Can't generate 500v for geiger counter

Discussion in 'The Projects Forum' started by rustypigeon, Apr 7, 2011.

  1. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    Hello all. I am trying to build a Geiger Counter circuit. I am using the schematic and instructions from the book "Electronic Sensors for the Evil Genius". I have followed the directions exactly, yet can't get my voltage up to 500. The most I can get is 240v.

    This circuit uses a 4049 Hex Inverter to generate a square wave. This square wave switches current on and off to a Mosfet IRF830, which in turn switches current to the transformer.

    I am not sure what the problem is. I double checked everything and it seems my circuit matches the schematic. I am wondering if I chose the wrong transformer even though I follow the specs in the book. (the book just says to use a 8 to 1200 ohm audio interstage transformer) The transformer I used has 810 primary windings and 80 secondary windings. I tapped it halfway through the secondary windings to essentially give me an 810/40 ratio. I then ran the transformer through the secondary windings up the voltage into the primary. I am only getting 240 volts out the primary.

    Any suggestions? Do you think my problem is in the transformer? Do you know what else could cause the lack of voltage? Thanks in advance. I am new to this electronics stuff so please go easy on me.

    The transformer I am using...
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMv0IfuNuy2LUd1oaj1xuSl7ovuhlClV6Fw=

    The schematic for the Geiger Counter...
    http://www.freeinfosociety.com/electronics/schemview.php?id=2064
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Try swapping the leads of just the transformer primary or secondary.

    You may also need more secondary turns.

    Where did you get the geiger-muller tube? I have not seen many listed by the surplus sellers lately.
     
  3. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I bought it from the manufacturer directly. They will sell directly to consumers but there is a $100 minimum purchase. So I ended up buying 2 tubes. I am using the popular LND 712 tube.

    http://www.lndinc.com/
     
  4. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    Tried that. Still getting 240v.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,785
    945
    What freq is the sq wave you are generating? Your transformer may have poor high freq response. Try lowering the freq of the 555 and see if there is any increase in voltage.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
  7. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I tried running the 9v battery directly to the transformer (bypassing the 7805 regulator that the rest of the circuit is on). This made the 830 Mosfet very hot right away. I disconnected it before I measured the voltage so I am not sure what voltage was produced. But I do know it would have destroyed the 830 Mosfet very quickly.

    I am not sure what frequncey the square wave is running at. The book does not specify and I don't have the means to measure it.

    The frequency is adjusted with the 5k pot. I tried changing the resistor next to the 5k pot to see if I could up the voltage but it tops out at 240v no matter what resistance I put in its place.
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Sorry I was attempting to edit my post & deleted instead.

    On reflection the operation is more complex than the simple voltage doubling case I first thought.

    The behavior will depend on many factors including the frequency, transformer inductance, high voltage loading ...

    The operation will more resemble a fly-back performance. The "doubling" type circuitry will boost the output by taking advantage of the on drive state induced voltage.

    It probably comes back to the transformer specs - particularly the inductance and operating frequency. If you know the transformer specs in greater detail then it might be possible to improve the results.

    Are you still getting 240V in the unloaded state - i.e. with no connection to the tube?
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Yes - that suggests the drive side was saturating at the higher 9V supply value.
     
  10. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I get 240v with no connection to the tube.

    Here are the transformer frequency specs from the data sheet. Keep in mind I an new to electronics and I don't understand how to apply these numbers into the circuit.

    Frequency response: +/- 3db, 300Hz-3.4KHz @ 1KHz 0dB
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Another question - what do you get if you use the full 80 turns on the drive side instead of 40 turns?
     
  12. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I get 120v using 80.
     
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    OK - I checked the data sheet you linked and it shows the impedances at 1KHz for both primary & secondary.

    These are Zp=1.2KΩ @ 1KHZ and Zs=8Ω @ 1KHz

    These are predominantly inductive (based on the resistances also supplied)

    So at the 80 turns inductance Ls=1.27mH and therefore ~318uH (one quarter) at the 40 turns tap.

    The primary inductance would be 190mH. There's no information about the saturation issue - or rather the duration over which the driven winding can support the 5V before saturation occurs. One could make some assumptions and try to simulate the circuit behavior. I'll have a look and see if I can suggest anything.
     
  14. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Best i can read on the circuit are D4/5/6 are IN5261/71/81 Zenner diodes 45/100/200V these dont ad up to 500V try removing & see what the voltage is. Circuit is in first post.
     
    Last edited: Apr 8, 2011
  15. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    post your circuit please.
     
  16. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I certainly appreaciate all the help!

    I looked for other transformers that meet the same 8-1.2k ohm specs that the book calls for. The transformer I have seems to be the only one that meets the 8-1.2k spec.
     
  17. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    It is hard to read but there are actually two 1N5281 diodes and one 1N5271 for the total of 500v. 1N5281 (200), 1N5281 (200), 1N5271 (100) =500v for the regulator.
     
  18. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    I ran some simulations - guessing the saturation conditions and other factors. The result is strongly dependent on the type of core material and the frequency & mode of operation.

    Interestingly your transformer has a bandwidth of about 3kHz but your drive oscillator looks to have a frequency of around 20-30 KHz - based on the 4049 oscillator timing components of 5.6KΩ and 0.0047uF.

    There is a significant gap between the nominal frequency range for the transformer and the drive frequency.

    You might try bring the frequency down to see what happens - say by increasing either the timing R or C value by (say) 20%. Use a 6.8K instead of a 5.6K. But you may just run into the same problem with the core saturation and drive FET overheating.

    Other options might include finding a transformer with wider bandwidth and higher saturation current. One could wind something from scratch using a suitable ferrite core. Again that involves some design work.

    The control pot looks to be more like a pulse width control for the FET drive on time rather than a frequency adjustment.
     
    Last edited: Apr 8, 2011
  19. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    It's just occurred to me to ask where and with what you are measuring the 240V DC.

    If you are doing this with a DVM at the J1 terminal then you may not have a problem at all.

    If you are using a DVM with an input resistance of ~10MΩ on the J1 side of R4 then with 500V on the other side of R4 you would expect to get a reading of about half the actual value. The input resistance of the DVM would form a 50/50 voltage divider with R4 in such a measurement situation.

    Just a thought in case you are chasing a problem that may not exist.
     
  20. rustypigeon

    Thread Starter New Member

    Jul 25, 2010
    21
    0
    I am reading the 240v at the junction of D2 and C4... Just past the voltage doubler.

    I am using at Craftsman digital multimeter with 10M ohm impedence. I did try to read the voltage at J1 and of course got about 120v since the 10M resistor was acting as a divider.

    Again I certainly appreciate all the help and the time you have taken. I will try changing the resistance and cap values and see what happens. Thanks!
     
Loading...