Fuse placement and sizing

Discussion in 'General Electronics Chat' started by RandyFL, Jun 5, 2015.

  1. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Hello All,
    I am new to this forum. I was interested in the subject of fuses... Where would you put a fuse on a breadboard? I have built a Johnson cascade counter using a 555 two 4017 s and a 4081 that flash 16 Led s in sequence. My first build I accidently used a low power 555 from RS and when I connected to a 12 volt lawnmower battery it went pffft ( it fried in a picosecond ). I just checked the fuse I have in the red positive cord and its 10 amps ( way too much amperage ). So my question is I would like circuit protection up front but I don't want to keep changing fuses every time I connect to a strong 12 volt battery. The reason for the 12 Lawnmower battery is I have a BDX53c ( 8 amp max ) transistor I want to kick in at some point in my build... The breadboard is only rated at 1 amp and I had the BDX53c in a breadboard next to the breadboard that has 555 etc. Hence when I connected to the 12 volt lawnmower battery yesterday it blew the 10000 uF 25 volt cap, 555 and everything south of that... Plan B ...Today I rebuilt the circuit and I'm using a 9 volt battery to power the 555 etc.. Getting back to my question how do I calculate my fuse.... Here's the list of parts to the circuit... the fuse, a diode, the cap. the 555 etc. ....do I calculate my weakest ( or maybe the cheapest ) part... I will use the 9 volt for the semiconductors on the 1st breadboard and I will use the 12 volt lawnmower battery for the BDX53s and use a barrier strip ( block ) with the 10 amp fuse there... The 555 is configured in Astable mode so there is a 10 nf and a 100nf caps on the 555...the circuit came from looking at different 555 circuits and looking at the data sheet for the 4017 s. I could have started with the lowest rated fuse and work my way up but instead I wanted to find more experienced electronics people and how they would tackle the question...

    All the Best
    Randy
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    All of the parts you mentioned will tolerate 12V so a 12V battery, regardless of amperage, shouldn't have fried anything. Look for circuit/wiring issues that would have issues with a higher amperage power source.

    Do some research on fuses. You'll find that they won't work in the manner you desire. A typical 1A fuse will handle 2X that for longer than you'd think. It would probably be better for you to use or build a current limited power supply, or use a PTC fuse. A PTC fuse won't react quickly, won't interrupt current, and will vary in resistance based on current.

    Also, you should start your own thread instead of hijacking one.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    I've split this off from the following thread:

    http://forum.allaboutcircuits.com/threads/fuses.111114/

    While you were, indeed, talking about fuses, the TS of the original thread was asking a question about fuses that was completely unrelated to you query.

    Now you have your own thread so that the discussions don't get comingled, which results in chaos.

    Welcome to AAC!
     
  4. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Thank You.
    TS ( ? )
    As per my original post ( if that's the right word ( question )). My intent was to generally protect the circuit en mass ( as other circuit designer's do/did ). The original 555 ( that I bought in haste ) was a low power 555 and I just used it quickly to build a circuit using 555's. I used a 9 volt battery un fused and partially dead to build the original Johnson cascade ( to get it to work correctly ). I use all my partially dead 9v to power circuits so I don't have to put resistors in front of the Led s ( and use resistors on fresh batteries ). The circuit that I built works whether its a 9 volt or 12 volt lawnmower battery ( I've tested it ). I'm working backwards... when I knew I was going to be using a heavier 12 volt battery I purchased a fuse cable and a fuse was already there ( or I bought some )... anyway there was a 10 amp fuse in there. That original Low power 555, hooked up to the 12 lawnmower battery, burned in a second but also burnt the breadboard...but it could have also burned our dinning room table and possibly my house if I physically wasn't there...........My question is this... I want my circuits to be safe. And I want my main defense against shorts or stupidity to be fast, accurate and simple to maintain. How do I calculate the energy used in a circuit ( or do I just calculate the weakest part and fuse for it ) and fuse one times that. What I had planned to do was just get a circuit breaker ( two actually ( one rated at 1 amp and the other one .5 amp )) and then put a thru hole 150 mA fuse after it... then build my circuit. I know that would just solve the problem but I want to " understand " the problem...

