Converter

Discussion in 'General Electronics Chat' started by EAUX, Jul 31, 2011.

  1. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    how would u modify this circuit with minimum components to give 0 or -22vdc output @ IO2 by the actuation of s1?

    Thanks in advance.

    [​IMG]
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Remove VEE and replace it with VCC.
     
  3. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    Can't change those they are fixed values (in my other part of the ciruit), I was going after adding a resistor or capacitor somewhere to make it work.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    This will do it:

    [​IMG]

    R2 should be 22k instead of 1k.

    When S1 is connected to 5v, Out will be near 0v.
    When S1 is connected to ground, Out will be near -22v.

    Power dissipation in R3 is about 220mW due to 10mA current flow across 22v, which is why a 1/2 Watt resistor is required. If you need less current to Out, you can increase R3.

    BTW, the more positive voltages are usually labeled Vcc, and negative labeled Vee.

    Also, when drawing a schematic, inputs come from the left, and outputs to the right; more positive values towards the top, more negative towards the bottom. This makes schematics much easier to read/comprehend.
     
    Last edited: Jul 31, 2011
  5. eceblr2011

    New Member

    Jul 8, 2011
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    In this configuration you are exceeding the base-emitter voltage rating of 6VDC of 2N2222. This will cause the damage to the transistor.

     
  6. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    Thanks for your help!! Is there an easy fix that doesn't require another transistor?
    Also thanks for the circuit drawing tips. :)

    edit:
    I don't want the transistor to burn up, lol there must be another way.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    No, the Vbe of the 2N2222 won't go above about 0.8v.

    However, R2 should have been 22k instead of 1k; I have noted that in my previous reply.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    In the original schematic I posted, R2 should have been 22k instead of 1k.
    Other than that, it's OK. Vbe should not exceed about 0.8v.

    The problem you're having is due to your switch S1 being connected to either +5v or GND. If you could eliminate the connection to GND, then all you would need to add is a base return resistor (like R4 in my schematic). The base return resistor makes certain that the transistor turns off and stays off when the current source (path through S1) is removed.

    As it is, with your S1 connected to +5v, there is a difference in potential of about 26.3v from 5v to the base of your NPN, and when S1 is connected to GND, there is a difference in potential from GND to the base of around 20.3v. The way your switch is wired, you need another transistor used as a switch to break the current path; which is the purpose of the 2N2907 PNP transistor.

    If you want to replace everything with 2 parts, then use your switch to power the contacts of a SPDT relay who's coil is rated for 5v, and connect the common terminal as output, and the NC terminal to the voltage you want output the most; the NO terminal to the other voltage. That's one part - the other is a diode connected across the relay coil, cathode towards +.
     
  9. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    got you, thanks for this info.

    The relay option is ruled out bc it is going to be driving a BCD device so I can't have much mechanical bouncing going on.

    I don't want to built this circuit because there will be 20 of them. Do you know of any type driver available in the market that changes ttl logic to -22vdc logic?
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I never tested this but in theory it should be possible with an optocoupler, the simple ones with just one phototransistor, (without using the base).

    If you want to invert the logic you could do it at the input, connecting the LED on sourcing or sinking logic gate output.

    As an example I'd think of the 4N25, but CTR is poor and I don't know if it's fast enough for your application. Also, the LED forward current might be too high, but there exist low current devices, too.

    EDIT: just ran it through a simulator and it works, resistor configuration depends now on the current available at the input and the current requirements at the output and how fast it has to react. (I replaced your transistor with an optocoupler)
     
    Last edited: Jul 31, 2011
  11. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    I think that it might work too. :)

    Found this one @ digikey for $1.00 and has four channels. That makes a total of $5 plus shipping.

    [​IMG]

    http://media.digikey.com/pdf/Data Sheets/Lite-On PDFs/LTV-817_827_847(M,S,S_TA1).pdf

    For the output I'm thinking on landing the Vcc terminal to ground and the ground terminal to -22vdc. (this ground terminal would not be joined with the input ground.)

    The input direct to the 5vdc logic.

    The rise and fall time are listed with a max of 18us which is good.

    What do you think?
     
  12. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Problem is always the shipping cost... :(

    Should be ok. The output will be either on -22V + Vsat of the opto's transistor or on 0V. Since I don't know what logic you are driving this from I can't recommend any resistors for the optocoupler. Just make sure to configure them correctly. CTR is >50% at 5mA LED current the datasheet says.

    Just being curious, how is the -22V going to be used???
     
  13. EAUX

    Thread Starter New Member

    Jul 31, 2011
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    The logic comes from an arduino mega board, which can supply up to 40ma per channel. I will be using the -22vdc to drive a yesteryear piece of equipment with nixie tubes.

    I tested the circuit in multisim and it works,

    [​IMG]
     
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