Smart current sink for testing shorts?

Discussion in 'The Projects Forum' started by MATTY B, Nov 13, 2010.

  1. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    Hi there,

    I have a little project I want to put together to upgrade the very simple set up I have now. Backround on this is that I am an automotive electrician, not an EE I fix stuff after it breaks.

    One part of that is dealing with shorts on the vehicles harnesses. The first part of fixing it is actually locating it. As of right now, I have a little project box with an analog ammeter and a NO momentary switch that can plug into a fusepanel in place of the fuse. I can quickly hit the switch and watch the ammeter to see if there is a large hit on the circuit. A dead short will max the ammeter out right away. I DO NOT hold the switch just tap it quickly to see if it pegs while shaking down harnesses and removing components to isolate the short.

    The big issue with this is there really isnt any circuit protection for this other than just being quick on the switch so it doesnt heatup. I dont really trust a run of the mill circuit breaker as it could fail as well as possibly being over or undersized for even a normal current draw of a particular circuit eg: a heater circuit can be sized at 25-30A 12vdc while a dome light circuit could be 10A. Obviously a one sized fits all circuit breaker could damage some circuits and open up on heavier current circuits even if its not shorted.

    Testing by using continuity to ground is good only if the circuit is short to ground, it doesnt allow me to see if a component such as a motor is going bad and pulling too much current but isnt a dead short, just overloading the circuit.

    What I want to do is build some type of "smart current sink" that allows me to see current draw but also allows me protect the circuit from melting if a short is encountered. There are some automitve products like the "stinger" that beep and turn on a light if a short is found but it uses the continuity to ground method of determining the reason a fuse is blowing so it does the same thing my meter does without a display to show me the resisitance to ground. Theres also the headlight method which you wire in place of the fuse, if a short is present the bulb lights and its current sinking abilty protects the circuit IF the wiring is stout enough to handle the current of running a headlight (some circuits are 20 gauge wire and would melt handling the current draw of a standard 50w headlight).

    Obviously what I need is a combination of all these that can recognize the current draw and protect the circuit while allowing me to do my diagnostic tests.

    Ive though about a shunt but I figured that itd need to be a pretty big resistor or be a MOSFET that PMW the circuit as current increases. The fet method would be good IMO but I dont even know how I would design that circuit and I dont think it would allow me to see actual current draw.

    Maybe Im looking for something that isnt actually possible since in my head I dont know how you can monitor actual current draw while at the same time not allowing it to draw that current. Then again if theres a way to sense the requested current and somehow bleed it off safely to protect the actual circuit then Id be in business.

    Hopefully somebody understands what Im lloking to do and theres a circuit I can build that will be able to handle this. BTW max curent draw can easily exceed 60A on a dead short so it needs to turn off at some point so as not to compromise the main power circuits feeding the smaller circuits. I need safe first and second and third while allowing me to do my job.

    Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    I think the smart part of your circuit is going to have to be YOU. Would it work to have an adjustable current supply, so that you'd turn a dial to ramp up the current in a circuit? The max would depend on the circuit itself. I mean you can't force 50A thru the dome light because the voltage is limited. Your device would only see too much current on a shorted circuit, and you'd have to use your judgement about what "too much" is. I think you might want some safety switch such as a push button you'd have to hold down to get a current over, say 10A.

    This approach would be just like your momentary hit on the switch, except it would allow you to go slowly and see if the current is getting out of hand before damage occurs.
     
  3. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    0
    I guess thats what Im doing now lol. What I would hope for is be able to put my box in line and if theres some sort of massive amperage spike bleed it off and let me know whats happening.

    As of now I can see a normal draw and see if its 5 or 25 or 50+ amps and know if its kosher or not. A lighting circuit should draw between 5 and 15 amps and a motor circuit might spike at 30 and drift back to 15. A dead short is gonna go max obviously.

    What I would like to build IF EVEN POSSIBLE is a circuit that can tell the difference between a normal current draw and a short or something close to it. Shorts spike rapidly as do bad motors (motors spike hard sometimes but settle down after a second).

    Im wondering if theres a way to test current draw by how hard the draw is requested and open up if it exceeds a given amount while at the same time monitoring that draw to a regulated point.

    I dont need a carbon pile shunt to tell me amperage draw but something under 30A so I dont have to worry about frying wire while searching for the fault. It bothers me that if the switch sticks I could cause a huge problem that the fuse wouldve saved otherwise.

    Like I said I dunno if this is even possible but I figured there might be a way to regulate the current away from the short via solid state components while being able to monitor the requested current draw continuously.