    All the best

    ps I have a zillion questions and I don't want to hijack again...do I blunder my way thru to learn how to use this forum or is there some place that has the correct protocols...
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi RandyFL,

    Welcome to AAC. You'll find that you'll get more useful feedback if you post wiring information (e.g. a schematic, part numbers, etc) to help people visualize the question/problem. Don't just post a picture of your breadboard; few will take the time to trace every connection. I did this once for a poster who had a circuit that wasn't working and he never responded whether that was the problem, so I personally won't waste my time tracing.
    TS is Thread Starter which some of us who have used other forums call OP (Original Poster).
    I assume you mean a CMOS 555 such as LMC555 or TS555, which I mentioned in my original post are 12V tolerant.
    I assume this means this type of battery
    200px-Duracell_9_Volt_0849.jpg
    The amp hour rating of this type of battery is quite low and a weak battery wouldn't be able to cause much damage; or provide much power...
    As I mentioned earlier, all of the components you listed will operate at 12V or more, you should check for wiring issues.
    Did you search for information regarding fuses (normal fast acting or PTC)? If you did, you'll find that the type of fuse you mentioned in your original post will handle 2X the rated current for longer than you'd expect (minutes). At 10X the rated current, it will open in seconds. If you want to protect against fire, this is probably sufficient. If you want to protect components from damage, that's not the way to do it.
    I think it would be better for you to understand what caused the original problem before you get into solution space. Post a schematic of how you had things connected.

    You mentioned that you had a 10,000uF 25V cap that the 12V battery fried. About the only way a 12V (several amp hour) battery could damage the cap would be if it was connected backwards. A weak 9V transistor radio battery can't cause much damage in that situation. Also, if you're using that type of cap for decoupling, you'd get better results with a much smaller cap (like 220-470uF) with a 0.1uf ceramic cap in parallel (this cap has better high frequency response and will decouple switching transients).

    FWIW, when I'm concerned about frying parts, I use a current limited power supply. These are relatively inexpensive these days, or you could build your own based on an LM317. That would give you an inexpensive current limited, variable voltage power source.

    When you're breadboarding, be mindful of exposed leads creating inadvertent shorts. Without more information on your setup this, or some other connection problem, is my best guess as to what caused components to smoke.

    To answer your question... If you really want to use a fuse instead of understanding what caused the problem; calculate the maximum current that will be drawn by all of the components in your circuit and select a fuse of that capacity or slightly higher. Just be aware that even fast acting fuses won't interrupt current fast enough to protect all of the components.

    I'm of the opinion that fuses were intended to prevent fires, not protect components from damage. Recently I was troubleshooting a power supply and accidentally shorted one of the regulator inputs to ground. The fuse did blow, but not before 2 1A rectifier diodes had been fried. The short was brief (less than a second), but the diodes both developed shorts and that's what blew the fuse (2A slow blow).

    EDIT: Just noticed that you are in the habit of connecting LEDs without current limiting resistors. This is a questionable practice and you need to know what you're doing if you do this. A higher current supply could damage LEDs that aren't current limited and the associated drive circuitry; but not the big cap (I'd look for a wiring problem for the cap).
     
    Last edited: Jun 7, 2015
  6. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Hello All,

    Dennis,
    Thank you for your info... I have been studying fuses ( what you have suggested and from Electronics All in One - Doug Lowe ). Is there a pc program ( or anything where I could put the circuit as I have used it ) as I would like to be able to show ( post ) it...
    And yes it was 9 volt batteries as you have inferred ...Maybe the leads of the cap touched or since I didn't have a " load " connected to the BDX53 it ( whatevered )...Anyway I would like to draw out the circuit ( or take a circuit from the web and add to it ) and present it here to peruse...

    All the Best
     
  7. RandyFL

    Thread Starter Member

    Aug 28, 2014
    116
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    IMG_1935June7a.JPG IMG_1937june7b.JPG IMG_1938june7c.JPG IMG_1939june7d.JPG IMG_1940june7e.JPG IMG_1941june7f.JPG IMG_1942june7g.JPG

    The 555 that fried was TLC low-power Timer IC ...LinCMOS timer from RS # 276-1718
    the Fuse that's in the red cord is a slow-blow 10-amp 32 volt fuse
    and the cap that just fried is a Liberty 10000 uF 25v electrolyte - maybe I leaned into it and it touch the other lead. Which would cause...? ( everything south of it to fry...? )
    btw I have a video of the new circuit running on 9 volts but I don't know if this forum accepts videos...

    All the Best
    Randy
    PS what is ( FWIW) and [snip] and I wouldn't want anyone to have to trace from a breadboard by way of a picture but the video would have shown this circuit indeed runs.......
     
