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Thread: general electronics questions

  1. #1 general electronics questions 
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    Hi everyone I have some questions about batteries etc. Firstly it possible to take 2 12V battery and take 24v and 12v simultaneously?? I've tried it and it didn't work I had to use another 12v battery to get the separate 12v. Why is this so?? And why do different circuits which work together in a machine must have their grounds common??? And what is the proper way to solder without getting too big/thick solds??? And lastly is there any alternative to using Mosfet for PWM?? From my experience Mosfet get burned often. Thanks in advance


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  3. #2  
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    Quote Originally Posted by fine View Post
    Hi everyone I have some questions about batteries etc. Firstly it possible to take 2 12V battery and take 24v and 12v simultaneously??
    Sure. Make a connection to the junction of the two batteries. That will give you a 12V tap.


    I've tried it and it didn't work I had to use another 12v battery to get the separate 12v. Why is this so??
    Since you didn't tell us what you did, no one can say. But if you do as I describe above, it will work.


    And why do different circuits which work together in a machine must have their grounds common???
    They don't have to. There are many circumstances when you expressly desire to have two circuits completely isolated. It depends on what problem you're trying to solve.


    And what is the proper way to solder without getting too big/thick solds???
    Use less solder.


    And lastly is there any alternative to using Mosfet for PWM?? From my experience Mosfet get burned often. Thanks in advance
    If parts are burning up, that's not a MOSFET problem. It's a bad-design problem. To fix it, use a better design. Study all device specifications, and make sure your circuit stays within them under all operating conditions. Most important are voltage, current and power/thermal limitations.

    And no need to use multiple question marks. One will do.


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  4. #3  
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    I agree with tk421's remarks.

    Using less solder might involve changing your technique or using a thinner solder. I find thinner solder easier to control.

    If you're still having troubles, show us the schematic.
    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
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  5. #4  
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    OK, here's another of my terrible diagrams: This is two 12V batteries connected in series. A is the positive terminal of the top battery. At B the plus terminal of the lower battery connects to the minus terminal of the top battery. C is the minus terminal of the bottom battery.



    | A
    [] 12V
    | B
    [] 12V
    | C

    Voltages in series simply add, so between points A and C you will now have 24V, and between A and B or B and C you will have 12V.
    If you want both 12 and 24 volts in the same circuit, take the 12V from B and C, and the 24V from A and C.
    C is the common earth terminal.

    Soldering:
    Make sure that what you are soldering is clean and free from grease etc.
    Make sure your iron's tip is also clean.
    Use solder with flux inside it.
    Wait for the iron to reach operating temperature, a couple of minutes.
    Dab a little bit of solder onto the tip of the iron - it should melt and coat the tip surface.
    With a damp sponge, wipe the tip on the sponge - you should be left with a clean tip that has been 'tinned' with a thin layer of solder.
    Dab a little more solder onto the tip to make a slightly thicker layer.
    Place the iron tip underneath the junction to be soldered if possible.
    Start trying to melt solder onto the other side of the junction and as soon as it flows, add just enough to flow into the junction.
    Don't add any more solder.
    Take the iron away.
    Don't disturb the junction for 10 seconds.
    DON'T blow on it, let it cool by itself.

    A good solder joint should be shiny and the solder should have flowed into and should have coated all the joint. If it is dull, or sat like a blob on top of the joint, it is a bad joint, which we call 'dry'.

    Repeat wiping the iron's tip on the damp sponge and reapply a small amount of solder every time before you solder a new joint. Many soldering iron stands have a sponge holder for this purpose.

    If you are still struggling, you might need a better or hotter iron. Cheap ones do not get hot enough, meaning that when you put the iron onto the joint, too much heat is taken away from the iron tip for the solder to melt and flow.

    Practice practice practice, it is a bit of an art and skill.

    Good luck!

    OB
    Last edited by One beer; June 18th, 2014 at 05:34 AM.
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  6. #5  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by One beer View Post
    Soldering...
    Imagine that I clicked like. And it worked...
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Thank you Strange, I've just added to my post slightly. I hope fine finds it useful too.

