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PSU Sugestions

Discussion in 'Power Electronics' started by solo2racr, May 2, 2014.

  1. solo2racr

    solo2racr

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    Aug 21, 2013
    Hey guys. I'm back with another project that needs a (possibly) weird PSU. Dual voltages again (maybe) but not the same ones as before (+/-15 and +48vdc if you remember). This time, I am doing this project (a subwoofer phase control) and want to put in the same case as a active audio crossover. The phase control requires +/-15vdc. The crossover is one I have from my old car audio days that uses 12vdc. Now, here is the fun part. There is a switch mode PSU in the crossover that I am just about 100% sure puts out +/-15vdc. OK.......but I doubt that it would handle the extra load of the phase control. I figure I can either come up with a dual voltage supply or (with your help) sort out the switch more PSU and cut the traces so I can build a single +/-15vdc PSU for both circuits. I know zero about switch mode supplies. I do know at least a couple ways to do a dual voltage supply though. It just seems like a single supply would be a much cleaner design though. Below is the schematic of the phase controller. Later, I will post both sides of the active crossover pcb.

    p103-f1.gif
     
  2. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    OK....I got some pics taken of both the component side and the trace side of the PCB. On the trace side, I flipped it so that it matches the component placement.

    Alpha Comp Side.JPG

    Alpha Trace Side.JPG
     
  3. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    I think I may have answered my own question about the switch mode PSU on the active crossover in the pics. While I know zero (well...almost zero) about switch mode PSU's, I recognized the 4 diodes in the top right as a rectifier. I traced these forward to pins 4 and 11 on the JRC7001 on the right hand side of the PCB and according to the datasheet, these are the Vdd and Vss pins. I will need to power it up and measure at the + side of C53 and the - side of C54 but, I'll bet I come up with the +/- voltages required.


    I couldn't wait to test. I came up with +14.18vdc and -14.22vdc with a +12.48vdc power supply voltage going in. The data sheet shows absolute max at 18volt so I should be good with a +/-15vdc supply.

    I have attached the datasheet to help anyone that would care to confirm my findings.
     

    Attached Files:

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I don't think those ICs are NJU7001. The NJU7001 is the single op-amp version in an 8-pin package; those are 14-pin packages which are most likely quad op-amps. I looked on the jrc.com website and couldn't find anything else with 7001 in it, so I don't know what that IC is, but it sounds like it's a quad op-amp in a 14-pin DIP with the standard pinout a la LM324, TL084 etc, as you said.

    On that circuit board that you've shown, I don't see any switching power supply. Is it powered from an AC adapter via the connector with the three exposed pins?
     
  5. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    Hi Kris. Thanks for helping me again. I agree that they are quad opamps. The datasheet shows the quads as a 7004 and not 7001, (a 8 pin single opamp). I am making a bit of an assumption on them, being that they are the old JRC. It's hard to read flipped over but the date of manufacture is 1988.

    The original power supply is 12vdc. It's an active crossover designed for car audio (mentioned in the OP). The pins are POS, NEG, REM (remote turn on). Most car stereo things that I have messed with have a switching PSU to either get a +/- voltage or just a higher DC voltage, depending on the design and function.

    Measuring the voltage where I did leads me to think that if I lift the rectifier diodes and tie in the +/- voltage there, it should be fine. Ground, or 0 volts, would be the common trace that both C53 and C54 share.

    My concern is that this crossover is not replaceable and I really don't want to fry it.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Is the top line on those ICs "074D"? If so they're probably TL074s. See http://www.njr.com/search/search_e.php?part=tl074&x=0&y=0 and the "7001" is probably a date code. I don't think the TL074 existed back in 1970 but I could be wrong. Or maybe JRC didn't use the usual yyww or wwyy date code format.

    (The web site is njr.com not jrc.com as I said in post #4.)

    OK, I see what you mean now. The circuitry in the back left corner is a switching supply that drives the ring core transformer, which has a centre-tapped secondary with the centre-tap connected to 0V. The ends of the secondary go across the back of the board and feed the bridge rectifier, which generates positive and negative supplies across the two electrolytics, which are commoned to 0V. There is copper running from the common point of the electrolytics to the rear panel connectors.