    If not Ill stick to my method I have now although I am learning not to like it.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
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    Since you are an auto tech, do you not use modern dedicated auto test multimeter?

    Most of these have a dedicated low ohms range for just your purpose, along with other useful functions to the automotive industry.

    It is also possible to construct a low ohms ohmeter using an ordinary multimeter.

    Normal multimeters use a series circuit, as you are trying. The secret is to use a parallel circuit. The test resistor is connected in parallel to a standard resistors, which is supplied through another standard resistor or constant current supply.
    The measuring circuit is thus self protecting and can be left on indefinitely. It is easy to measure from 100 ohms down to 0.001 ohms in this manner.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    I think I understand exactly what you're after -- and I've occasionally wished I had such a tool also when working on the 12 V systems of cars and RVs. A tool that uses MOSFETs to control DC current is a DC load; here's a unit that I've used and I'm pretty sure with a laptop and this device sitting on a bench, it could be made to do what you want it to do. But it would be very clumsy to use (actually, a PITA if you were crawling around under or inside a car) and you'd probably want to write a GUI software tool to control it. Not to mention that it's too expensive.

    I would think a hefty-current MOSFET on a heat sink in a project box with some control circuitry could make a nice portable test unit. It would be a device that would be used to present a pulsed, adjustable load. There'd be a knob to adjust the maximum allowed current and a knob to adjust the time width of the pulse. There'd be a momentary pushbutton to connect the load to the suspected circuit and either a red or green LED would be lit up after the button was pushed. Red would indicate that the current drawn equaled the set limit for the time allotted; green would mean it did not. There could be two adjustable current limit ranges: say, 0 to 2 A and 0 to 20 A. The time could be adjusted from, say, 1 ms to 100 ms.

    I'd imagine such a tool would most easily be made with a low-power microprocessor. Pressing the test button would power it up, make it go read the knob settings, perform the test, and light the LEDs.

    To use the device, you'd set it to a short time, then set the current knob to maximum current (or whatever you think is appropriate for the circuit under test). Press the button. If the light was green, turn the knob to half the set current value and perform a binary search for the current drawn.

    A more deluxe device would also make the current measurement and display it on an analog meter or an LED bar graph. It would also have a buffered TTL output to allow it to trigger or drive other devices; a switch would select whether the output would go high if the red or the green LED came on.

    If equipped with an analog meter and if it integrated the current during the time window, it would be essentially a ballistic galvanometer with a memory. The reading displayed would be proportional to the charge that went through the tested circuit; it would be handy to have it be a logarithmic display in amp*seconds.

    If the design was made to handle AC currents and line voltages (it would integrate either the absolute value or RMS value of the current), the device could be used to measure AC inrush currents for the set integration time.

    The nice feature of these short current pulses is that they can substantially exceed the continuous-current limit of a conductor, yet you don't have to worry about burning something up because the current is on for only a short time. With a human doing the testing, the duty cycle would remain pretty low, which would help protect both the conductor and the test unit's MOSFET from overheating.

    This sounds like a useful piece of test equipment to have -- if we're lucky, one or more of the design wizards on this board will agree and we'll see a flurry of activity, arguments, fist fights, etc., and the result will be an eminently usable piece of test gear... :p
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Are you only trying to identify which wire it is in the harness, or are you searching for a dead short?

    A networking toner (Fox and Hound) can be used to ID wires, clipping the black lead to ground, and the red to the wire of interest, then chase down the other end by following the tone.
     
  7. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    First, Im trying to locate a dead short on the particular circuit that happens to be blowing a fuse. At the same time I have come up against loads such as motors that are bad or going bad and will either blow a fuse or actually melt it from sitting at a current level that is at the borderline for actually blowing the fuse.

    Using a multimeter as I said can test for dead shorts quite easily. All you gotta do is probe the circuit at the fuse panel and test for continuity to ground. This doesnt help if you have a bad component that is drawing too much current though as alot of integrated circuits can do with say a defective controller shorting out a transistor or a blower motor that is pulling too much current compared to its specs.

    Having a smart device that could differentiate between dead short, dangerous current draw and normal current draw is what Id like to put together. It would allow me to better identify certain failures without the risk of damage to the wiring.

    Someonesdad understands exactly what Im looking for and probably has run up against the same problems from the sounds of it. As simple as using a meter sounds, sometimes it doesnt give you the full picture and if it does something is smoking lol.

