Help with 24+ circuit

Discussion in 'The Projects Forum' started by drycounty71, Feb 7, 2012.

  1. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    I have a circuit that is in need of 24+ power regulation and a 7824 regulator in place. I have a power transformer with two windings that is capable of producing 15vac 2A wired in series and after rectifier bridge is putting out 48vdc. I put a 2k dropping resistor before the 7824 and brought the voltage down to 31 volts - the circuit will not power up. I put another transformer producing about 34 vdc 92ma and the circuit powers up. I reviewed the spec sheet for the 7824 and didn't see where I was missing anything. The 7824 produces 1 A output, am I overloading it with the 2 A vs. 92 ma? Any suggestions or a fresh set of eyes - so to speak, would help.

    Thanks

    DC
     
  2. Meixner

    Member

    Sep 26, 2011
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    You can't use a 2K dropping resistor before a regulator, you will lose the regulation. The 7824 cannot handle 48 Volts on the input. You need to use a different transformer or wire the existing one differently to get the Voltage down closer to 30 - 35 Volts.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The maximum allowed input to a 7824 is 40V and you are feeding it 48V so you are lucky it is not destroyed.
    The 2k resistor in series does not reduce the voltage, it reduces the current when there is a load.

    A 7824 can pass a load current up to 1A, it does not produce 1A. The current required by the load determines its output current.
    if the voltage from input to output is high and the load current is high then the regulator IC will get too hot which causes it to shut down.
    You can power a 7824 with 27V at 1000A and it will draw only as much current an the load requires, up to about 1.5A.

    Simply reduce the input voltage to the 7824 to about 27V-30V and it will work fine.
     
  4. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    thanks for your responses, luckily the 7824 is fine and I have removed the resistor and I am using the transformer that is producing voltages more to the 7824's liking. However, downstream there is circuitry that is not being powered such as the reverb subcircuit connected to the +24v circuit. I am scratching my head on that one but I figure not enough mA from the transformer's secondary. I have simulated the circuit with Circuitmaker and best I can figure the original transformer for the LV side was providing ~500 - 800 mA which the current transformer as stated above is allowing only 92mA. I will throw a LM317 in place of the 7824 pending pin-out arrangement and program it down to 24. That should do it!
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Changing the regulator won't have any effect on the maximum current available. If you require 800 mA then you need a transformer that will deliver at least that much.
     
  6. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    In relation to the other transformer which I stated, I have on tap 2A to supply for this circuit. But it does have the 48vdc supply to the 7824 which cannot be handled. Hence the LM317 regulator which will have an effect on not letting the smoke out.;)
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    An LM317 reduces its maximum output current when the voltage from its input to its output exceeds 15V and when it is hot. Its max current might be 500mA with your input voltage that is too high. Then your entire circuit that draws 800mA will not work.

    Simply calculate how much heating the poor little LM317 regulator will have:
    The voltage from input to output= 24V.
    The reduced current = 500mA.
    Then the heating is 24 x 0.5= 12W which is a lot of heat in the little thing. It will need a huge heatsink and maybe a fan.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Just buy a proper power supply. A few bucks spent and you are DONE.
    I just don't understand why so many people try to roll their own supplies from crap they have laying around when pre-built ones are a dime a dozen now.
     
  9. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    LOL, that kinda contradicts what this site is about. If were into buying anything and everything we need then we don't have any working knowledge of how the stuff works and that is what this site is all about. BTW, the power supply is in my tube amp and is prebuilt, just needs tweaking to work with the transformer secondary output.
     
  10. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    According to manufacturer:

    * Guaranteed 1.5A output current

    The LM317 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range.
     
  11. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    I was able to finally get a hold of the manufacturer for the original transformer and the specs are as follows.

    300vac 200mA HV
    28vac 200mA LV

    So this is a better basis for any conversion that needs to be done between current transformer voltage reduction and manufacture's circuit design.

    All things considered I am still trying to bring down from 48 to around 33 vdc into the 24+ circuit at 200mA and the HV side I have already converted with a mosfet b+ reducer circuit.

    Knowing this, recommendations on a component and complimentary components - if necessary, that is robust enough and efficient enough to resolve differences in the 15v variance but maintain the mA rating.

    Thanks in advance

    DC
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,652
    2,348
    Hello,

    Did you see this graph in the datasheet?

    [​IMG]

    The current will drop as the voltage increases.

    Bertus
     
  13. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    So much for guarentees.
     
  14. BSomer

    Member

    Dec 28, 2011
    433
    106
    I had a citcuit board come across my bench for repair at work that I was a little confused as to why someone designed it this way. It had 10 1n4007 diodes in series before a 7805 regulator. I understand that they were clipping the voltage down approx 0.7 V per diode to reduce the voltage getting to the regulator. I assume it was to reduce the heat dissipated by the regulator though I am uncertain as I did not have the complete system the board was connected to.

    If you have the room you could do something like this. Though it would be ugly and inefficient.
     
  15. drycounty71

    Thread Starter Member

    Jan 16, 2012
    30
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    That was what I went through with the mosfet b+ reducer circuit which works really well. I guess it's possible to drop this voltage in a similar nature with maybe a tip120 pre-regulator buffer as someone discribed to me previously. These suggestions were given to me as a "better" "cleaner" alternative to the "bunch of diodes in series" that I had thought about previously as well. I am learning a lot here guys. Thanks for your pointers and suggestions all of them - I mean it, it helps.
     
  16. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    How much current does your actual circuit require/draw?
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    Read the details in its datasheet:
    1) The LM317 reduces its maximum output current when the voltage from its input to its output exceeds 15V. It says so and shows a graph.
    2) The LM317 gets too hot when it has an output current of 1.5A and when the voltage from its input to its output exceeds 6.7V, even when it has a pretyty big heatsink. Its max allowed internal temperature is 125 degrees C.
    Your ambient temperature might be 30 degrees C so it is allowed to have an internal temperature rise of 95 degrees C.
    3) Its thermal resistance from junction to case is 4, an insulator beween its case and a heatsink is about 1 and a pretty big heatsink will have a thermal resistance of 2 degrees C per Watt.

    So with 10W of heat, its internal temperature is higher than its max allowed temperature which will cause it to shut down.

    With a maximum output of 37V then its input is about 39V.
    With an output current of 1.5A then it will overheat when its output voltage is less than about 32V.
     
  18. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    I am not sure the best way to determine it. I am a hobbiest so I haven't gotten into all that Ohms Law and what not. I can say I added up all the resistors and components in the circuit and came up with about 2.4M ohms but I don't know if I am approaching it the right way by doing this. I figure if the ratings the manufacturer shared with me about the original transformer are even close it has 200mA available for the 24+ circuit so within that range and maybe more for good measure.
     
  19. drycounty71

    Thread Starter Member

    Jan 16, 2012
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    So what would be a better alternative component? A LM350 or LM338?
     
  20. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    This thread is worthless without a circuit schematic or actually knowing how much current your load requires.. We can't correctly suggest anything before you have the basics nailed down.
     
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