    Last edited: Jun 7, 2015
  8. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Lastly...I am very much interested in knowing what caused the frying...and acquiring a DC power supply ( and to build one or others )...but I would like to bring the 10 amp fuse to a reasonable amperage while I continue on

    All the Best
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd recommend a PTC fuse instead of the melting wire type to reduce cost and waste. PTC fuses don't interrupt current, but they reduce it significantly and are self resetting when the current through them is reduced. The downside is that they have resistance that increases with temperature.

    I use the free version of Eagle for schematics.
     
  10. RandyFL

    Thread Starter Member

    Aug 28, 2014
    116
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    Hello

    Another question...How would you calculate the maximum current that would/will be drawn by all of the components in my/your circuit...
    The datasheet on each component or an ammeter or...? after watching a wire melt, a 555 catch on fire and cap smoke... I would be leery in ammeter ing thru

    All the Best
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    Add up the worst case currents used by each component. Supply current requirements will be given in datasheets (plus any loads you add).

    The 555 timer you used probably draws less than 1mA, plus whatever load you're driving. The CMOS logic you were using is probably in the same ballpark. Without current limit resistors for the LEDs you used, you can't calculate a worst case current. When operating from batteries, you use the full charge battery voltage for calculations.

    PTC fuses have a normal current (won't trip), trip current (will trip), and resistance (larger than wire fuses). They are also more limited in maximum voltage.
     
  12. RandyFL

    Thread Starter Member

    Aug 28, 2014
    116
    0
    Dennis,
    I thank you for your help in this... List: 2 Led s ( 5mm Red clear 624nm 30 deg ), 2 470 ohm 1/4 watt resistors ( for the Led s :), 1-555 2-4017 s 1-4081, 4 black 22 awg solid hook up wire ( neg. rails ), 4 red 22 awg solid hook up wire ( pos. rails ), 2 green awg solid hook up wire ( for the 555 ), 1 large cap 10000uF 25V - 1 104 ceramic cap - 1 103 tantalum cap, 1 1meg pot. with a 2.2k resistor, 3 green awg solid hook up wire for the pot., 5 breadboard pre made hook up wires, 1 diode 1N4181 and 2 large hook up wire ( one black and one red ) and a 10 amp fuse... I don't expect you to look up each component ( and I wouldn't ask you to ).

    ex. 1 diode 1N4181
    datasheet
    http://www.vishay.com/docs/81857/1n4148.pdf


    1. what rating in the datasheet would you use... ( I would pick the peak forward surge of 2 amps ) if every component has similar ratings it could easily pass the 10 amp of the fuse I already have in there... so I am confused...

    2. The " load " would be everything hooked up to battery...correct?

    3. Would you suggest a DC power supply or a DC power supply schematic...

    All the best

    PS I noticed the triangle at the bottom says Report...Am I still doing something wrong?

    Lastly...the circuit does work ( I must have shorted the large cap's wires ) is there a way to cut and paste and then work on the circuit in Eagle... or do you painstakingly draw each part once... then paste it when you do a build...
     
    Last edited: Jun 8, 2015
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    You don't use the maximums from the datasheet, you use the maximum based on how you have it connected. If you post a schematic, I can help you calculate. It doesn't need to be exact. Everything you mentioned should be less than 100 mA, then there's the power transistor you mentioned. A 1 or 2 amp fuse should suffice; but the intent is to prevent a fire, not prevent components from being destroyed.
    I don't have any experience with recent power supplies. My newest is probably 20-30 years old. Get a datasheet for LM317 and it'll have some examples of an adjustable supply. None of the current limiting I've seen in LM317 datasheets was what I'd consider good, but I can help you with that.
    No, that's for others to report an issue with the post.
    It isn't likely that anything other than connecting the large cap backwards would damage it. If you shorted the cap, it would stress the battery; but the cap would be undamaged.
    For single components, you just click Copy and then click on what you want to copy and click again to place it. For a group of things, you use Group (dashed box) and draw a box over the group. Then you Cut (scissors), click on the group, click Paste, and click to place the selected group. You can paste a group multiple times. It may mess up component designators or do it in an unexpected way.
     
