Help connecting cap in series with battery using transistors please

Discussion in 'General Electronics Chat' started by warp, Jan 20, 2014.

  1. warp

    Thread Starter New Member

    Jan 20, 2014
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    Hi,
    Im pretty new to electronics and i like to play around with little homemade motors and stuff.

    I was needing to charge a cap and then switch it to the battery to discharge through the motor to give it extra pep if necessary. I used a tip3055 npn to charge the cap to 12V battery voltage ok, but when using the same type of transistor to connect the cap to the positive, with 12 volts on the base I couldnt get my 24 volts.?

    Do I need to go to a transistor that can switch 24 volts with 12V or less on the base. If so can someone please recommend a suitable one, Also even though it is between the battery and the cap and they are on the high side of the motor coil, would I need pnp or p channel.?

    I would probably prefer a bpj because they are cheap. I was reading about a buz71 that looked cheap enough would that do?

    Thanks in advance.
     
  2. warp

    Thread Starter New Member

    Jan 20, 2014
    18
    0
    I forgot to mention I would like to be able to switch up to 5A.

    Thanks again.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A cap and one transistor cannot do what you want to do. I am only guessing because your description is not clear. Could you start with describing your goal.

    Then show the schematic of what is not working - with a description.

    It sounds to me like you plan to get double the voltage from a circuit just because you add a cap and a transistor to a circuit. And that the cap will be a able to increase the current. And you want to continuously run a motor of higher power than your battery can supply just because you add a cap.

    Correct what is wrong with my paragraph or write your own.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How long do you expect the capacitor to boost the motor voltage each time you switch the capacitor in series with the battery?

    Here is a simulation showing how impractical this idea is! I have taken a 1 Farad capacitor, charged it to 12V (same as battery voltage ), and then connected it in-series with the battery to supply a constant 5A load. The 5A load is not a very good model for your motor; it is a worst case scenario.

    I have added a diode to prevent a reverse polarity across the capacitor after it completely discharges.

    I plot the motor voltage, the battery voltage and the diode current.

    Note that the charge stored in a 1Farad capacitor can only provide a boost voltage for about 2.4 seconds. To further convince you that this is not a practical way to go, look at the prices of such large electrolytic capacitors, here.
     
    Last edited: Jan 20, 2014
  5. warp

    Thread Starter New Member

    Jan 20, 2014
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    Thanks guys
    The motor is home made and the coil operates on a 50% duty cycle or less.
    I was intending to charge it on the off time and discharge on the on time continuiosly charging and discharging. When discharging it will be competing with the battery current. And yes I would use blocking diodes. I would also put in a manual switch so I would could choose more grunt or not. I dont expect to run the motor at twice battery voltage continuosly.

    I also want to make variable voltage buck boost or dc to dc converter to run little nick nacks of mine at different voltages over battery voltages.

    There have also been other things Ive wanted to try and have needed to connect battery to cap with a transistor.

    What I am hoping to see on the bench before I can proceed is. A 12V battery
    connected to a transistor which is connected to a capacitor. When the transistor is on I want to see close to doulble battery voltage across the two in series. Ive tried TIP3055 and its complimentary partner but the voltages have been far to low.

    If someone recommends suitable transistor so I can achieve this I am more than happy to discuss the ins and outs of the rest of the circuit/circuits.

    Thanks again.
     
  6. warp

    Thread Starter New Member

    Jan 20, 2014
    18
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    I forgot to mention that the capacitor would be charged to battery voltage
     
  7. warp

    Thread Starter New Member

    Jan 20, 2014
    18
    0
    Actually Thanks MikeMl for running that simulation, I can see from that I would need to charge it on an independantly timed circuit probably into the kHz to make an impact on a motor that size. Probably more hassle that what its worth.
    That means I will be needing the variable voltage dc to dc converter. I want to have the fun of making it myself so I am still keen to know a good transistor for my original question.

    Thanks again.
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It sounds easy, but alas it is not.:(
    In order to charge the cap you need to hook it in parallel with the battery. Then once charged hook it in series with it.
    A little boost regulator might be a better solution.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    How about a second battery or a bigger motor that could run on less voltage?
     
  10. warp

    Thread Starter New Member

    Jan 20, 2014
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    Hi inwo,
    I was starting to get a bit annoyed with this situation but with a sense of humor like yours youve made me smile. I too pull most of my ideas from my but,..... but I try to limit that to just ideas
    .
    The motor idea is off the table and the variable voltage dc to dc converter is on.

    Im not sure just how hard this is going to get. What have I got so far?

    1 An ne555 configuered as a variable frequecy astable multivibrator set to 50% duty cyle. check
    2 Two intermittantly toggling (while one is off the other is on) transistors at the output ne555 capable of supplying upto an entire amp of switching bias current at battery voltage 12 V if required check

    3 One transistor capable of allowing capacitor to be charged to 12v battery voltage when placed in the ciruit where I want it, and will suffice for now check

    4 One cheap transistor for prototype that can switch comfortably 24 volts with only 12V bias current for connecting battery to capacitor so that increased voltage can be introduced to buffering capacitor. No check, nadda, nothing

    A part of me once thought that most of the harder stuff had been taken care of, but now I am starting to think otherwise. But I will keep trying.

    Can anyone plese reccomend a suitable transistor that can be used as number 4 on my checklist.

    I have to do things my way and learn from my own mistakes. Inwo dont ever stop kidding around you make this world a brighter place.
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Take a look at this one.

    The 555 and transistors charge the cap to 24 volts. The pFET adds it to the +12.
     
  12. warp

    Thread Starter New Member

    Jan 20, 2014
    18
    0
    Wow Ok. Thankyou very much RonV. Actually thanks to all you guys.

    The simulation and other suugestions all very helpful. This circuit goes about things in a much smarter way than I would have, thats probably why its taken me a while to get my head around it. Sometimes I get annoyed with myself for being so silly.

    Toggling the npn and the pnp of the same bias connection, I never would have thought of that. I cant help but be a little curious about Q2 and why its not the complimentary pair partner to Q1. the numbers seem too different, but I will do it as shown.

    Sort of wierd when I tried the tip3055 to connect them with I got the low voltage reading.

    Im thinking I will snip the circuit just before d2 and just after d3 making sure the battery is still connected to the positive side of c8 via d4, then connect d3 to a small cap and let it rip for a few cycles with no load. The voltage on the small cap should confirm that all is well.

    Then I will have something that is very useful to me that I can build onto. That circuit addresses precisely where i was getting hung up.
    Ive been at it for to or three days now and its been a long road to seeing the light at the end of the tunnel. I couldnt be more greatful.
     
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