Hello!
I have a glow plug igniter for RC cars etc. and I want to switch a led on so I know the glow plug is good. Problem is the glow plug voltage is around 1.5v and not enough voltage to light the led. So I need to detect current flow and use a transistor to switch the led on from the glow plug igniter battery. Appreciate any ideas on how I can make this work.
Thanks,
Kevin
That's a 5V battery with a resistor in series with the glow plug?
You should have ~3.5V across the glow-plug resistor when the glow plug is good. If the glow plug is open, you'll have 5V across the glow plug and zero volts across that resistor.
If you put an LED with its current-limiting resistor in parallel with the glow-plug resistor, the LED should light-up when the glow plug good.
Or, you can put the LED and resistor and in parallel with the glow plug, the LED will light-up when the glow plug is open.
I used the battery as just a representation of the glow plug driver. If you read the description you would have read that I only have 1.5v to work with coming out of the glow plug driver..
Updated schematic attached
I only have 1.5v to work with coming out of the glow plug driver.
Then you will need another voltage source to provide the voltage to light the LED.
The LED connects to the voltage source and then to a resistor and finally to the collector of the transistor. Then a resistor to the base of the transistor to the positive end of your glow plug. Finally emitter of the transistor, negitave of your voltage source and ground of your glow plug must all be connected together.
That lights the led as soon as power is applied to the glow plug and will light either bad or good!
Seems to me current sensing would have to be used from across the resistor, just not sure how to pull it off. I guess I will read up on it.
Use a 1:2 to 1:3 voltage divider across the glow plug, with it's tap to the base of an NPN transistor.
Emitter to glowplug/supply ground, LED from (5volt) supply to collector (with CL resistor).
1k and 2k7 could work. 1k from base to ground, 2k7 from glowplug(+) to base.
If the glowplug is still ok (<= 1.5volt across), then there is not enough voltage on the base to switch the warning LED on.
Leo..
I will give that a try! THANKS
WAWA,
The circuit worked great. I had to make it a equal voltage devider for the general purpose transistor I am using. Base voltage to turn it on was .6 - .7 volts. The led is on all the time until you hook up the glow plug and it goes off. If the glow plug is open the light will be on. This might be backwards from how you mentioned but it works for me!
I totally didn't think about a circuit like this but makes perfect since now. The glow plug drawing more current robs the voltage devider from any current flow.
I thought I may have to end up using some circuits I studied up on about a small current resistor and a current sensing chip( differential amp) to pull it off.
Thanks for all your help! You have helped me out greatly on other posts!
Kevin
Glad you got it working but
The glow plug drawing more current robs the voltage devider from any current flow.
Is not what is happening.
It is more like your supply can not support the current the glow plug is demanding and so it drops the voltage.
If you had a better power supply then this soloution will not work. Therefore if you change the glowplug’s power supply then this circuit may not function anymore.
It is fine as you have it but you have to realise that your soloution is a kludge and you should not apply what you think is happening to any other design.
Hmm! My supply for the glow plug is a bench supply that can deliver up to 10amps. When the glow plug is hooked up the voltage doesn't drop.
This is the same thing as putting a motor and a led strip in parallel, the motor will take all of the current and leave the led strip dead.
Ok what is a better solution?
This is the same thing as putting a motor and a led strip in parallel, the motor will take all of the current and leave the led strip dead.
No it is not, that is not how electricity works. You seem not to have a grasp of how current and voltage work with kershoff's laws, see Kirchhoff's circuit laws - Wikipedia
Please draw a circuit diagram of what you have and include the part numbers, and we will see if we can explain to you what is happening.
Ok teacher, explain to me why the circuit wouldn't work? Also explain to me how the circuit relates to Kirchoffs law 1&2.
You also have not suggested another option.
I also want to mention I am not a newbie, so I expect respect!
Ok teacher, explain to me why the circuit wouldn't work?
OK then please comply with the request in reply #10.
KevinRoach:
The circuit worked great. I had to make it a equal voltage devider for the general purpose transistor I am using. Base voltage to turn it on was .6 - .7 volts. The led is on all the time until you hook up the glow plug and it goes off. If the glow plug is open the light will be on. This might be backwards from how you mentioned but it works for me!
You probably didn't use it as I explained.
The LED should be off with the glow plug installed.
Divided voltage (on the base) should be <0.5volt with <2volt on the glow plug,
and >0.7volt with >3volt on the glow plug.
Leo..
Wawa,
Yes it is off when the glow plug is hooked up. So explian to me why this works? Is it as I mentioned in the previous posts?
Not sure if I understand what you mean.
The base of an NPN transistor has a threshold voltage of about 0.5volt.
Below that there is (almost) no collector current flowing (LED).
The voltage divider makes sure it's below that threshold when the voltage on the glow plug is less than
(1k+2.7K)*0.5volt = 1.85volt.
For driving a LED to 20mA, with a low gain transistor, ~200uA base current is needed.
650uA is 'lost' in the 1k resistor to ground, assuming 0.65volt BE.
So 850uA is needed for the voltage divider >>> Led full on at ~3volt @ glow plug.
Leo..
Hi,
Welcome to the forum.
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Thanks.. Tom... 
Here you go Grumpy Mike! I have posted a pic and it is just like Wawa explained for me to do. Now explain why it won't work and how it applies to Kirchoffs law and what are an alternative circuits? I did my part by posting the circuit.
Why doesn't it stay on all the time? What happens when the glow is hooked up and good and causes the light to go out? I am trying understand exactly how it works.
My theory has been dismissed by others that the glow plug Rob's the current from the voltage divider.
The circuit relies on some resistance in the glow plug supply (resistor in diagram post#0),
so voltage across the glow plug will increase when plug is removed or open circuit.
Don't worry about a 3700 ohm voltage divider across a very low resistance glow plug.
What is the resistance of your glow plug.
Is the circuit (voltage divider) close to the glow plug (lowest voltage is near the glow plug).
Post a picture of the setup.
Leo..
