Flight box power panel question

[Gary Dowler]

This spring I bought a used flight box with a Thunder Tiger power panel.  Works great, just a question about it. Checking it with my volt meter in the various outlets showed exactly what I expected, except for the outlet for the glow plug. This has a rheostat to control it, and you can visually see a change in the element as the knob is turned. However, the bolt meter causes questions here. No matter where the knob is set, the volt meter says 9.95 volts.  Given that glow plugs are 1.5 volt units, how is it, even if this is simply an amperage control, that it functions on a fixed 9.95 volts?

[Brett Buck]

This spring I bought a used flight box with a Thunder Tiger power panel.  Works great, just a question about it. Checking it with my volt meter in the various outlets showed exactly what I expected, except for the outlet for the glow plug. This has a rheostat to control it, and you can visually see a change in the element as the knob is turned. However, the bolt meter causes questions here. No matter where the knob is set, the volt meter says 9.95 volts.  Given that glow plugs are 1.5 volt units, how is it, even if this is simply an amperage control, that it functions on a fixed 9.95 volts?

     The meter is broken?  When in use, the voltage across the glow plug is about 1.1-1.4 volts. I think Howard's constant-voltage source is nominally set to 1.0 volts as well.

     Alternately, there could be some sort of component failure that puts the full battery voltage across the meter, which probably means the meter will be dead pretty soon, too.

     There are at least three types of circuits used in these panels that I have seen, ranging from really good to dismal, and I am sure there are lots of variations. Unfortunately, without a schematic or a very clear detailed picture of the guts, anything we might tell you is probably speculation.

         Brett

[dave siegler]

most power panels that are switching supplies.  They pulse on and so the average is the voltage you need on the plug. 
It isn't a DC value.

Many DC volt meters are fooled by the switching supply. 

If you connect a meter to it open circuit, no plug you may see about 10 volts. 
if you looks at the voltage while a plug is being driven, you may see the correct value.  It depends on your meter. 
If the meter is in the panel, are you sure it is trying to measure glow plug voltage?  It may always measure battery voltage.

The panel usually measures current to the plug.  you turn the knob till it glows the way you want

[Brian Hampton]

I'm not at all electrically minded but the panel has a little gizmo in it that switches the full battery voltage on and off very quickly so you get 12 volts for an instant then zero volts for a longer instant and the knob just alters those instants. Also you can hear a bit of a buzzing sound which changes a little when you turn the knob and the gizmo does its thing. IOW, pretty much what Dave said .

[Randy Cuberly]

The switching supply will likely be dependent on the total resistance in the circuit to give a proper reading.
See what Dave Siegler said above.
I would simply risk frying an old plug by putting it in the circuit and see what happens .  Turn the rheostat down to the lowest setting and put the plug in the circuit.  If nothing happens, slowly turn the rheostat up until something does.  Watch the plug if it begins to glow normally somewhere around the middle of the "twisting" you know it's probably OK.
This is a reasonably cheap way to see if it works.  Personally I doubt you could repair the unit if it's bad.  It's likely "solid state" and cast in a block!

Randy Cuberly

[Brett Buck]

I'm not at all electrically minded but the panel has a little gizmo in it that switches the full battery voltage on and off very quickly so you get 12 volts for an instant then zero volts for a longer instant and the knob just alters those instants. Also you can hear a bit of a buzzing sound which changes a little when you turn the knob and the gizmo does its thing. IOW, pretty much what Dave said .

    Right, that's what Dave was trying to describe. It's called a switching or PWM (pulse-width-modulated) power supply. The same thing is going on in a lot of really small "wall-wart" chargers like you get with your cell phone and in RC servos (which is why it was called "digital proportional" control). Also in powered computer speakers, which are called "Class D" amplifiers, basically doing the same thing as twisting your dial, except electronically, and in response to the music input signal.

   The good power panels work that way. The less good ones are pretty rudimentary and described in another thread.

     Brett

[Gary Dowler]

The switching supply will likely be dependent on the total resistance in the circuit to give a proper reading.
See what Dave Siegler said above.
I would simply risk frying an old plug by putting it in the circuit and see what happens .  Turn the rheostat down to the lowest setting and put the plug in the circuit.  If nothing happens, slowly turn the rheostat up until something does.  Watch the plug if it begins to glow normally somewhere around the middle of the "twisting" you know it's probably OK.
This is a reasonably cheap way to see if it works.  Personally I doubt you could repair the unit if it's bad.  It's likely "solid state" and cast in a block!

