Adding diodes in series

Discussion in 'DIY Discussion' started by locky, Jun 20, 2005.

  1. locky

    locky

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    Another question on the PSU theme...I presently have +-39vdc and want to reduce this to +-30vdc by adding in some diodes in series (A voltage regulator circuit seems a bit complex). My question is do I have to place the diodes on both rails of the circuit or can I simply place them on the positive rail? If I go for the latter then I presume I will have less voltage on the positive rail than the negative. Is this the case?
     
    locky, Jun 20, 2005
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  2. locky

    LiloLee Blah, Blah, Blah.........

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    I'm sure diodes will not reduce voltage.

    Depending on your circuit, if it uses a voltage regulator, you may be able to reduce it there, or you need to use a high wattage resistor in series. Now don't ask me what value because I was never very good at calculating things like that.
     
    LiloLee, Jun 20, 2005
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  3. locky

    T-bone Sanchez

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    I presume he means zener diodes used after a resistor, I think they only go up to about 8v so he's looking at the series way. Thats what I think anyway.

    I would presume you'd need them on both rails.
     
    Last edited by a moderator: Jun 20, 2005
    T-bone Sanchez, Jun 20, 2005
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  4. locky

    ChrisPa

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    Diodes will reduce voltage. There is a forward voltage drop of about 0.6V on a normal silicon diode, so if you put 10 of them in series you wll get a voltage drop of ~6V - 39V would drop to 33V.

    In answer to Locky's question, yes you would need it on both rails to maintain symmetry, but...

    Gut reaction is don't do it - the 'complexity' of a regulator should be compensated by a much higher quality (perfomance) PSU - the multiple diodes will still leave you with an unregulated supply with associated ripple and lack of performance. Bite the bullet and build yourself your first regulator. Simple 3-pin chip regulators aren't hard to implement
     
    ChrisPa, Jun 20, 2005
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  5. locky

    Graham C

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    Also, 10 diodes is 19 more dodgy solder joints and points of failure
     
    Graham C, Jun 20, 2005
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  6. locky

    locky

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    I hear what you're saying...can anyone point me to a simple 3 pin regulator that would do for 39v per rail at 8-8 A ?
     
    locky, Jun 20, 2005
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  7. locky

    LiloLee Blah, Blah, Blah.........

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    Well I was nearly right :D and I'd need loads to drop the voltage in my valve amp :eek:

    I would some implementaion of a LM317 http://www.national.com/ds/LM/LM117.pdf

    Hang on, I just re-read your post. Is that 8 Amps? Wow what are you trying to weld :confused: The 317 is only good for 1.5A
     
    LiloLee, Jun 20, 2005
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  8. locky

    locky

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    Let me give you the full picture...I have a transformer which is giving about -+ 28vac at 165VA which makes about 6 amps. This is being rectified by a bridge diode rectifier to give 39 vdc once it has been smoothed out by the capacitors. This is a little high, as I need 35vdc maximum. I need to reduce the dc voltage to +-35vdc. Does anyone know of a regulator that will handle this kind of current?
     
    locky, Jun 20, 2005
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  9. locky

    themadhippy seen it done it smokin it

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    if you need that kind of current then your looking at making regulator out of power transistors,have a look for amplified zenner circuits,the zenner sets the voltage on the base of a power transistor.another dirty way is to reduce the A.C using capacitors, droping 4 volts at 8 amps is 0.5 ohms,so a 0.00636 farad capacitors in series of each power rail should work,dont use polarized caps though
     
    Last edited by a moderator: Jun 22, 2005
    themadhippy, Jun 20, 2005
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  10. locky

    LiloLee Blah, Blah, Blah.........

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    Dawned on me that we are looking at this the wrong way around.

    What is it you plan on powering? I ask this because the xfr may be able to produce 6A, but of course you may only be power a DAC for instance which is unlikely to draw more than a few 100mA's.

    And BTW I still go for the power resistors to cut the voltage if you need high current. I think the calculation is
    (Initial Voltage - Required Voltage)/Current, so (39-35)/6 = 0.7ohm which needs to be rated at 16watts for 4A, 12w for 3A etc. CPC do a 12w 1R which may do.

    BTW this is what TMH was talking about http://www.allaboutcircuits.com/vol_3/chpt_3/9.html
     
    LiloLee, Jun 20, 2005
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  11. locky

    zanash

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    Lots of diodes will add noise into the system....not good
    Big power resistors will add/ affect the sound produced by the psu minimally
    Power transistor will also add noise but no more than any other voltage regulation.
    I 've not checked so could be wrong but the must be a myriad of voltage regs. suitable to drop 8vdc at 3amp....

    Though I'd be tempted by the simple resistor approach.
     
    zanash, Jun 21, 2005
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  12. locky

    wadia-miester Mighty Rearranger

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    If you go that route, then use a caddock 1% thick film unit (with heat sink say 6.8C/W) aint heard a better off the shelf unit, ultra low noise about the £6 mark not cheap, but then good stuff never is. Wm
     
    wadia-miester, Jun 21, 2005
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  13. locky

    NRG

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    As mentioned what is the current draw of the device you are powering? The LM338 is rated for 5A and 7A peak.
     
    NRG, Jun 21, 2005
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  14. locky

    LiloLee Blah, Blah, Blah.........

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    Of the top of my head, the LM338 can't handle 39v input voltage.
     
    LiloLee, Jun 22, 2005
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  15. locky

    NRG

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    NRG, Jun 22, 2005
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  16. locky

    locky

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    Thanks for all the ideas...I'm tempted to go down the power resistor route. Does a 1ohm 30W resistor plus heatsink sound reasonable? I'm presuming that I will need one for each rail? By the way, the psu is powering a D-class amp from 41hz.
     
    locky, Jun 22, 2005
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  17. locky

    LiloLee Blah, Blah, Blah.........

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    Doh :) I'll check the datasheet and not my memory next time.

    So it can provide up to 32V, which means up to 72V input voltage.

    But locky what are you building and what is the current draw?
     
    LiloLee, Jun 22, 2005
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  18. locky

    locky

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    It's to power a DIY amp module. See http://41hz.com/main.aspx?pageID=88
    I'm not sure what the draw of the module is, but the amp needs a 30-35vdc supply as well as a +5vdc supply.
     
    locky, Jun 22, 2005
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  19. locky

    wadia-miester Mighty Rearranger

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    The 5v supply is for the dsp chipset and needs to be as clean as possible
     
    wadia-miester, Jun 22, 2005
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  20. locky

    NRG

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    If you are going to drive 50W into 4R with +/-30v rails a 300Va TX is the minimum needed. Ia peak is 5A into load, I would regulate the supply with the LM338. Heatsink well.
     
    NRG, Jun 22, 2005
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