    That seems a bit strange. I would have kept all the power supply circuitry together, and avoided sending two AC signals from one side of the board to the other. But it looks like that's what they've done.

    So do those two electrolytics in the back right corner connect directly to the VCC and VEE pins of the quad op-amps? There are no regulators between them?

    I don't think there would be a problem powering an additional dual op-amp from those supplies. You can measure the current that's being drawn from the electrolytics by the existing circuitry and assume you can add at least 10% of that without causing a problem. What op-amp is recommended for the phase adjuster circuit? You can use a low-current one like the Texas Instruments OPA2137 (0.45 mA at ±15V supply).
     
  7. solo2racr

    solo2racr

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    0
    Aug 21, 2013
    Thanks Kris. Yes, they do say "074D" in the top line. I clicked on your link and it shows a +/- supply requirement for those. That is what threw me on using the JRC7001 number. The PDF I attached says it needs a single supply and not a split supply.

    I drew the same conclusion about the traces running from one side of the PCB to the other....Strange.

    Yes....the two electrolytic caps connect directly to pins 4 & 11. Pin 4 by the brown wire and pin 11 through the fat trace going down the right side (third trace in), across jumper J13 and on to the pin.

    The opamp recommended for the phase controller is a TL072 (dual opamp). I was considering maybe using a OPA2134 or a NE5532P. I understand that these are suppose to be more "low noise" the the TL072.

    Using the existing +/- PSU in the crossover would be great. The only concern I had was drawing to much from it. On the other hand, building a new PSU that I know will handle both isn't a problem either.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    There's no such thing as an op-amp that "needs" a single supply. Some op-amps are described as single-supply-capable but you can also use them with split supplies.

    I wouldn't worry about noise; the signal levels are going to be pretty high. If you're concerned about overloading the power supply, go for a dual op-amp with low current consumption. The TL072's current consumption for both op-amps is 2.8 mA typical, 4 mA maximum. But as I said, you can measure how much load there is on each side of the power supply at the moment, and assume that you can safely add at least 10% more.
     
  9. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    OK.....I powered it up and with no music signal passing, it measured 180mA. Of course 10% would be 18mA. At max (4mA) that's just over 2% increased load. It looks like it would be fine.

    This will work out well as I have a 12vdc wall wort rated for 500mA.

    Edit....I just checked it's unloaded voltage and it showed 20.3vdc. Good idea to use a LM7812 with it?
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    180 mA sounds very high for a simple low-power circuit like that. How did you measure the current? You have to measure current by breaking the circuit and connecting the multimeter across the break. If you connect the multimeter across the electrolytic, the way you do to measure voltage, you will actually be measuring the short circuit current of the power supply.

    So you need to cut the track that runs from the electrolytic to the rest of the circuit, and put the multimeter (set to a DC current range) across the break. With that circuit, you need to measure both currents separately, and temporarily bridge the other break, so positive and negative supplies will be present on the op-amps. Disconnect the incoming power between every change.

    In other words:
    1. Disconnect incoming power
    2. Cut both tracks from the electrolytics to the rest of the board
    3. Temporarily bridge one track break
    4. Connect the multimeter on DC current range across the other break
    5. Power up
    6. Record the current you measure
    7. Power down
    8. Move the temporary bridge to the break that you just measured
    9. Move the multimeter to the other track that was temporarily bridged
    10. Power up
    11. Record the current you measure
    12. Power down
    13. Bypass the breaks in the tracks permanently.

    Ideally both supply rails from the electrolytics should go through regulators, but it's not really necessary. Make sure the op-amps (the ones already on the board, and the one you use in your phase shifter circuit) are rated for the voltage across the electrolytics.
     
  11. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    What I did was to measure the current, with the DMM, in series with the +12vdc input power. Here's a pic I drew up quickly that shows better.... TEST.png

    But, I think I will go ahead and build a +/-15vdc PSU. The reason being is that I also have a small 6 band EQ that requires +/-15vdc to operate. This particular EQ is strange in that it is designed for proprietary use the the same brand amp. The EQ gets it's power through a 5 pin DIN plug that carries the right/left audio signal as well as the +/- power and ground. Since I no longer have any amp that can power it through the DIN plug, I believe that this will be a good use for it. Plus, I will have plenty of room inside the 1U rack case that I intend to use.
     
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