    Shorts on a PCB are typically easy to identify since you can jump to each part of the circuit with out any physical dexterity. In a car,truck, RV or boat you may have over 25 feet of circuit to deal with that may have shorted at either one spot or in some instances multiple spots due to usually shoddy assembly. If a dead short is constant a meter can be used and just shake down or break down the circuit till you pinpoint the failure. Other times there are intermittent failures from say a rubbed through wire that is intermittently shorting to ground from movement. Its the cases in particular that in the event of shaking a harness down that you need to constantly hold current on the circuit until in spikes. If it spikes you and shaking it down you can pretty typically locate the issue. Im scared that while doing this my test box sticks and instead of a fuse blowing I burn the harness up. Obviously that could be really f'n bad and someting I want to proactively avoid.

    Someonesdad explained what Im wanting to do perfectly and if someone knew what to do I think a few people could benefit.
     
  8. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    actually by enough google clicks I came across a product that does what I want
    http://www.ipatools.com/9000.html ask and you shall recieve I suppose lol. I guess Id be happier if I could build one for less than retail asking price but I guess someone already did the hard work.

    I think Ill buy one unless someone here knows of a circuit or could design one that would be comparable. Otherwise I think it does absolutely everything I was looking for, which obviously means that theres a need beyond a meter or a short detector.
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
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    That commercial unit looks appropriate, but I didn't look at it in any detail after seeing the price. A person making his living troubleshooting things could certainly justify the price, but not a hobbyist like me.

    However, I don't think it would be terribly hard to build a simple go/no go device like I described above. The one shot pulse can be gotten from a 555 circuit and it just needs to drive a MOSFET like an IRF540N. A comparator could be used to test the peak current in the pulse window (a later version can have an op amp integrator). I think I have all the required parts on-hand, so that might make a good winter project.

    One thing such a tool would be useful for is characterizing circuits. You'd measure the starting currents of normally-operating circuits and stick them in a log book. Later, when you're troubleshooting, a quick measurement would give you an idea whether something was working normally or not.

    Another tool I used once or twice in the early 80's was the HP current tracer (the kid that sat next to me at work had one). This was used by digital EEs to troubleshoot digital logic problems and could trace shorts. A modern version of this would be a nice tool to have. You'd put it up next to a shorted conductor and use the companion HP 546A pulser to provide a current. You'd then adjust a pot on the tracer to adjust the sensitivity to see the current, then use the tip to follow the trace that was sourcing or sinking current. Such a tool would be helpful in finding shorts in low voltage DC circuits like cars, boats, etc. I'd imagine this also wouldn't be terribly difficult to build. The key would be e.g. a small ferrite toroid with a gap to detect the magnetic field of the pulsing current. Obviously, the first thing to try would be a magnetic head from a surplus magnetic recording device.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    $469.95

    ouch.
     
  11. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    yeah ultra expensive... Some of the "features" provided I could do without.

    So saying this is the end all be all of what I was looking to build, is there a way to build this with out some of the more gee whiz doo dads?

    Say you choose an amperage limit and open the circuit. If the current exceeds the level it turns on a light or led to show the current has exceeded the selected level. I could build from this with the 555 timer and a digital ammeter.

    Does anyone know of a circuit that would do this? I dont want to spend 400 on a tool unless Ive exhausted every other possibilty of making my own tool.
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I always look at equipment purchases like this: If the income from using the tool meets or exceeds the cost of a DIY tool, I buy it. This is a break even when working with high end items such as Fluke meters as well as oscilloscopes.

    If your time can be cut in half or more on finding defective circuits, the unit will pay for itself in fewer "book hours" than it would take you to actually design/build and debug a similar circuit.
     
  13. MATTY B

    Thread Starter Active Member

    Jan 26, 2008
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    this is true but my $30 tool has done alright for about 15 years. My weekends are my own and this kind of thing keeps me away from the tv.. well except for the Red Wings games.

    I would love to spend some weekend time tinkering to make a toold that could help out and not be too expensive.

    Granted Im not gonna try to machine a ratchet or even make a wrench but this kind of project is fun and is ultimately useful.

    Ive looked around and found circuits for digital circuit breakers and such, some being much more complicated than others, but even a variable fuse would be a good starting point for me.

    Maybe someone can point me in the dirrection of a few circuits that would do the job?
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Well, you can always search for the patent for the device, build one, and attempt to improve it, using the extant patent as "prior art" (e.g. building a copy for yourself). As long as you don't attempt to sell multiple copies, you should be fine.

    The biggest issue is locating the correct patent, there are usually several on any given topic, some of which do not even work, or omit component values (which makes the entire patent system sorta worthless).

    An account at www.patentstorm.us is about the most handy research tool around, and it is free!
     
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