  14. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    Here's where I got the circuit
    http://www.electronic-circuits-diagrams.com/simple-variable-frequency-oscillator-circuit/
    lets use all of the components in this circuit plus an led and a 470 ohm resistor
    List:
    1k 1/4 watt resistor, 1 meg pot, two .01 cap s, 470 ohm resistor and a led...
    9 volt battery ( as you pictured )


    omit the BDX53 or any type of relay...

    9 volt battery
    A. 1k ohm 1/4 watt resistor ( 0.009 amperage = 9 mA / 0.08099 watt = 80.99 mW )
    B. 1 meg ohm 1/4 watt pot ( 0.000009 mA / 0.000081 mW )
    C. 2 .01 ( 103 ) ceramic cap s ?
    D. Led ?
    E. 470 ohm resistor ( 0.01914 mA / 0.1723404 mW
    F. hook up wire ?
    G. breadboard resistance ?
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    There are some components you can ignore for power calculations; namely the capacitors, hookup wire, breadboard resistance, and the 1M pot. You ignore the caps because of their value (and other reasons), the resistance of wire/breadboard (because they're very small), and the pot (because highest power is when it's adjusted to zero, and it's a "large" resistance).

    The datasheet for TI's LMC555 says supply current at 12V is 400uA (selected voltage closest to what you're using). R1 will dissipate max power when the power is initially applied (because both terminals of C will be at ground and it will begin charging towards supply voltage); 9mA, very briefly. You could also calculate max current in R1 by looking at the block diagram for the 555. Pin 7 is the discharge pin and is periodically switched to ground by the discharge transistor (MOSFET in this case, but they're also called transistors; Metal Oxide Semiconductor Field Effect Transistor). Timer output will alternate between ground and supply voltage, so maximum current will be about (9V-1.5V)/470 ohms = 16mA. Total current will be around 25mA; notice I ignored the current used by the timer (because it's small). I'd use a 1A fuse and call it good. There are 1/32A fuses, but you're really trying to prevent a fire, not protect the components. The downside to a 1A fuse is that it will carry 2A for quite awhile and would probably open at 10A in a few seconds.

    For (some) component protection, a current limited power supply is a better option.
     
  16. dl324

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    Could you post a PDF? I use an old version of Eagle and don't care to upgrade.
     
  17. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    I made the sch. on the fly...so if you could adjust it ( if it needs it )
    And if you have a sch. of a power supply...

    cheers
     
    Last edited: Jun 9, 2015
  18. dl324

    Distinguished Member

    Mar 30, 2015
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    I have Foxit installed for a PDF reader and it has a plug-in that lets any application print to PDF. It also lets you modify PDF's and it's free!
    Look at the LM317 datasheet or find an application note and it will give some examples. If you can't figure out a current limiting scheme you like, I'll help you modify your circuit.
    You got it. Higher voltage gives higher operating currents.
    They do have a small resistance when forward biased, but it's not always negligible. When you have a current limiting resistor, it determines the current in the LED.
    If you want others to read it, it would be best to start a new thread as anyone not interested in fuses or this thread might not read it.
     
  19. dl324

    Distinguished Member

    Mar 30, 2015
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    Not bad for your first try. Here's my, hopefully, constructive criticism:
    1. You have pins 2 and 6 wired to the wrong side of C3
    2. You don't have the wiper on R1 connected
    There's an active thread regarding schematic guidelines that you should read. It will suggest that:
    1. C3 should be placed "upright"
    2. R's, C's, and supplies should have values
    3. There should be a dot at the intersection of pin7, R1, and R2. It's understood that there's a connection, but the dot makes it explicit.
    4. I don't like that symbol for NE555. The one in the URL you referenced above is much easier to understand. You can make your own for Eagle if you're going to use 555 timers enough for it to matter.
    There are other supply symbols that will give you the ones that are more typical in the US (I assume FL is Florida). In my old version of Eagle, it's under 'supply2' There's also a different symbol for the pot, a resistor with an arrow representing the wiper 'pot -> TRIM_US-'.
     
  20. RandyFL

    Thread Starter Member

    Aug 28, 2014
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    http://www.ti.com/lit/ds/symlink/lm317.pdf
    On the LM317 front...
    If I read it correctly... on note 8.1 it states " The LM317 device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an out-put voltage range of 1.25 to 37V "...

    What's the top range of current and I assume it starts at 1.5 A...?
    what sort of transformer and bridge rectification...? is there a filter cap before it gets to the " Tex inst. circuit "
    what's the input voltage ( from the transformer )

    cheers
     
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