Light sensing relay - for weld fume extractor

Discussion in 'The Projects Forum' started by Pump Parts, Feb 18, 2016.

  1. Pump Parts

    Thread Starter New Member

    Feb 12, 2016
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    Hi All,

    I need a little help designing a basic light sensing relay to turn on and off a welding fume extractor whilst welding.

    I believe a fast acting photo diode, controlling a simple timer relay is probable the best approach.
    The fan has a 5KW - 3 phase contactor to run it with a 24v dc coil, so ideally I would like to use that 24v dc supply to also power the photodiode circuit. ( if that's easy, otherwise I can put in a second power supply to suit )

    I am fine with the ac side of things and an off delay relay to allow the fan to run on for a while after the welding stops, but need a little help with the dc side.

    Can any of you electrical Wizards suggest a little circuit that would detect the light from the welding torch and engage a small timer relay to power the 24v dc starter ? ( possibly with a some form of sensitivity adjustment, so I can prevent it false triggering in sunlight. )
     
  2. blocco a spirale

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  3. Alec_t

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    Gas welding or arc welding?
     
  4. Pump Parts

    Thread Starter New Member

    Feb 12, 2016
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    The welding is Mig, all at quite high power, so its pretty bight when welding.
    The extractor is overhead, on a 4m long flexible boom arm, I want to mount a light sensor on the tip so its turns itself on and off automatically without and electrical cables trailing over the workshop floor.
     
  5. GopherT

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    Something like this will work. Your light will definately turn it on/off without need for hysteresis - I suggest to keep it simple like this.

    Note that you need some extra parts if the coil current is more than 30 mA. Do you already have DC supply or do you need that too (made from 24VAC)?

    Most any common photo transistor or photo resistor should work.

    image.jpg
     
  6. blocco a spirale

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    Then, simply attach a current sensing switch onto one of the incoming power cables to the welder, it will be far more reliable and easier to implement than trying to use opto sensing in that environment.
     
  7. Pump Parts

    Thread Starter New Member

    Feb 12, 2016
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    Thanks GopherT,

    I have a DC supply.
    Which is better... a Photo transistor or Photo resistor ?
    Would something like ... Excelitas Technologies VT43N2 LDR (Light Dependent Resistor) 8Ω Light, 300kΩ Dark, 2-Pin be OK ?
    Cheers
    P xx
     
  8. GopherT

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    Yes it should work but, like any project, be prepared for some debugging. You may need a small tube around it shield from ambient light or other details. Few projects work perfectly right out of the gate. Also, you may need a regular cleaning schedule to remove metal dust from the photo-resistor. These are DIY projects, not commercial products.

    I do normally recommend using a commercial product if you need reliability or if there is a big concern for safety (device must absolutely work; device his high voltage; device must be in explosion proof case;...). But, if you are looking for a DIY project to build youself, then this is a start with a risk of failure. The risk is not high (fairly cheap parts and fairly low risk if the fan does not start).

    Good luck and check back in if you need help or want to report positive or negative result.
     
  9. shortbus

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    As a welder myself I question this projects usefulness. Air flow doesn't start and stop as fast as electricity. There will be a lag time from when the weld starts to when the 'fumes' start moving. Wouldn't it be more effective to tie the extractor into the welders main on-off switch? Many industrial welding power supplies even have a switched outlet on them for this and water cooling for the torch, if needed. Any fume extractor I've been around is on all the time during welding, to keep air moving.
     
  10. Pump Parts

    Thread Starter New Member

    Feb 12, 2016
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    Dear shortbus,
    The extractors are very fast to start from zero to full suck in under one second. I have included an delay off timer relay in my design to allow the extractor to run on for a couple of minutes after the weld. However with 3 welding bays running all the time, they are quite noisy and draw a lot of the heat out of the building, which is why I want to automate them.
    If you have any constructive thought on actual circuit then I be delighted to read them.
    Thanx P x
     
  11. Alec_t

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    If you go for optical sensing then I think the circuit should have some hysteresis added; otherwise slight variations in light level could cause your extractor to be off and on repeatedly, like a tart's drawers :).
     
  12. shortbus

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    I was just saying the way it was done in places I worked over the years. If you think there will be a energy saving by turning them on and off constantly go for it. Does the cost of heat loss equal the electrical cost of starting a motor hundreds of times in a shift? The start current in a motor is higher than the running current.

    Instead of a "light sensor", why not tap off of the signal from the torch trigger? Either the gas flow solenoid or the wire feed motor. That way it would only work for that single welder when it is laying down a bead.
     
  13. Pump Parts

    Thread Starter New Member

    Feb 12, 2016
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    Hi Alex,
    You may have a point, however have you got any suggestions what to add to GophersT's circuit to achieve that ?
    P x
     
  14. Alec_t

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    Something like 100k between the comparator's output and its non-inverting input would probably do the trick.
     
  15. Pump Parts

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    Feb 12, 2016
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    Thanks Alec_t

    P x :)
     
  16. gerty

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    In our weld shop, we recently installed individual fume extraction booths. The fan motor can be turned on manually, and if it isn't , there is some sort of hall effect sensor attached to the ground cable that will turn it on, should you forget. The sensor will only let it run for about 5 minutes before it times out. Re-striking an arc will reset the timer. The sensor is wired as an input to a small controller which also serves as a filter shaker output and timer to indicate when the filters need to be removed and washed out. The welders themselves are usually left running all day long, much to the dismay of the instructor..This is the only pic I have in this 'puter showing what the booths look like. 2 booths share one extraction system. DSCF0139.JPG
     
  17. shortbus

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    I agree about most places having only one extractor blower for many welding bays. That is the way it was done in the different places I worked over the years too.
     
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