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Problem with TTL ignition circuit.

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jbeuckm

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I attached my schematic that is supposed to fire the igniter. The LED lights but I only measure about 2.5V at the igniter terminals and the igniter doesn't light. My battery is actually around 11.3 volts - not 9 - by using eight AA batteries in series.

How can I get the voltage up to battery level (or close) and make sure my igniter lights?
Screen Shot 2017-02-20 at 6.17.35 PM.png
 

UhClem

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The threshold voltage for that device is in the range of 2V to 4V. (drain current is a tiny 250uA at threshold) Note that the source is connected to the load and a NFET is controlled by the gate to source voltage. To get the voltage at the load up to what the battery will provide you must drive the gate to a voltage greater than the battery voltage by more than the threshold voltage. 10V more than the battery voltage would be nice.

A high side NFET driver would help.
 

jbeuckm

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A high side NFET driver would help.
Does this mean that I should move the igniter between the battery and FET? I think that is how the original schematic was that I essentially copied. My concern was that the igniter would then sit at the battery voltage so it would be easier to light accidentally by touching ground.
 

keithp

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A common plan is to use the FET on the low side of the igniter; that way, you can use whatever voltage you want on the high side.
 

UhClem

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Does this mean that I should move the igniter between the battery and FET? I think that is how the original schematic was that I essentially copied. My concern was that the igniter would then sit at the battery voltage so it would be easier to light accidentally by touching ground.
Yes. But the IRF510 is an old FET and will provide limited current with a Vgs of 5V. There are logic level FETs available with lower threshold voltage that will do better.
 

jbeuckm

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Sorry for the confusion. The schemo says IRF but I'm actually using IRL510.
 

cerving

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Geneally the voltage passing through the igniter isn't that important, it's the current. You can easily light just about any ematch with a decent 1S 3.7V LiPo. As long as you can get a few amps through the igniter, you're good. If you put the igniter on the +9V side of the FET and just shunt the drain to GND you'll be fine; if you put the LED and resistor across the source and drain you can use it as a continuity check. 330 ohms is probably too low; get a decent LED and you can light it with 1 mA, so 820 ohms or 1K will be fine and use less current. Also, you might want to move that 10K pulldown resistor directly to the gate instead 0f in front of the current-limiting resistor.
 

jbeuckm

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Thanks for all your help. I kludged the igniter between +V and the FET and had a good ignition tonight. My next prototype will look like this:
Screen Shot 2017-02-21 at 8.47.03 PM.png
 

OverTheTop

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The first circuit you had you were using the FET in a source-follower mode, so the output will be the gate voltage minus the Vgs at the current it stabilises at. It isn't a logic level FET, so a higher voltage on the gate will help greatly in getting it going. As it is the drive is marginal.

Your last circuit is the typical setup for an N-channel FET. It is used as a switch to ground, based on a high voltage on the gate. If you are still using only 5V for the gate it will not be turned fully on, but will possibly do the job.

If you change to a FET that quote a Vgsth (turn on threshold) of of 3 or 4V or less it is a "logic level FET" and will be slammed on solidly.

One word of caution if you happen to use source-followers again (this is to anyone who is listening!). They can oscillate if the situation is right, even though the nominal gain is less than one. A capacitor from gate to ground helps heaps usually, and a resistor in series with the gate I call mandatory. I have seen this on two occasions in 18 years. If the circuit was connected to an igniter it could have led to "exciting times".
 

jbeuckm

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if you put the LED and resistor across the source and drain you can use it as a continuity check.
Related: How can I detect the igniter continuity electronically? I am using ATMEGA microcontrollers that have A2D conversion or I could use a simple TTL signal. I guess one way would be to replace your continuity LED with an optoisolator and use that output. But there should be a simpler way...?
 

cerving

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Related: How can I detect the igniter continuity electronically? I am using ATMEGA microcontrollers that have A2D conversion or I could use a simple TTL signal. I guess one way would be to replace your continuity LED with an optoisolator and use that output. But there should be a simpler way...?
Do what I do with the Eggtimer altimeters and substitute the LED for an optoisolator. The receiver end (i.e. phototransistor) goes to a processor input port. Alternatively, you can just set up a voltage divider between the source side of the igniter and GND, and connect the middle to an analog input, use 22K to the high side and 1K to the low side; that will keep the input to < 1V up to 23V on the deployment side.
 
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