LTSpice Help -48V to 5V

Discussion in 'General Electronics Chat' started by mcgyvr, Jan 27, 2012.

  1. mcgyvr

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    Need a little help here trying to simulate this..Ive got -48V input and need 5V output.. Tried in LTspice (newbie so be gentle) put a probe on the input pin of the IC and a probe on my 5V output tag.. input on the scope shows as -48V but the output probe shows as 0V

    schematic attached.
     
    Last edited: Jan 27, 2012
  2. mik3

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    Feb 4, 2008
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    The regulator you are using cannot regulate a negative input voltage.

    Also, generally linear regulators cannot convert positive voltage to negative and vice versa.
     
  3. mcgyvr

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    The circuit is actually working just fine in real life (I didn't design it but its been working for years).. Now I have a similar application and wanted to start by putting the schematic into ltspice. Right now I have a -48VDC supply and am using the 5v output from the regulator (above schematic) to feed the V+ input of multiple op amps to get a 0-5V output from a 50mV current shunt.

    I think in reality the 5V is really 5V above the input.
     
    Last edited: Jan 27, 2012
  4. mik3

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    In real life it works because you have the source connected in reverse.

    Connect the minus terminal to Vin and the positive terminal to GND in LTSpice.
     
  5. mcgyvr

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    But my source is a negative 48VDC.. not +48V.. also wouldn't that have caused the C1 cap to blow as its + side would be more negative that way..

    What I want is an output that is 5V "more positive" than my input. In the original guys schematic the + of the battery is going to the Vin terminal and the - of the battery becomes my ground reference (I guess it becomes a virtual ground for everything else down the line from this regulator is referenced to that and powered from the 5V output pin).
     
  6. mik3

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  7. mcgyvr

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    Where is the SGT when you need him.. I'm thinking this is a limitation with LTspice as I know for sure that this is functioning perfectly in the real world. It IS being wired as shown in the schematic and is generating an output voltage that is 5V more positive than the -48V side of the battery (ie you measure between the - of the battery and the output of the LT3010-5 you read 5VDC).
     
  8. mcgyvr

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    ahh shucks... I'm retarded today.. Thank god its Friday or I'd feel totally silly.. Thanks Mik for actually making me re-think 3+ times..
    my source is -48V but in LTspice I actually set the DC value of the battery -48V (it should have just been 48V and polarity is just how you wire it) and since I attached the +terminal of the source to the input and the -terminal was my ground I was actually feeding it with a +48V but since i had the -48 as the dc value it was causing my stupidity... All is well now. Thanks again Mik.. time to go home..
     
  9. w2aew

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    I'm certain that it is NOT wired exactly has you've drawn it. If it was, you'd smoke the input electrolytic capacitor because the polarity is wrong. And, the linear reg like this can NOT perform as you've described. Is your -48V from a floating source, and maybe this circuit's ground is actually connected to the negative side of your source. I'd triple check your actual circuit.
     
  10. mcgyvr

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    yeah..yeah.. (see the post above yours) But that is how its wired but my source was just incorrectly labeled/set as -48V instead of positive.. I mean it really starts out as a -48VDC source but I switch the wires at the input to the regulator so its in essence a +48V source as far as LT is concerned. Took me actually firing up one of the boards and shorting out the regulator to figure it out too.. Sparks on a friday are fun.. vaporized pin 1/2 (Vout/sense) on that IC in the process :D

    Corrected schematic attached..
     
  11. mik3

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    That is why I said you have to swap the battery terminals before.
     
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