Chip Protection.....Inductive Kick
tek_man
Posts: 9
I am in the process of using the BS2 controller to drive automotive fuel injectors for training purposes. I have the code finshed and working the way I want it, and the basic circuit is built using LEDs to simulate the injectors.
Now I would like to go ahead and wire up the actual injectors.
I had a couple of questions regarding what transistors/mosfets to use to drive my injectors.
The injectors are 14.6 ohms and I will be driving them with 12VDC, which will give a current of slightly less than 1 AMP. I went to Radio shack and picked up a couple of different possibilities: a TIP120 Darlington Transistor rated at 5A continuous, base current 120mA and a IRF510 MOSFET.
The individual injectors will probably be running ar about a 10% duty cycle with an ON TIME of 5-30mS
The next question I have is about the inductive kick that the injectors will generate. I understand the inductive kick and how to clamp·it using a diode across the injector.
My problem is is that I WANT the kick to be there. (It is used for diagnostic purposes while scoping injectors on live vehicles)
I know that the Darlington has a built in diode across the collector/emmiter...Will this provide decent protection or do I need to use some sort of an opto-isolator?
The inductive kick from most automotive injectors ranges from 30-90 Volts. Auto manufacturers design the circuit in one of 2 ways, they sometimes "clip" the spike to a lower voltage (using a zener ?) or they just let the field collapse naturally with a resulting ~60V kick.
If this works as I think it will, I would like to make another similar project using ignition coils which generate ~400-600V kick and that circuit HAS to be unclamped to ensure a good secondary spark...
I can scope'em and diagnose'em, but I am still learning the "Design'em" part of this..
I have attached 2 waveforms from actual automotive injectors, one is "clamped' to ~35 V and the other collapses normally(Ignore annotations on waveform, I have used it in training for trigger setup)
Any thoughts would be greatly appreciated
Lee
Now I would like to go ahead and wire up the actual injectors.
I had a couple of questions regarding what transistors/mosfets to use to drive my injectors.
The injectors are 14.6 ohms and I will be driving them with 12VDC, which will give a current of slightly less than 1 AMP. I went to Radio shack and picked up a couple of different possibilities: a TIP120 Darlington Transistor rated at 5A continuous, base current 120mA and a IRF510 MOSFET.
The individual injectors will probably be running ar about a 10% duty cycle with an ON TIME of 5-30mS
The next question I have is about the inductive kick that the injectors will generate. I understand the inductive kick and how to clamp·it using a diode across the injector.
My problem is is that I WANT the kick to be there. (It is used for diagnostic purposes while scoping injectors on live vehicles)
I know that the Darlington has a built in diode across the collector/emmiter...Will this provide decent protection or do I need to use some sort of an opto-isolator?
The inductive kick from most automotive injectors ranges from 30-90 Volts. Auto manufacturers design the circuit in one of 2 ways, they sometimes "clip" the spike to a lower voltage (using a zener ?) or they just let the field collapse naturally with a resulting ~60V kick.
If this works as I think it will, I would like to make another similar project using ignition coils which generate ~400-600V kick and that circuit HAS to be unclamped to ensure a good secondary spark...
I can scope'em and diagnose'em, but I am still learning the "Design'em" part of this..
I have attached 2 waveforms from actual automotive injectors, one is "clamped' to ~35 V and the other collapses normally(Ignore annotations on waveform, I have used it in training for trigger setup)
Any thoughts would be greatly appreciated
Lee
Comments
·
·· Leaving the EMF Spike there means you’ll need a part that can handle the maximum voltages you expect to see there. ·As for the TIP120 requiring 120mA, it would actually only be 1mA if you’re driving 1A, since the gain is 1000 (if memory serves). ·Since you’re not putting the diode across the injector there is the possibility of breakdown of the TIP120 (even with the internal diode) which could cause damage to the I/O pin of the Stamp Module, however since you only need 1mA you could use a 1K resistor and even if the transistor failed in such a way that it shorted back through to the I/O pin the current would be insufficient to damage it. ·So I don’t think you need an Opto for this circuit.· Take care.
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Chris Savage
Parallax Tech Support
My personal favorite for driving solenoids are IRLZ44 logic level fets from IR, no biasing just direct connection to your stamp pins. If you use them you should however still install a current limiter from the stamp pin, and I use a 4.7k pull down resistor on the gate so it doesnt run out of control on power up. They have internal clamping diodes and a 55-60V rating, on a scope they seem to clamp out at around 70 volts so still seeing the spike for your class shouldnt be an issue.
Lee