My 24v old-school tester

Discussion in 'General Electronics Chat' started by english.g, Apr 9, 2018.

  1. english.g

    Thread Starter New Member

    Apr 9, 2018
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    I am a truck Electrician and use this(homemade with old parts from workshop)

    for testing wiring on 24v system, ie to see if wire is good , not open circuit , corroded etc .

    back in the day we called it belling out (used old door bell )

    it works well apart from size 10in x 3in and bulbs keep getting broken

    it tests just the wire to see if capable to cope with 3 to 4 amps load

    so would like to build electronic one if possible with leds green for good and red for bad
    and if poss a buzzer for good as well, but must have the 3-4 amp load test .

    i am not electronicly good at working out circuits but can build it

    so any help if it can be made and poss diagram
    thanks graham

    wire tester.jpg


    Mods Note:
    Please don't use Big fonts on all post.
     
    Last edited by a moderator: Apr 9, 2018
  2. cmrincon

    Member

    Oct 25, 2017
    38
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    Hi, im a electronics engeneer student. if the transistor is shortcircuited like the image D1 lights, if there isn't shortcircuit D2 lights and D1 doesn't. You should use a bc546 transistor. Can this help?

    imagen para leds.jpg
     
  3. cmrincon

    Member

    Oct 25, 2017
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    note: it only say you if there is connection from one end to another. It doent test the cable for 4 amps
     
  4. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Three 21 watt bulbs is 63 watts. At 24 volts that's only 2.6 amps. But I'd suppose that if you have a wire that is capable of handling that current it can likely handle whatever current the wire diameter is designed to handle. In other words, if it's designed to handle 15 amps then if it handles 2.6 amps it's very likely it will handle the full 15 amps.

    Now, you want a load to pull a current through the wire under test. Mmmmmm. How would I do that? Lightbulbs produce a lot of heat. A small motor running a fan can demand that much current and NOT produce the heat. But it seems like you are asking for two sorts of testers. One to test if the circuit is complete (or open). LED's can easily handle that. But to have an indication of - lets say - Green for good, all you need is a green LED and an appropriate resistor for the voltage supplied. But for - lets say - Red, you'll need a constant power source because an open circuit isn't going to give you any power to light an LED. Therefore to light an indicator that tells you the circuit is open you'll need an LED, the appropriate resistor and a transistor powered from a small (likely 9v) battery. There's more to the circuitry than just a transistor and a battery, but for sake of pointing out the needs - - - .

    As for testing for current, you simply need to know the line is good first. Then you need to present a load. You could use a resistor. You want to test for current capabilities at 4 amps. Well, at 24 volts you'll need (24 ÷ 4 =) 6Ω @ 100 Watts minimum. And recommended wattage would be at least 150 W. That's a pretty big resistor.

    For ease of construction, I'd stick with what you have. Just mount your light bulbs in an enclosure that can protect the bulbs from being destroyed.

    Let me pose some questions: First, why are you testing for current capabilities? Aside from proving out a connection or switch, you're not really getting to know anything else about the wire. Well, there IS the fact that you are proving out that there's a complete circuit. But if that's all you need to do then an LED with a resistor (a single LED) should suffice. At 24 volts, minus 3 volts forward (the voltage drop across the LED)(assumed) is 21 volts. Likely you want to run the LED at 15 mA (0.015 amps). 21 ÷ 0.015 = 1400Ω, so a 1.4KΩ resistor will prevent the LED from being burned out when testing a 24 volt circuit. At 15 mA the LED should be bright enough to be seen even in daylight.

    As for testing for current, if the circuit tests out good (rings out good - or as you call it "Bells" out) then the trailer lights (assumed) should light. If they don't light then there's a weak connection somewhere. To ring that out you'll have to break the circuit in the middle and test for current using your old trusty lights. [edit] And assuming there are no shorts.

    @cmrincon showed you a diagram. Yeah, it needs a power source. But if you just put it away in your tool box it will sit there telling you there's an open circuit because it's not connected to anything. You'd need to add a switch to turn it on and off. And if you forget to turn it off then you run your battery down. Also, I didn't see where you'd connect the wire under test. Unless it's the long wire on the bottom of his diagram. And I don't see how one LED will light under a bad test and two LED's under a good test. Perhaps cmrincon can post another drawing to clarify.

    Finally, there are a lot of really talented people here. I bet I'm wrong about my current testing methods and someone may have an elegant and beautiful solution. Perhaps using a MOSFET as a load to test the current. But now I'm thinking the circuit will become even more complex. But somehow I bet it's doable. Just beyond what I know.
     