    OB
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  8. #7  
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    Quote Originally Posted by tk421 View Post
    If parts are burning up, that's not a MOSFET problem. It's a bad-design problem. To fix it, use a better design. Study all device specifications, and make sure your circuit stays within them under all operating conditions. Most important are voltage, current and power/thermal limitations.
    Yes, all operating conditions.
    Keep in mind that driving an output device with an extended slope input multiplies the power dissipation by keeping the device in its active region for a longer average time. Inductive kickback can exceed device voltage ratings and instantly destroy the device if not clamped.
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  9. #8  
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    Hey fine,

    If people take time to read your question and write you a detailed reply, it would be polite to say thank you or at least acknowledge their time in some way.

    OB
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  10. #9  
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    Quote Originally Posted by fine View Post
    Hi everyone I have some questions about batteries etc. Firstly it possible to take 2 12V battery and take 24v and 12v simultaneously?
    Yes, done all the time
    And why do different circuits which work together in a machine must have their grounds common?
    They don't. However if you want 12 volts to device A to be seen as 12 volts by device B they have to have common grounds. You can get around this with various isolation designs but they are complex.
    And lastly is there any alternative to using Mosfet for PWM?? From my experience Mosfet get burned often.
    Sure. Bipolars, IGBT's, triacs, SCR's - all can work. However MOSFET's are usually an excellent choice up to about 200 volts.
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  11. #10  
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    Quote Originally Posted by G O R T View Post
    Quote Originally Posted by tk421 View Post
    If parts are burning up, that's not a MOSFET problem. It's a bad-design problem. To fix it, use a better design. Study all device specifications, and make sure your circuit stays within them under all operating conditions. Most important are voltage, current and power/thermal limitations.
    Yes, all operating conditions.
    Keep in mind that driving an output device with an extended slope input multiplies the power dissipation by keeping the device in its active region for a longer average time. Inductive kickback can exceed device voltage ratings and instantly destroy the device if not clamped.
    well the circuits involving MOSFET that I've used often have the same issue i.e the MOSFET get really hot and eventually burn out. i have used several different schematics for several different purposes involving MOSFET but it's usually the same result. Sometimes the problem is fixed if the MOSFET is changed.
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  12. #11  
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    Sorry OB i was out of town. I really found your post to be extremely helpful
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  13. #12  
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    The issue I had with the batteries was that i needed a 12V supply and a 24V one, I used two 12V batteries in series and got 24V as i should be, then i tried to get 12V from one of the two batteries and it didn't work. Using OB's design maybe i can explain what happened.
    | A
    [] 12V
    | B
    [] 12V
    | C

    i got 24V from A and C, then i tried to get 12V from A and B. A terminal of both supplies was made common and i expected to get 24V from C and 12V from B but I got 12V from both of them and then something short circuited and went kabloom
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  14. #13  
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    Quote Originally Posted by fine View Post
    The issue I had with the batteries was that i needed a 12V supply and a 24V one, I used two 12V batteries in series and got 24V as i should be, then i tried to get 12V from one of the two batteries and it didn't work. Using OB's design maybe i can explain what happened.
    | A
    [] 12V
    | B
    [] 12V
    | C

    i got 24V from A and C, then i tried to get 12V from A and B. A terminal of both supplies was made common and i expected to get 24V from C and 12V from B but I got 12V from both of them and then something short circuited and went kabloom
    You didn't label any polarities. Here's how you should have connected them:

    +A (will give you +24V)
    -A +B (will give you +12)
    -B (this is your zero volt reference -- "ground") [ETA: Eliminated reference to a non-existent battery C; here, A and B are the batteries -- sorry for differing in notation from OB's]

    I'm willing to bet that you did not hook things up this way.
    Last edited by tk421; July 14th, 2014 at 10:52 AM.
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    umm no i didn't. The A terminal was black (negative) B was a combo of 1 battery's negative and one battery's positive. C is the other battery's positive. Which one will be the ground reference? I think it will be A am I right?
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    OK fine, good to see you back.

