Problem with fotodiode.

Discussion in 'General Electronics Chat' started by leksa-s, Dec 14, 2010.

  1. leksa-s

    Thread Starter New Member

    May 31, 2010
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    Hi,

    this is a picture of webasto bbw46 flame sensor circuit. http://img262.imageshack.us/img262/7519/fotod.png

    When there is no flame, fotodiode isn't "open", and when there is, steel near fotodiode starts to glow causing the voltage difference around the diode drops to about zero.

    My problem is that when weather is cold, it takes too long for steel to get red hot causing that webasto thinks there is some problem with fuel or glow plug etc. Voltage difference only gets to about 8V. So what I need is a small and simple circuit to react this 4v change of the diode. Then it could be used with small relay or mosfet in parallel with original fotodiode. Then it's fooling the control box there is flame by making short circuit in parallel with original fotodiode.

    Very annoing problem, what would you say, what's the best way to do this?

    Thanks and sorry for my english.
    Greetings from icy cold Finland (-22F)
     
  2. #12

    Expert

    Nov 30, 2010
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    Welcome to all about circuits, but...

    The best way to do it is to get the furnace repaired with the original parts in proper working order. Trying to fool a furnace is among the most dangerous things you could do.

    Then again, maybe it's not a furnace? What's a webasto?
     
  3. leksa-s

    Thread Starter New Member

    May 31, 2010
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    Gas (same what regular car uses) fired, and glow coil ignitor.

    #12, I see what you're saying but:
    1. Webasto is vehicle heater. Google will tell you more =)
    2. Parts are brand new, they just don't work well when it's very cold.
    3. Not trying to fool it that way, idea was just make it notice the flame earlier. It takes too long for the steel to get red hot, so I want it so see there is flame when steel is just mild yellow.

    Operation in basic: 30sec glowing, then fuel pump on, after 5 seconds combustion air fan gets activated. Glowing continues and fuel ignites in couple of seconds. Steel around the flame sensor starts to heat, but control box gives only 30 sec (what is not enough in cold temperatures) time before it shuts down fuel pump.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    So what you are basically looking for is a hobbyist work around for an important safety circuit in your furnace. I wonder why the furnace designer picked 8V, if 4 V would be enough? Maybe it has something to do with excess gasoline getting into the firebox during cycling and then explosion?

    You need to call professional service for your furnace, find out what is really wrong, and not risk your house and family to being burned.

    John
     
  5. leksa-s

    Thread Starter New Member

    May 31, 2010
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    Again, it's not furnace. It's vehicle heater size of a quarter qallon coke bottle. I really appreciate your help, but I'm afraid you're not getting the point?

    First is very good point, and I have been thinking about it. It would be safety, we both agree. However, it would be safer if I removed the heater from my car.

    Last, when the burning stops, voltage over fotodiode gets to 12v in couple of seconds. That because cold air is blowed to the combustion chamber.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Sorry, I thought you were using the vehicle heater in a shed or home. In northern Minnesota (also very cold), it was not uncommon to use such auxiliary heaters in dwellings.

    Still, a flame's temperature is pretty hot and the time to warm a metal strip shouldn't be that much different with a change in outside temperature of 20°F. I wonder if the switch is simply timing out too soon. In other words, the problem is with the switch and not the flame/sensor. I've had commercial heaters that depended on sensing infrared from the flame in a similar fashion. Early versions of those sensors would go bad in 4 or 5 years. You could get a little extra life out of them by cleaning the window to the photocell.

    John
     
  7. leksa-s

    Thread Starter New Member

    May 31, 2010
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    :) thanks.

    I'm studying physics and been dealing with automotive electronics and mechanics many years. Maybe hard to believe because of my english..

    "Still, a flame's temperature is pretty hot and the time to warm a metal strip shouldn't be that much different with a change in outside temperature of 20°F."

    Yes, you're right. Still, problem seems to go with temperature..

    "I wonder if the switch is simply timing out too soon. In other words, the problem is with the switch and not the flame/sensor."

    So, it would be the control unit then.. I must borrow working somewhere and try with it.

    Sensor is just 1 month old. About week ago, I cleaned it with wire brush. It worked little better for a while, but now again the same. And to be mentioned, there was just little amount of dirt. Still it did better for a while.. Weird.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You should check it VERY carefully for an intermittent connection somewhere. Crimped on terminals sometimes become very resistive and corroded inside. The fact that you were 'inside' it and it worked better for 'a short time' tells me that this is very likely.
     
  9. leksa-s

    Thread Starter New Member

    May 31, 2010
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    So true it sounds like corrosion or something. But. I didn't really go "inside" I just openend one connector (2 wires in it) and then one screw. And the voltage I've been speaking of is measured from this connector very close to the sensor.

    I agree it's better to fix the real problem not the symptom, but I have pretty much cheked everything connected to the sensor. Of course every kind of help is greatly appreciated! Great forum I've got to admit!

    By the way, what should be the colour of flame when burner is operating properly? Very light yellow or maybe bluish? If anyone is familiar with these kind of heaters. Not the first one i'm dealing with, but the first where I can watch the combustion operation through tiny window.
     
  10. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Natural gas burners are bluish flames. My kerosene portable is blue with a little yellow. I have no experience with gasoline burners. I would be pretty worried about an oxygen-starved gasoline flame, particularly if it is not well vented. I assume your auto heater is well vented, though. John
     
  11. leksa-s

    Thread Starter New Member

    May 31, 2010
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    I was just thinking that if it runs too rich causing lower heating power and then it takes too long for sensor to get red hot. Maybe I get the car for webasto shop for CO2 measurement. Should be 10% of exhaust.

    Sure it`s well ventilated, I also take safety notices of the installition manual seriously, so there`s no problem.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    A blue flame means complete combustion. You're probably better off if there's just a little yellow in the flame; if you get too lean it may be difficult to light initially, or "pop" a lot.
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    First, let me say that I am just guessing because I don't know the first thing about a Webasto heater.

    But, if the problem is as you seem to describe above, the problem may be in the control box rather than the photodiode. If the equipment is brand new, perhaps the control box is not properly adjusted and it's shutting off the fuel pump too soon. Have you contacted the manufacturer? I would guess they would replace the control unit very quickly if there is any chance of a safety issue.
     
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