    Last edited: Apr 9, 2018
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Nope.

    A 24 Vdc, 3 A fan motor will produce a lot of heat. It is less noticeable because it is swept away in the air stream, but it is there. I've seen MIL systems fail thermal testing because the fans are intake, pumping pre-heated air *into* the chassis, rather than exhaust. This can be a tricky problem if one of the requirements is a positive-pressure system to prevent contaminants from being drawn in through every micro-opening.
    Because connections that test perfectly at 10 mA but fail at 2 A are very common in old truck wiring.

    ak
     
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  6. be80be

    AAC Fanatic!

    Jul 5, 2008
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    He is wanting to load the wire which is a good idea because on a car or truck the wire can test good but when loaded
    corrosion breaks the circuit.

    I had a sunbrid that started when it wanted to and everything look great it was corrosion in a sealed cable connector
    You couldn't see it and it never shows up till the starter try's to start.
     
  7. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    The simplest solution would be to put the bulbs inside a box with a window (or a clear box). As the only contents of the box would be three bulbs and their sockets I'm sure it could be a lot smaller than that.
     
  8. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Yeah, I have to admit you're right. However, the motor won't heat up as fast as a light bulb will. AND motors are far less susceptible to shattering than a glass bulb.

    Watts is watts. Whether it's a bulb or a motor. Or a MOSFET dissipating heat, it all comes down to conservation of energy. It's going to exist in one form or another. So yes, it's going to be there. But a fan will keep the motor cooler than just light bulbs. And I suppose the fan blades can be damaged too.

    Just thought of it - I have a radiator cooling fan in my garage. Came off my wife's wrecked Celica. Don't know why for the life of me I kept it. But it's there. Perhaps one day I'll attach it to a car battery mounted on a cart. The cart is what I use to haul my 4th of July gear out into the field. We set up camp and wait for the fireworks. Wouldn't be bad to have a car radio and the fan out there. The fan to blow air on us hot souls while we listen to music. But that's getting off topic.

    Maybe a high amperage rated MOSFET could act as a 4 amp load. Again, there will be heat. But you avoid the "Breakage" issue. And adding an LED to it would be easy enough. But I lack experience with FET's.
     
  9. english.g

    Thread Starter New Member

    Apr 9, 2018
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    thats correct .
    we get a lot of pin holes in the insulation and water gets everwhere and after a bit corrosion
    test with ohm meter could show ok but put 3/4 amp down and fails .

    power supply not problem as use truck 24v 234.jpg
     
  10. Tonyr1084

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    Sep 24, 2015
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    I'm wondering what the amperage would be using a 120 volt light bulb. It surely won't glow as brightly as three 24 volt 21 watt bulbs would, but it's probably close to 6 ohms. Of course, as it heats up the resistance goes up too.

    Speaking on the subject of having good continuity but a failed circuit - I repaired my neighbor's sprinklers. One valve wasn't opening. I tested for voltage and it was there. So I thought the valve was not functioning. I replaced the diaphragm but that didn't solve the problem. So I put a new solenoid on it. That didn't solve the problem. So I swapped two wires and the failure moved with the swap. So even though there was voltage obviously there wasn't enough current to actuate the solenoid. Started digging up the line and discovered butt splices buried under ground. REGULAR butt splices! No wonder there was no current. Corrosion took its toll. I ended up just replacing the whole run with a single run - no splices anywhere. So yes, I understand the need to prove out a circuit including the ability to deliver the current. On a tractor trailer, those trailer connectors do corrode. You can get voltage and continuity through them but the lights just don't want to light. Trailers are such a problem. I rewired a trailer for a friend. Side markers, brake and turn signals as well. I believe it was a seven wire system. Been a few years. But if there's continuity but the trailer lights are not lighting up then you have a connectivity issue. And yes, I've even seen wires develop corrosion internally, leading to high resistance. So I DO see the need for testing a wire for current as well. Sometimes I live in a perfect world. Inside my head anyway.
     
  11. cmrincon

    Member

    Oct 25, 2017
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    Hi! the test cable in my circuit is the one that connects the 300k ohm resistor with the negative pole. Maybe instead of 3k resistor would be better 2k resistor or 1.4K like was proposed.

    The circuit works like this: when there are connection from one end to another, current flow through D1 is 7mA and lights and D2 doesn't. When there is no connection, current flow through D1 is 0.1 mA and doesn't light and the transistor saturates and D2 lights.

    Is difficult work with 60+ watts...sometimes the simpler way is the better one.

