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Author Topic: More questions for the EE guys.  (Read 356 times)

Offline frank mccune

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More questions for the EE guys.
« on: July 11, 2017, 07:09:35 AM »
       Hello all:

        I have two problems that may have the same solution.  How can I check the power going to my glow plug without burning it out?

        I have been using a HD power panel that is very popular with fliers and seen on many flight boxes.  This is the ubiquitous type with outputs for glow plug, starter fuel pump etc.  All of a sudden, as soon as I touch a glow plug lead to a glow plug, it goes off like a flash bulb!  This happens even with the pot turned all the way to what used to be very low output.  I checked the outputs and they all had 12 volts with no load.  I did place a load on the glow plug via a resistor, and I could adjust the power out put to the resistor..  This went from 0 to 1.5 volts as it should.  I am hesitant to use a glow plug for a test as I do not want to blow more plugs.

      Is there a way that I can test the output under load. Testing with a glow plug becomes very expensive. Lol  How much resistance does a glow plug produce?  I think that I must do my tests with something that produces the same resistance of a glow plug to obtain a valid result.  What would make the power panel start delivering 12 volts to the glow plug?  I looked and could not see any signs of damage. 

      Oh yes, is there a way to reduce the current from a glow driver to make it much less sensitive while adjusting the output?  I have about 5 degrees of latitude on the output knob between no glow and blown plug. A resistor in series with the glow plug leads?  Perhaps a resistor in the glow plug lead would help.  If so, what value would be appropriate?  A pot may be the cat's pajamas in this application.

     If this problem becomes insurmountable. I guess that I could resort to using a 2v sealed battery and a Fireball plug that is rated to 3v or one of those rechargeable batteries that lock onto the glow plug.  Perhaps I am just making things difficult for myself! Lol

     Tia for any suggestions or comments.

                                                                                                       Be well,

                                                                                                       Frank McCune



     


Online Brett Buck

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Re: More questions for the EE guys.
« Reply #1 on: July 11, 2017, 08:49:48 AM »
       Hello all:

        I have two problems that may have the same solution.  How can I check the power going to my glow plug without burning it out?

        I have been using a HD power panel that is very popular with fliers and seen on many flight boxes.  This is the ubiquitous type with outputs for glow plug, starter fuel pump etc.  All of a sudden, as soon as I touch a glow plug lead to a glow plug, it goes off like a flash bulb!  This happens even with the pot turned all the way to what used to be very low output.  I checked the outputs and they all had 12 volts with no load.  I did place a load on the glow plug via a resistor, and I could adjust the power out put to the resistor..  This went from 0 to 1.5 volts as it should.  I am hesitant to use a glow plug for a test as I do not want to blow more plugs.

      Is there a way that I can test the output under load. Testing with a glow plug becomes very expensive. Lol  How much resistance does a glow plug produce?  I think that I must do my tests with something that produces the same resistance of a glow plug to obtain a valid result.  What would make the power panel start delivering 12 volts to the glow plug?  I looked and could not see any signs of damage. 

      Oh yes, is there a way to reduce the current from a glow driver to make it much less sensitive while adjusting the output?  I have about 5 degrees of latitude on the output knob between no glow and blown plug. A resistor in series with the glow plug leads?  Perhaps a resistor in the glow plug lead would help.  If so, what value would be appropriate?  A pot may be the cat's pajamas in this application.

     If this problem becomes insurmountable. I guess that I could resort to using a 2v sealed battery and a Fireball plug that is rated to 3v or one of those rechargeable batteries that lock onto the glow plug.  Perhaps I am just making things difficult for myself! Lol
     

    Without a schematic or a very clear picture of the back of the panel, we'd just be guessing. I have worked on several variants and the glow driver for the very cheapest types is a pretty crude transistor with a pot that sets the base or gate current, more-or-less open loop. As such, it is very prone to the transistor changing its characteristics and doing either nothing, or turning full on at the slightest inclination as soon as the voltage crosses some threshold, which is like yours. Replacing the transistor usually restores the function, however, the design is such that you sometimes wind up with nearly random results and you have to fiddle with the external resistors to trim it for your particular transistor. Many of the common replacement sources like NTE are using either rejects from someone else's process that happen to fall within a country mile of some other transistor and are listed as replacements for yours by fate more than design.

   The simplest solution is to just go get another power panel, they are cheap. A perhaps better solution is to go get two McDaniel batteries and a charger like most of us use. The worst solution that probably will require some re-engineering is to go find a replacement transistor and try to repair the current panel. If you want to test, get a 1/2 ohm resistor rated at about 25W (which in the worst case will get VERY HOT), a fuse clip and a package of 5 amp fuses, hook the fuse in series with the resistor and use that as a dummy load. And don't have anything flammable around when you test.

