Can this be done?  Voltage divider from a 24vDC wall wart - Page 1
Forum section: electronics / projects

author: gooseEL34
author_rank: Regular Contributor
timestamp: September 10, 2013, 04:16:55 pm
content: Ok.I have a guitar effects processor that needs a new power supply. The original has a transformer 120vAC to 24vAC Center Tapped..Inside the pedal, the AC is rectified for two 12v rails (+12vDC, -12vDC).Question...If I use a 120vAC to 24vDC wall wart, can I do the following to get +12/-12?If I run a pair of resistors in series and then run that between +24vDC and ground, it would appear that I can reference the junction of those series resistors to ground and then the + lead of the wall wart supply would be +12 from ground reference and the negative lead would be -12vQuestion is this: Can I do that. What values of the resistors would I use? What wattage?Assume load is drawing 0.45mAIn the bad pic below A=B (resistors)from wall wart....----------->*------------------>V+*A**-----------> C (ground)**B**from wall wart neg---------->-------------------->V-

author: gooseEL34
author_rank: Regular Contributor
timestamp: September 10, 2013, 04:20:27 pm
content: I am handy with a soldering iron, can read a schematic and use a Fluke for measurements.Not an EE but booksmart and understand EE basics. I am seeing the above as a voltage divider and if I am correct, referencing the mid point of the divider to ground in this scenario would give an effective -12 to the other "leg" of the divider.

author: krivx
author_rank: Frequent Contributor
timestamp: September 10, 2013, 04:56:45 pm
content: You can do this, I don't know if it will work well. Your two voltage source are likely to become unbalanced unless the current draw is identical on both. If you buffer the virtual ground you will get much better performance, there's some interesting stuff here:http://tangentsoft.net/elec/vgrounds.htmlIs it worth it? I just bought a transformer with dual 12V windings that is rated to 6VA and I think it cost me around 6euro.Also, I really, really doubt your multi-fx is drawing only 0.45mA...

author: rsjsouza
author_rank: Super Contributor
timestamp: September 10, 2013, 05:05:27 pm
content: Yes, this can be done; however it is not the best approach for several reasons:- the resistance and power dissipation of A and B will have to take into consideration the difference between the current drained by each rail- this will almost certainly introduce signal distortion when the circuit draws a variable amount of current during its normal operation - the center "ground" will fluctuate, unless the processor has some beefy capacitors in its input supply.- if the current drawn by the circuit is too high, excessive power will be dissipated in the resistors and you will have to use a much beefier power supply.All that said, why not get a 12+12V transformer?

author: tszaboo
author_rank: Super Contributor
timestamp: September 10, 2013, 05:34:04 pm
content: Just use ahttp://www.ti.com/product/tle2426. For this load and application, it is perfect.

author: dannyf
author_rank: Super Contributor
timestamp: September 10, 2013, 11:56:57 pm
content: QuoteIt can work - whether it will work or not will depend on your particular requirement.I would also put on two capacitors in parallel with those resistors. I would also size the resistors so that at least 10x of the required current flows through the resistors. That means 12v/0.45ma / 10 = 2k.A better approach is to use a follower to establish the virtural ground: put a opamp, its inverting input tied to its output and its non-inverting input to the middle of the resistors. The opamp's output would be the new virtual ground. Depending on your load, the opamp can be a small opamp (ne5532 for example), or a power amp (tda2030 for example).

