Glow Driver and Meter
 Roy S. Appleton
9-21-2003


 
I've been flying combat for about 3 or 4 years and early on there were so many things to get right to just fly in an event.  I decided that I had to come up with an answer for at least one of my problems.  The thing that was so irritating was to have the horn blow for the 90 second launch and not be able to get my engine to fire and find out that the glow plug was gone.  You've got to be sharp to guess that the plug is bad and get it changed in 90 seconds and that wasn't usually the case for me.  I had a Ni-Starter with a built-in meter that I had used in the past for sport flying but eventually the meter got vibrated to death and you couldn't fix it nor for that matter could you replace the nicad battery in it if it went bad. I'd seen a couple of self contained glow starters with leds lights to let you know that you were drawing current and your plug was good but they were way too much money. 

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So one day I was in Tanner Electronics and I noticed that they had just gotten in several cases of cheap digital multimeters for under $10 each.  Tanners is a surplus electronics store in Carrollton, Texas.  I looked the multimeter over and found that it was a little unusual in that it had a 10amp range in the ammeter function.  Most of the time you find meters like this maybe go up to 1 amp or 1000 milliamp.  A glow plug will draw between 2 and 4 amps depending on manufacturer and whether it's a hot or cold plug.  So here was the answer to checking for bad plugs.  All I had to do was add a nicad battery and a cable and I'd have a glow driver and digital readout of what current the plug was drawing.  If the meter showed current then the plug must be at least glowing.  Plus I might be able to actually tell the condition of the plug by the amount of current it was drawing!
I had several C sized nicad batteries that I'd recovered from a commercial cordless drill pack.  They were somewhere in the range of 1500 to 1800mah batteries and one of them would work perfect for this project.  I took the old insulating cardboard sleeve off the battery and replaced it with a piece of fresh heat shrink tubing.  Since I was going to just "hang" it off the back of the meter I didn't want it shorting out against something in my tool box.  Then I used some goop adhesive to stick the battery to the back of the meter.  After letting that set up over night I soldered an old glow starter cable to it and I was ready to test it out.  Well it worked like a champ.  I just flipped the selector to the 10 amp position and stuck a glow plug in the connector, the plug glowed and the meter read 3.00 amps!  I've since found that various plugs read and draw different amounts of current but if it's reading a solid number in the 2 to 4 amp range you know your plug is glowing.  If you wanted to you could probably keep track of the current readings from when the plug was new and see if the readings change with time and age on the plug.  I'm not sure if the resistance and the heat of the plug at tied together so that you could tell when a plug is going bad or not.  Mine are usually just fine and then the next flight the coil is damaged or broken.  This has been more of a problem with Open B and pushing the OS 25fx to the higher rpms then it's been so far with SSC and  the 15 size engines we're running in SSC.  In any case it sure is easy to check the plug right after a round and see if you've got a reading and change the plug before you're under the gun and the 90 seconds is ticking away.  You can use the driver without the meter turned on.  If the meter is turned off the battery is still connected to the plug connector, you just don't get a meter reading.  So I do a quick check after a flight with the meter on to see the condition of the plug and then just leave the meter off the rest of the time including when the start of match is called.  I've never had a plug check good and then five minutes later be bad.  That way I don't leave the meter on all the time.  Which can be important as I'll explain a little later.
I've included a diagram and a couple of photos of my meter so you can see what I've done.  Starting with the diagram, figure 1, you see that all you have to do is put the battery and the meter in series with the cable.  You can use the banana plugs and wire that came with the meter to connect to the meter.  I just cut the probes off and soldered the black one to the negative size of the  battery  and the positive (red wire) to the glow plug cable.  I soldered the other wire of the plug cable to the positive side of the battery.  There really isn't any polarity to a glow plug so you could turn the battery around and the set up would still work just fine.  The meter would simply show a negative current flow instead of positive.  The glow plug is like a light bulb and doesn't care which way the current is flowing.  The only thing you've got to do right is put the battery in series with the meter and not parallel.

You can use about any meter you find that has the ability to read 5 amp or more.  It can be an analog meter as well as a digital meter.  As shown in figure 2,  you need to look for a meter with the ammeter function, a 5 amp or greater scale and it will usually have connection separate from the voltage or resistance connection for the probes when measuring ampage.  Figures 3 & 4 show the battery and wiring connections.  You'll also notice that there's a 9v battery on the back of the meter.  After about a year of using the meter and several times forgetting to turn it off and leaving it on for a day or two, the internal battery that runs the meter died.  That might be the advantage of an analog meter instead of digital.  As an analog doesn't need a battery.  In any case, when I opened the case on the meter to see what kind of battery it took I found a stack of 1.5v watch batteries.  Well the cost of replacing those watch batteries would have been about half of what I'd paid for the meter to start with.  Fortunately the stack of batteries made 9 volts and so I just stuck a standard 9v battery on the back of the meter with some double back foam tape and wired it in place of the internal batteries.  Even if I forget to turn it off occasionally, I figure that it's good for a couple of years and a 9v battery is only a buck. 
As for charging the battery, you can connect the plug connector to whatever charger you'd normally charge your "Ni-starter" with.  The meter doesn't have to be turned on when you're charging the battery but if you're curious what the charge current is the meter will also show you how much instantaneous current you're putting back in the battery.  It's not going to tell you how much total current the battery took like a battery charger might.  I normally leave the meter off.  No sense running down that 9v battery any sooner.  One day at an event I'd forgotten to charge the battery the night before and so I used my "Hanger 9 Ni-Charge Quick Charger" that runs off 12v to charge the battery enough for a couple more rounds.  Then I left the meter on to see just how much of a charge the unit gave the battery.  The Hanger 9 unit is handy.  It runs off 12v and has a timer function that gives your "Ni-Starter" about a 20 minute charge at 1 amp.  I usually give my "Ni-Starters" one or two shots with the unit and I'm good to go the rest of the afternoon.

Speaking of charging the battery, another nice thing about this glow driver is that it's fairly easy to tell when your battery is about out of charge.  Let say that you're using a plug that normally draws between 2.9 and 3.0 amps.  When you're checking the condition of a plug after a round, if  you keep it on for about 15 seconds or so and you see the current reading start to drop off slowly, 2.5, 2.20, 1.9, etc.  That's not a bad plug, that's your battery running out of juice!  It's time to grab your fully charged backup "Ni-Starter".  You do carry a backup starter don't you?  Instead of spending 45 seconds of those precious 90 seconds finding out and then having to beg a starter from a another pilot!  Winning is all about paying attention to details!
That's about it.  I think that I've explained about everything.  If there's anything that didn't make sense or if you have any other questions drop me a line and I'll try and help you out.  Just click on my name at the top of the article to send me an email.  I love my meter and wouldn't be without it!