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