   There are also more sophisticated glow-driver circuits that use PWM switching power supplies with a variable duty cycle, or constant current systems, that are beyond diagnosis with no schematic over the internet; If it makes a buzzing sound as you connect it, that's what you have. Those are a much better design overall and the problem might be a lot simpler like a failed capacitor, but that even wilder speculation.

      Brett
« Last Edit: July 11, 2017, 09:11:20 AM by Brett Buck »

Offline frank mccune

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Re: More questions for the EE guys.
« Reply #2 on: July 11, 2017, 09:08:25 AM »
      Hello Mr. Buck:

      Thank you so very much for taking the time to hare your knowledge.  I do have another panel in the attic that I will swap into flight box.  I think that I will use the 0-5 amp ammeter to monitor the current on my glow driver.  As it is now set up, there is no means to monitor the amount of current flow to the plug.  This may provide me with an indication when I approach the ideal current to the plug.  As it is now set up there is a piolet lamp the is supposed to turn on when the correct amount of current is flowing to the plug.  This is on the jagged edge of destroying a glow plug as it is very critical to what the correct setting is on the driver.

      Why am I punishing myself?  Just hook up 3 or 4 "D" cells and call it a day! Lol  The glow driver is neat as it will light a plug even when immersed in water!  I take this as in indication that there will not be a flooded plug in my life ever again! Lol

                                                                                Be well my friend.

                                                                                Frank McCune

       

Offline Tim Wescott

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Re: More questions for the EE guys.
« Reply #3 on: July 11, 2017, 01:23:49 PM »
Dunno what's going on, because if I'm not mistaken you're saying you see reasonable voltages with a load.  One way or another, it sounds like the panel is borked, though.

Brett's description of cheap panels disappoints me, but on reflection I guess I shouldn't be surprised.  I'd be interested in seeing a schematic.
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Offline frank mccune

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Re: More questions for the EE guys.
« Reply #4 on: July 11, 2017, 02:47:58 PM »
    Hi Tim:

     I misspoke re. testing the voltages.  I checked all of the outputs WITHOUT a load and as normal I got 12 volts on all.

     Now here is the mystery for me.  When the output for the glow plug was tested WITH a load, I could adjust the amount of voltage driving the plug with the pot on the panel. I could advance the voltage to 1.5v on the 4 resistors of varying  value. However, if I connect  glow plug to the glow plug output, I get a burned out plug instantly.  This leads me to think that a plug has very little resistance.  I will check on this as soon as I complete this reply.

      I think that Brett is on to something.  I have two other panels in the attic to substitute for the one that is not working properly.

                                                                                                           Thanks for the reply,

                                                                                                            Frank McCune

Offline Tim Wescott

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Re: More questions for the EE guys.
« Reply #5 on: July 11, 2017, 02:59:21 PM »
    Hi Tim:

     I misspoke re. testing the voltages.  I checked all of the outputs WITHOUT a load and as normal I got 12 volts on all.

     Now here is the mystery for me.  When the output for the glow plug was tested WITH a load, I could adjust the amount of voltage driving the plug with the pot on the panel. I could advance the voltage to 1.5v on the 4 resistors of varying  value. However, if I connect  glow plug to the glow plug output, I get a burned out plug instantly.  This leads me to think that a plug has very little resistance.  I will check on this as soon as I complete this reply.

      I think that Brett is on to something.  I have two other panels in the attic to substitute for the one that is not working properly.

                                                                                                           Thanks for the reply,

                                                                                                            Frank McCune

By all means swap out the panel.  The fact that you can get a good voltage on a resistor, but will burn out a glow plug, is strange.  If you DO want to do further testing (to find out how it's borked -- you already know it is borked), then use the 1/2 ohm resistor Brett suggested, or an old 7" sealed beam car headlight with the low and high beams connected together.
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Online Brett Buck

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Re: More questions for the EE guys.
« Reply #6 on: July 11, 2017, 08:23:48 PM »
    Hi Tim:

     I misspoke re. testing the voltages.  I checked all of the outputs WITHOUT a load and as normal I got 12 volts on all.


   That is not abnormal, open-loop, there will be no consequential voltage drop across any active components.  12-ish (maybe minus .7 volts for the transistor forward drop) sounds about right.

Quote


     Now here is the mystery for me.  When the output for the glow plug was tested WITH a load, I could adjust the amount of voltage driving the plug with the pot on the panel. I could advance the voltage to 1.5v on the 4 resistors of varying  value. However, if I connect  glow plug to the glow plug output, I get a burned out plug instantly.  This leads me to think that a plug has very little resistance.  I will check on this as soon as I complete this reply.
                                                                                                            Frank McCune

     The plug should be around 1/2 ohm but it varies depending on the temperature. I am not sure what value the other resistors might be.


   Brett


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