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 11, 2013, 02:00:39 am
content: It'll work well up until the instant you start to draw any current from the middle of the voltage divider. As soon as you try to pull any current, then the current through the top resistor will be different from the current through the bottom resistor. Since the currents won't match, the voltages in the two resistors won't match, so the voltage at junction is no longer halfway between the rails.Lesson: a voltage divider is always a high impedance voltage source, and never appropriate if you want the voltage to be constant with varying current. The equations for the voltage divider only work if no current is drawn.You can configure an op-amp as a voltage following buffer to fix this problem. The rail splitter IC which NANDBlog suggested is essentially a voltage divider with a voltage following buffer, all built into a single chip. It is designed to solve precisely this common problem.

author: echen1024
author_rank: Super Contributor
timestamp: September 11, 2013, 02:04:28 am
content: You will be getting eitherA) Very high currentorB) Very high ripple

author: edavid
author_rank: Super Contributor
timestamp: September 11, 2013, 02:41:01 am
content: You should have just added this on to the other thread instead of starting a new one.Anyway, this won't work because the +12V current draw is much higher than the -12V (among other things, it has the filaments connected to it). As I said in the other thread, you should get a 12VDC wall wart and add a small DC-DC converter to generate the -12V. Or, just use a second 12V wall wart.(The TLE-2426 won't work either.)Here's the previous thread with the schematic:https://www.eevblog.com/forum/beginners/best-way-to-replace-a-wall-wart-power-supply

author: dannyf
author_rank: Super Contributor
timestamp: September 11, 2013, 11:02:40 am
content: QuoteTrue. But the variation depends on the relative current off balance vs. the current flowing through the resistors. If you size the resistors right, the fluctuation can be sufficient small. As the load is here < 1ma, that is very easy to do.

author: IanJ
author_rank: Unknown
timestamp: September 11, 2013, 11:19:44 am
content: Hi,Tread carefully:-Get it wrong (depending on your abilities/experience) you may damage your Fx processor. Much easier/safer just to get a new PSU.You may be able to use the resistor method but it would only be ok for a constant load on the rails.....i.e. as soon as you switch on/off an effect then the load could change and thus the voltage could be all over the place.You may have 0vdc referencing issues, i.e. if you tie-in a few pedals not to mention your guitar/amp.Ian.

author: crispytato
author_rank: Regular Contributor
timestamp: September 11, 2013, 03:48:37 pm
content: Another vote for the TI TLE-2426. I've used it for virtual grounds in headphone amps and other small low current circuits, it works well. Check out Tangentsoft's pages on buffered virtual ground, he provides several useful circuits. buffering TLE-2426 with BUF-634 has worked really well for me. It can handle a decent amount of current, there is also a virtual ground circuit in the BUF-634 datasheet that may be worth looking at if you need more current than a buffered 2426 can provide.peacechris

author: dannyf
author_rank: Super Contributor
timestamp: September 11, 2013, 05:04:21 pm
content: Another approach you may try is to use a 12v 3-terminal regulator (doesn't matter it is either positive or negative). Tie the input / gnd to the 24v sources and the output is your virtual ground.It has the advantage of better regulation and higher current capabilities.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 11, 2013, 06:18:32 pm
content: Quote from: dannyf on September 11, 2013, 05:04:21 pmBe careful. That only works if the current is always going the same direction on your center rail.I don't know the full schematic for this device in question, but my guess is that this is used for some sort of audio amplifier, and at different times during the waveform cycle, the center rail might be called upon to alternately source and sink current. A typical linear regulator isn't designed to sink current at its output while maintaining regulation.

author: dannyf
author_rank: Super Contributor
timestamp: September 11, 2013, 06:19:37 pm
content: QuoteNo.

author: edavid
author_rank: Super Contributor
timestamp: September 11, 2013, 06:26:48 pm
content: Quote from: dannyf on September 11, 2013, 05:04:21 pmNo, that won't work!

author: TerminalJack505
author_rank: Super Contributor
timestamp: September 12, 2013, 01:08:03 pm
content: I'm sure this schematic won't mean anything to you, OP, but this is basically what you need to do to get +12V and -12V DC rails from a single 24V DC source.Note that it establishes a ground using a resister divider but then has to use active components to ensure that that particular reference point (virtual ground) is low impedance. Note also how the virtual ground can source or sink current. (Alotmore current than a resister divider can.)Even still, notice that there's a fair bit of ripple on the two voltage rails--about +/-100mV--so it's not as good as doing it properly.