    Go back and read my post #4 VERY carefully, I answered your question there. Wire your batteries as I say:-

    A is the + of the 'top' battery.
    B is a junction between the - of the 'top' battery and the + of the 'bottom' battery.
    C is the - of the 'bottom' battery.

    The 24V comes from A (+) and C (-).
    The 12V comes from B (+) and C (-).

    C is the common terminal. It is the minus of both the 24V and the 12V circuits joined together.


    OB

    PS, tk421 He needs to take the 12V from B and C, using C as the common earth. If he takes 12V from A and B, in the same circuit as the 24V and 'commons' both negatives, he will short the 'bottom' battery.
    Last edited by One beer; July 13th, 2014 at 05:32 AM.
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    I've tried to draw the full circuit, but the system removes the spaces I put in so it garbles my diagram.

    Anyway, if you wire the batteries and circuits as I've said, it will work.


    OB
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  18. #17  
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    Quote Originally Posted by One beer View Post
    PS, tk421 He needs to take the 12V from B and C, using C as the common earth. If he takes 12V from A and B, in the same circuit as the 24V and 'commons' both negatives, he will short the 'bottom' battery.
    Yes, indeed. Sorry for both switching notation (in an attempt to bring individual polarities explicitly into the drawing) and introducing an error. In my (intended) notation, A and B represent the batteries. Thus "+A" is the positive terminal of one battery, and "-B" is the negative terminal of the other. I will edit my previous post to eliminate a spurious reference to a C.
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  19. #18  
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    No problem.

    In my posts, the A B C refer to the terminals/junctions.

    My crude drawing of a battery is this:

    |
    [] 12V
    |

    The box shape next to the '12V' label is a battery, and the vertical lines are terminals coming out above and below it. The top line is the positive terminal, the bottom line is the negative terminal.
    I connected two batteries together in series and labelled the terminals/junctions A, B, C.

    Perhaps I should have drawn it like this;

    A
    |+
    [] 12 V battery
    |-
    B
    |+
    [] 12V battery
    |-
    C

    So;

    A is the +(positive) terminal of the top battery.

    B is a connection between the -(minus) terminal of the top battery and the +(positive) terminal of the bottom battery.

    C is the -(minus) terminal of the bottom battery.


    So to derive both voltages the OP needs;

    take 24V from A(+) and C(-)

    take 12V from B(+) and C(-)


    A will feed the 24V users/circuit

    B will feed the 12V users/circuit

    C will be common return for both circuits


    If I could only draw a proper diagram, and put it on this forum, it would make it a lot clearer.

    OB
    Last edited by One beer; July 15th, 2014 at 12:29 PM.
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  20. #19  
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    Quote Originally Posted by One beer View Post
    No problem.

    If I could only draw a proper diagram, it might make it clearer to the OP.

    OB
    It's too bad that we can't post pics; it would make things a lot easier. If I weren't so lazy, I bet I could find a drawing on the net somewhere, and post a link to it.

    ETA: Here's one: http://bus.getdave.com/Docs/12Von24V/Pics/Split.gif

    @fine: "bus ground" is the terminal that is common to both +12 and +24V supplies. It is the zero volt reference terminal.
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  21. #20  
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    Yeah, that's near enough.

    Thanks.

    OB
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    fine, you have asked so many questions, I’ll probably have to refer you to an electronics and electrical circuit basics. Anyways, to answer a few, you can take out 12V from a 24V battery pack but it’s not recommended as the batteries/battery being used for 12V will get depleted quickly and that will affect the 24V power source that you want. Grounds have to be common so that voltage levels across different devices are comparable with each other, some at a higher potential, and some at lower potentials.

    circuit board manufacturing
    Last edited by arin; October 1st, 2014 at 12:54 PM.
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