Voltage Switch

Discussion in 'The Projects Forum' started by emmy, Aug 14, 2006.

  1. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Greetings yet again,

    My project progresses slowly an I have now come across a new challenge.

    I have a 24V supply.
    This charges 2 X 4700uf Caps.

    Hence I have them charged and ready to go.

    I am working on a simple charging and charged LED indicators as I type.

    Currently a row of toggle switches would release the now abundant voltage to a fuse and bang goes the fireworks.

    I need to replace these toggle switches with a device that can be triggered via a logic circuit.

    1, Toggle switches are expensive and cumbersome
    2, This will allow either a small data switch 5 or 12V (Toggles are rated a 5 amps) or a logic circuit say 555 timer for a chase etc.

    There are either 24 or 32 seperate circuits per unit so a PCB solution would be an advantage.

    OK my question is what should I use as the logic switch SCR/TRIAC/Power Transistor ?

    Max power will be maybe 3 amps @ 24 Volts
    Fuses require 800mA at 4ms for firing
    Fuses are save at less than 180mA for 5 Mins
    The fuse heads do have a habit of shorting so the trigger must switch on and then off. (This I believe may be a problem for SCR's)
    A timed switch might be an option say closed for 300 - 500 ms then open.

    If there is any other info please ask I believe I can find some additional info on firing pules etc on the fuses if required .

    Any suggestions which particular path I should try walking ?

    Thanks

    Emmy
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Since you seem to be dealing with pyrotechnics, I believe the term is "fuze". A fuse is a circuit protection device.

    You might consider a double pole double throw relay. In the deenergized position, it can hold the line out to the fuze shorted, insuring a bit more safety. When energized, the capacitor bank can dump power to the fuze. A 555 timer arranged as a triggered one-shot can pull in the relay/s for the required time.
     
  3. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Beenthere,

    We actually call them fuseheads.
    But I get your point.
    They have safe working voltages and if you exceed the safe working voltage they go bang just like a fuse except they are wrapped in cool magnesium and stuff that then inturn set fire to black powder etc.

    Some specs

    Bridge wire resistance 1.5 - 2.5 Ohms
    No Fire current 0.18 Amp / 5 min
    Fire current 0.8 Amp / 4 msec
    no firing impulse 0.8 mj ohm -1
    firing impulse 2.5 mj ohm -1

    As to relays I had thought od that but would like to get away from them if I could.
    SCR's and other semiconductors seemed a better option if I want to use TTL logic later down the line to control the switch.
    Also the cost on many relays may price them out of contention as I need to make several of these device. Space is also a concern I would like to keep it as small as possiable.

    Emmy
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Might try an IRF630, IRF644, IRF840 or similar. They can be had surplus for a buck or two apiece. Make darn sure of that firing window timing - the MOSFETs can make fireworks of their own when their current rating is exceeded.
     
  5. Spoggles

    Well-Known Member

    Dec 2, 2005
    67
    0
    Emmy:

    Do these things open after they fire?

    Seems like you could use a SCR with a constant current limiter. It would also not be too difficult to provide a I limit of 1A for say 1sec and reduce it after this interval if the circuit has not for some reason opened. (or is shorted)

    (As you have indicated that they sometimes short).

    Spoggles
     
  6. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Spoggles,

    That's the theory but in practice 1 in 10 will fire and end up short. Basically fires and fuses the thing to short Circuit.

    This is why a timed circuit is required or a switch that is only on when triggered an SCR may cause a problem as once on I believe the have to be triggered off again.

    Maybe this might do it ?
    http://pdf1.alldatasheet.com/datasheet-pdf/view/24331/STMICROELECTRONICS/STP16NE06.html

    One of the problems here in Aus is standard componets like IRF510 can cost $4 to $6 each.
    The one I have linked to I can get for $2.00 But will it do the job ?

    Emmy
     
  7. Spoggles

    Well-Known Member

    Dec 2, 2005
    67
    0
    Emmy:
    The datasheet for the STP16NE06 indicates that Ids (current Drain to Source) with 5V applied from gate to drain is about 2.5 amps and is independent of power supply voltage.(ie constant current)
    (Ids goes to 0A at 4v). Could you not just apply a TTL level pulse for the required time? A little 'tweeking' with a voltage divider might get you to around 0.8A.
    Operating in the 'linear region' means that no matter what happens to the load or the power supply voltage (<<60vdc), you will never pull more current than you design for. 1 amp Ids is on the lower limit of linear transfer and you may have some problems tweaking this a keeping it where you want it. You should be just fine at 2.0A however.

    Spoggles
     
  8. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Spoogles,

    Have tried it and need to play with a bit but I think we may have a solution.
    Linear switching works fine I will play a little with the TTL later.
    Thanks for your help.

    Emmy
     
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