Randy Cuberly

Did exactly that early on, just to determine where the safe cut off point was, which I then marked on the dial. That led me to the curiosity of what the voltage was at this point. Quite surprised to read 9.95 on my digital volt meter. This is where the whole question begins.  There is an amp meter on the panel, which reads about .5 amp with a plug in place. I can buy a half amp of current draw with a plug in place.

[Randy Cuberly]

Did exactly that early on, just to determine where the safe cut off point was, which I then marked on the dial. That led me to the curiosity of what the voltage was at this point. Quite surprised to read 9.95 on my digital volt meter. This is where the whole question begins.  There is an amp meter on the panel, which reads about .5 amp with a plug in place. I can buy a half amp of current draw with a plug in place.

Hmmmm.  Sounds suspiciously like it's working exactly as it was designed to!!! 

Randy Cuberly

[dave siegler]

Did exactly that early on, just to determine where the safe cut off point was, which I then marked on the dial. That led me to the curiosity of what the voltage was at this point. Quite surprised to read 9.95 on my digital volt meter. This is where the whole question begins.  There is an amp meter on the panel, which reads about .5 amp with a plug in place. I can buy a half amp of current draw with a plug in place.

DVM's don't work on a PWM signal.  You need a scope, the meter is trying to measure a average value of a time varying signal.  The plug doesn't care, the voltage switches faster than the plug can cool off. 

But a DVM cant measure that accurately.  Seems to be working correctly. 

[Brett Buck]

DVM's don't work on a PWM signal.  You need a scope, the meter is trying to measure a average value of a time varying signal.  The plug doesn't care, the voltage switches faster than the plug can cool off. 

But a DVM cant measure that accurately.  Seems to be working correctly.

    It probably would if someone put a capacitor across the leads, in addition to the glow plug,  but I am not sure what problem we are trying to resolve any more. The 10v is probably the 12V battery voltage minus two or three diode drops (~1.4-2.1 volts), i.e. probably the "on" voltage of the PWM signal.

   I misread the original post, this is not talking about a panel meter on the unit, it's just sticking a DVM across the glow plug outlet (possibly open-circuit, i.e. with no plug attached). As mentioned, that's not likely to tell you anything useful.

    Brett

[Fredvon4]

Over the years I have had many of the newer and progressively better "Power Panels" from the usual suspects

Twice, some Hobbico version failed...  with the result of fried plug... seems they tend (OK small sample) to fail closed circuit

I am pretty smart electronics guy with both analog and digital good Fluke or Simpson meters ---and still can not figure the circuit and no help for the source....block diagram is a super secret   so I just made Hobbico replace under warranty

I corresponded years ago with Howard Rush about his design--- But before I could source all the parts, several commercial solutions for very good igniters came on the market....My primary one is the RCATS version
I have a combat box with 2 Cyclon 2VDC  5Ahr primary PB cells that I control with wire diameter and length

MY RC box still has one of the variable current versions.  I too got stumped why testing with no load, or under load, I was seeing well over 1.5 Vdc

Until it occurred to me that all the glow plug cautions about 1.5 or 2 Vdc were because MOST typical power sources were primary cell batteries that have no current control except internal resistance....

The plug does not care how much voltage... but will only resist so much current.... a clue

E I R anyone?

[Garf]

Check this circuit.

[Fredvon4]

To Add---- a glow plug acts like a variable resistor in two ways.... the heat changes its value, as well as being wet, (from cooling and slight conductivity)  and the rate of change is non linear

I love my RCATS... the only condition it can not deal with is an open plug element

One of the things that frustrated me back when I was comparing notes with Howard was there is not a standard or any description for any brand of glow plug for us to know the design resistance for a plug

But I am helping to stray a long way from Gary's observation....that with the "working great" Thunder Tiger panel the indicated voltage is confusing given that we have been told to never exceed 1.5 or 2Vdc  (depending on plug design)

Now if I could just get a reliable clip that works for every different plug type I have

[Cody bishop]

I have the same thing as long as you have a good igniter it will not burn plugs