    Regards
     
  12. Tonyr1084

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    Sep 24, 2015
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  13. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    If breakage is a concern, I would go with 2 100 W and 1 50 W, aluminum body, chassis mount power resistors in series to form the voltage divider for the comparator, or for the LED connected directly across the lower resistor.

    Add a little CPU cooling fan blowing across them. 2 or 3 CFM would make a huge difference in surface temperature.

    ak
     
  14. Tonyr1084

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    Sep 24, 2015
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    OK, how about this: And someone who's familiar with building constant current devices can fill in the blanks:

    24 volt CC tester.jpg
     
  15. english.g

    Thread Starter New Member

    Apr 9, 2018
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    THAT LOOKS GOOD BUT IS THERE LOAD ON TEST PROBES, IE LOOKS LIKE IF JOIN PROBES WILL WORK WITH OUT LOAD
     
  16. english.g

    Thread Starter New Member

    Apr 9, 2018
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    COULD YOU DRAW IT PLEASE . IT ONLY TESTS FOR A FEW SECONDS SO DONT THINK WOULD GET HOT 10SECS MAX
     
  17. Tonyr1084

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    Sep 24, 2015
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    The intent of the probes is to short out K1. If the probes are open then K1 closes and lights the red LED (LED2). If you short the probes then K1 falls out and sends 24 volts to the buzzer and to the green LED (LED1). However, I'm unsure how effective this tester will be. If there's 1 amp flowing through the wire under test then there are three amps available to continue to allow K1 to hold the red LED lit. Somewhere in-between there's a point where you may get a false trigger. Probably the best way to test for continuity is to test with a digital meter set to amps. If you flow all four amps through the wire then your contacts and connections are good. If it's less than 4 amps then there's high resistance somewhere in the circuit. Possibly a switch or plug - or corroded wires. Testing weak wires is not easy. If I were testing the circuit I'd test for voltage at every junction all the way back to the bulbs. If I'm reading more than some percentage of a voltage drop then I'd consider replacing the wire. I know that's not cheap. But it is what it is.

    You're asking for a solution and setting parameters. I'm trying to stay within your parameters. With my last circuit, if the wire is conducting a reduced amount of current then the voltage at the bulbs on the trailer (I keep thinking trailer) if the voltage is below 80% then I'd replace the wire. Why 80%? I don't have a good reason. Just an arbitrary test. Let the trailer lights be your test light. After all, if they work then the circuit is good. If not (or if dim) you need to replace the wire. Obviously SOME power is getting there. Just not enough.

    I serviced a 32 volt emergency lighting system in a store. I think it was an old K-Mart. The 32 volt bulbs at the end of the run were barely lit. The electrician who wired them in used too small a gauge wire, and the line resistance was high enough that the voltage dropped the further you went. So we replaced some of the bulbs with 24 volt bulbs and still others with 12 volt bulbs. It was that or pull the whole ceiling down and replace the wiring with proper gauge wire. Hey! They worked. The correct fix was to replace the wires, but that wasn't going to happen. In your case, if a bulb has low voltage then either a connector is corroded or a bad switch, or a bad wire. I think you're trying to come up with a solution that doesn't really have a problem for it. But since you ask - the constant current device will send no more than 4 amps through the leads. Just make sure your test leads and internal wiring all can handle the 4 amps. It'd suck if you built something then burned it up because of undersize wire.
     
  18. Tonyr1084

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    Sep 24, 2015
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    In my drawing: Short the test probes. K1 is shorted out and drops out, sending power to the buzzer and to LED1. If a wire is under test and it's good then it does the same thing. If there is a resistance in the wire then K1 is not completely shorted out. At some point (depending on the condition of the wire) K1 will kick in and shut down the buzzer and turn on LED2. So if your wire doesn't conduct the full 4 amps then it's likely K1 will remain on and indicate a fault condition.
     
  19. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Exactly right! I once found a wire that delivered 12 volts to my 30K ohms per volt meter but would not deliver enough to light an instrument panel bulb. Digging deeper revealed that the insulation was full of wet green copper chloride, not copper wire any more. A tester that draws some current reveals that flaw instantly.
     
  20. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    For the tester use a 5 ohm and one ohm resistor in series, with a green LED and it's own 74 ohm resistor across the big one ohm resistor.
    That is for the 4 amp check. Then for the low current tracing check, just a yellow LED with a 1200 ohm resistor to give a low current check. Add a NO pushbutton in series with the high current side to avoid a big heat build up.But it does not have the red LED to show open. That is why the yellow one is included.
     
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