheet of my 7915 has an error then. It says back connected to i/p
http://pdf1.alldatasheet.com/datasheet-pdf/view/22633/STMICROELECTRONICS/L7915CV.html

Anyone else had the issue that the guys on MFOS had of the negative Reg not working without load? I can just use kirchoff law to calculate what size resistor to add?
Title: Re: Bipolar power supply
Post by: DavenPaget on October 13, 2011, 09:31:58 AM
@Pablo I think the data sheet of my 7915 has an error then. It says back connected to i/p
http://pdf1.alldatasheet.com/datasheet-pdf/view/22633/STMICROELECTRONICS/L7915CV.html

Anyone else had the issue that the guys on MFOS had of the negative Reg not working without load? I can just use kirchoff law to calculate what size resistor to add?

There's no problem with the datasheet .
Oh i haven't tested mine yet , but i guess it's fine , each of my pedals do at least 8ma .
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 13, 2011, 01:29:34 PM
yes the data sheet is correct, if you look at my drawing the copper pad is the input voltage.
Title: Re: Bipolar power supply
Post by: DavenPaget on October 13, 2011, 07:54:47 PM
yes the data sheet is correct, if you look at my drawing the copper pad is the input voltage.
You basically had the entire board as the ground ? Sorry i have not seen someone do a double layer here .
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 13, 2011, 08:38:09 PM
no the Ground and negative input voltage pads are on both sides. I do dual layer all the time, I use proppelors alot for logic and controls and with that much I/O it would suck to try it on 1 layer, dual layers make everything simpler.
Title: Re: Bipolar power supply
Post by: jplebre on October 14, 2011, 01:36:51 AM
Slightly OT, but how to you line up your 2 layers????? :S

now back on topic, if I wanted to add some sort of fuse (say that I'm plugging it to something that is shorted, so I'd like to protect the power supply) how would I go about it? would I need 1 per rail?
http://uk.farnell.com/littelfuse/60r010xu/polyfuse-ptc-radial-0-1a/dp/1822246

My guess would be before the Vregs, after the caps. But what about the negative supply? There isn't any polarity either, right?
Title: Re: Bipolar power supply
Post by: DavenPaget on October 14, 2011, 06:19:24 AM
Slightly OT, but how to you line up your 2 layers????? :S

now back on topic, if I wanted to add some sort of fuse (say that I'm plugging it to something that is shorted, so I'd like to protect the power supply) how would I go about it? would I need 1 per rail?
http://uk.farnell.com/littelfuse/60r010xu/polyfuse-ptc-radial-0-1a/dp/1822246

My guess would be before the Vregs, after the caps. But what about the negative supply? There isn't any polarity either, right?


Wouldn't it be better to be before the transformer ? protects the transformer too .
Title: Re: Bipolar power supply
Post by: jplebre on October 14, 2011, 09:58:49 AM
Stopping it on the secondary side would also protect the transformer, no?
Just It would stop current on that side?

Also, I've put them on secondary side because someone told me we shouldn't mess about with the primary side.

Any opinions anyone?

Cheers
Title: Re: Bipolar power supply
Post by: EATyourGuitar on October 14, 2011, 11:41:16 AM
http://search.digikey.com/us/en/products/CC3-1212DF-E/445-2472-ND/920432 (http://search.digikey.com/us/en/products/CC3-1212DF-E/445-2472-ND/920432)

http://search.digikey.com/us/en/products/CC6-1212DF-E/445-2488-ND/920448 (http://search.digikey.com/us/en/products/CC6-1212DF-E/445-2488-ND/920448)

you wont find better specs at a better price for a small footprint assembled and tested DC to bipolar DC module. I will be using one of these for a 9.55VDC visual sound one spot powering some synth circuits in a pedal. its also great for increasing the gain and headroom of an op-amp. the smaller one is 100ma per rail. the one spot is 500ma. no problem.

I have seen all those AC to bipolar supplies posted everywhere and they are all similar in concept. you convert AC 110 to AC 12V or 16V. then you rectify each rail.. filter with huge caps. use 3 pin regulators for the end game. thats all good and great if you supply the wall wart with the pedal. I found some cheap wallwarts on jameco. there is a strip board version of the MFOS supply if you already have the transformer. its posted on electro-music forum.

remember kids, don't kill yourself building power supplies
Title: Re: Bipolar power supply
Post by: jplebre on October 14, 2011, 12:25:49 PM
http://search.digikey.com/us/en/products/CC3-1212DF-E/445-2472-ND/920432 (http://search.digikey.com/us/en/products/CC3-1212DF-E/445-2472-ND/920432)

http://search.digikey.com/us/en/products/CC6-1212DF-E/445-2488-ND/920448 (http://search.digikey.com/us/en/products/CC6-1212DF-E/445-2488-ND/920448)

you wont find better specs at a better price for a small footprint assembled and tested DC to bipolar DC module. I will be using one of these for a 9.55VDC visual sound one spot powering some synth circuits in a pedal. its also great for increasing the gain and headroom of an op-amp. the smaller one is 100ma per rail. the one spot is 500ma. no problem.