author: WarSim
author_rank: Frequent Contributor
timestamp: September 12, 2013, 05:46:43 pm
content: Solar there are post offering ideas, I won't get into which will work. What I haven't read is a reason for the 24v source requirement. I suspect that 24v sources was given solely by the original idea. I also suspect the change is inquired about to save the relatively small funds for a centre tap transformer.I think this thread is lacking this information from the op and has restricted the possible solution. Offering the dozens of possible solutions for an undefined criteria is a bit futile.With more information more options can be offered. Define what must be (schematic/block diagram of the load, etc) and the options you have for a source (details are important).To answer the original question, the voltage decider has a very poor chance of working. Rail use is rarely balanced and to save costs the load is rarely compensating nor filtered. Better options would increase cost. A good option would get very close to the cost of a DC-DC converter module if the load truly is under 50mA.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: September 12, 2013, 08:46:35 pm
content: This is the schematic that the OP supplied in his first thread about this situation...http://analogguru.an.ohost.de/193/schematics/Chandler_TubeDriver.gifThe original damaged 24V center tapped wall-wart transformer was noted as being connected to the guitar effects processor's enclosure simply by means of a three conductor cable.The filament of the 12AT7 vacuum tube will require a significant amount of current.Regards,Frank

author: dannyf
author_rank: Super Contributor
timestamp: September 12, 2013, 09:47:10 pm
content: QuoteThat's the issue with the particular design (C2/R4), not with the particular topology.

author: dannyf
author_rank: Super Contributor
timestamp: September 12, 2013, 09:59:01 pm
content: QuoteIn that schematic, the filament is powered by -13v - perfectly suited for a 7812 type regulator: tie its input to +24v, its gnd to gnd of the +24v source, and its output to the virtual ground. The gnd of the regulator will output a steady -12v vs. the virtual ground, regardless of load (as long as it stays within the performance envelop of the regulator).It cannot be simpler than that.BTW, the pi filter has a serial resistance of 1kohm so it doesn't draw much current.

author: WarSim
author_rank: Frequent Contributor
timestamp: September 12, 2013, 10:07:23 pm
content: Quote from: fpliuzzi on September 12, 2013, 08:46:35 pmOops I read 45mA I missed by a bit..Still is the only source available 24vDC.Is there another AC or DC source available?The actual load required for the +-12v is under 50mA all the rest is for the output amplifier which can be indirectly coupled.So why not a low power 20-30vDc to +-12vDC converter. Or a bipolar power supply from an AC source. Or repair/replace the original power supply.Yes a virtual reference will work but to get one that won't cause flutter will be dear. I assume the idea is to save money. This is the info I am missing.

author: edavid
author_rank: Super Contributor
timestamp: September 12, 2013, 10:17:34 pm
content: Quote from: dannyf on September 12, 2013, 09:59:01 pmCheck your glasses, the filament is powered by the positive supply, not the negative one. It would be necessary to use a negative regulator. I still don't see any reason to do it that way though.

author: TerminalJack505
author_rank: Super Contributor
timestamp: September 12, 2013, 11:34:59 pm
content: Quote from: dannyf on September 12, 2013, 09:47:10 pmThat's the best I could do and still keep it stable with a wide range of load capacitance. My op amp skills aren't the best, though.

author: TerminalJack505
author_rank: Super Contributor
timestamp: September 13, 2013, 02:30:44 am
content: Concerning the schematic I posted earlier, here's an AC model of the circuit with a lot better bandwidth. My question is, though, is it stable? The fact that the phase shoots down to nearly -180 degreeslookslike a bad thing but is it? Or is that just expected because of the big capacitance on the output?My common sense seems to be telling me that there's no way that the phase could shoot past -180 degrees but it's worrisome still.Edit: The parametrized value is the output capacitor, C1, by the way.