I have seen all those AC to bipolar supplies posted everywhere and they are all similar in concept. you convert AC 110 to AC 12V or 16V. then you rectify each rail.. filter with huge caps. use 3 pin regulators for the end game. thats all good and great if you supply the wall wart with the pedal. I found some cheap wallwarts on jameco. there is a strip board version of the MFOS supply if you already have the transformer. its posted on electro-music forum.

remember kids, don't kill yourself building power supplies

Need to look into those for pedals. This is for a rack unit with preamp, eq, compressor etc that I'm putting together from different schems. I do need a stable power supply. I can't go really higher than 15 because of one of the JFet's I'm using, so that wouldn't be good for me (I still need the AC conversion and tbh I'd like to design a safe'r power supply - hence taking the time in this forum, with very lovely people, to try and understand the schemos I found better, and to put them together to build a better one :)

Title: Re: Bipolar power supply
Post by: EATyourGuitar on October 14, 2011, 01:22:19 PM
if you are putting 110AC mains wiring inside the unit there is a lot to consider. I'm no expert so I'll leave it up to you to find good information. remember that if you dont do it right, you could die or burn down someones studio or house. if you can't do it right or your not sure if you did it right then don't do it.
Title: Re: Bipolar power supply
Post by: jplebre on October 14, 2011, 02:02:39 PM
Actually, I'm putting 240v :)
I live on the other side of the pond  ;D

Bump on the fuses: The rack will have an IEC lead i/p with fuse holder on the live terminal BUT I will still be doing testing. I built the small bear Wall Wart (240VAC to 9VDC) and it had a fuse on the + side of the secondary.
Any ideas? Fuse on both the + and - side on secondary or just a fuse on the primary? would the secondary also protect the transformer?

Title: Re: Bipolar power supply
Post by: Pablo1234 on October 14, 2011, 08:09:13 PM
you absolutely must use a fuse from the cord to the primary on all live wires, if its 240VAC single phase, this means a live and a neutral you only need one on the live side. If however you are doing 220 2 phase, that's 2 110VAC 180 Deg out of phase then you need one on both live wires. Almost every country is different and you really should look at how primary wiring is done in your country. Al ot of people add MOV's to their primary side also and a switch rated at your country line voltage and current draw is a must also. this is kinda where you start and finish your design, the regulation is secondary to the safety of the unit.

you can fuse the secondary side but its not required.
Title: Re: Bipolar power supply
Post by: Pablo1234 on October 14, 2011, 08:34:18 PM
Quote
Slightly OT, but how to you line up your 2 layers?Huh? :S

its not as hard as you might think. first you need to get your margins on the printer consistent from print to print, I use 4.5 on my brother laser printer and measure them on each print job. If it prints consistently withing +/- .005" then your ok and just mirror the top layer.

its helpful and more reliable to have a form to set everything up, I have a small 6" x 6" piece of sheet metal with a 90 deg bend on the top and side for alignment. you put the bottom print job facing up into the form then the dual side board then the top print job facing down and if its all pushed into the corner it will line up every time. then you just set your iron on it till its all stuck together and you can then move it about to really iron it together.

you must also make sure that your boards are square and straight.

Another option is too print out the bottom layer, lay it over the board and drill 2 holes, on in the top right corner and one in the bottom left corner. then you stick a needle through the printouts of the top laser print job that correspond to the board drill holes and ling it up then iron it on till its stuck and do the same for the bottom. this is how I used to do it and its not as difficult as it sounds.

Title: Re: Bipolar power supply
Post by: jplebre on October 15, 2011, 06:08:08 AM
Quote
you absolutely must use a fuse from the cord to the primary on all live wires, if its 240VAC single phase, this means a live and a neutral you only need one on the live side.
this is the one I'm using - 2 birds in one stone - full on IEC lead wich is what 19" rack equipment generally use, allowing me to earth the chassis straight there, and built in fuse holder.
http://www.rapidonline.com/SearchResults.aspx?kw=23-2100