Hi guys
I have a problem which I cant really get the hang of, and I thought that id maybe try my luck here.
Would just like help on 2 questions if anybody maybe knows?
1. for general interest -----> Why does one always switch the negative input of a automotive ignition coil? Meaning that the coil wants a permanent +12v and the pulsing is on the negative pin? why can it not work the other way around? The only coils that I have seen that work with a +12V pulse are later model coilpack coils that allready have embedded driver cct's and require a perm +12v aswell and a ground with the +12v pulse.
2. Then the main problem -----> I am trying to build a tester for these units, after thorough research i decided on using power mosfets, for the ease of pulsing from a microcontroller. I need this unit to have a fixed GND and generate +12v pulses for some coil packs, and a constant +12v with negative pulses(or switching to ground if you understand what im trying to say) for other coils.
All the MCU programming is complete, just struggling with the driver circuit.
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For the constant 12v and negative(GND) pulses, I used a N Channel mosfet, like described mostly over the net(In a common emitter type config). It works pretty well
and I get 11.9V which I think is pretty decent for a 12V source.
Here is the circuit I am using for the constant +12v and the negative (GND) pulses:
PLEASE NOTE I HAVE NOT ADDED IN THE SNUBBER PROTECTION CCT OVER THE "SIMULATION COIL", the circuit is just to give an idea.
________________________________________________________
However, for the constant GND and +12v pulses circuit, i use a P channel mosfet which according me needs a GND to open, and needs to be pulled up. So im driving it with a PNP transistor which switches the P channel gate to GND. Now for some reason I am getting a Max voltage of 10.6 over the load(coil) in simulation. I am quite new to mosfets and was under the impression this should switch the gate to open fully and give a 12v over the load, but no matter how I move my config it does not happen.
Here is the circuit I am using for the constant GND and the +12v pulses:
PLEASE NOTE I HAVE NOT ADDED IN THE SNUBBER PROTECTION CCT OVER THE "SIMULATION COIL", the circuit is just to give an idea.
______________________________________________________________
Is there any reason why this is like this? Anybody maybe know? like I said I am fairly new to mosfets, and would really appreciate help if possible?
3. Last question, is it better to use a proper RC Snubber over the coil in the above mentioned circuits, or will a simple Flywheel diode do the trick in protecting from back EMF? are there any advantages in using a proper snubber cct?
Many thanks in advance
Cheers
I have a problem which I cant really get the hang of, and I thought that id maybe try my luck here.
Would just like help on 2 questions if anybody maybe knows?
1. for general interest -----> Why does one always switch the negative input of a automotive ignition coil? Meaning that the coil wants a permanent +12v and the pulsing is on the negative pin? why can it not work the other way around? The only coils that I have seen that work with a +12V pulse are later model coilpack coils that allready have embedded driver cct's and require a perm +12v aswell and a ground with the +12v pulse.
2. Then the main problem -----> I am trying to build a tester for these units, after thorough research i decided on using power mosfets, for the ease of pulsing from a microcontroller. I need this unit to have a fixed GND and generate +12v pulses for some coil packs, and a constant +12v with negative pulses(or switching to ground if you understand what im trying to say) for other coils.
All the MCU programming is complete, just struggling with the driver circuit.
___________________________________________________
For the constant 12v and negative(GND) pulses, I used a N Channel mosfet, like described mostly over the net(In a common emitter type config). It works pretty well
and I get 11.9V which I think is pretty decent for a 12V source.
Here is the circuit I am using for the constant +12v and the negative (GND) pulses:
PLEASE NOTE I HAVE NOT ADDED IN THE SNUBBER PROTECTION CCT OVER THE "SIMULATION COIL", the circuit is just to give an idea.
________________________________________________________
However, for the constant GND and +12v pulses circuit, i use a P channel mosfet which according me needs a GND to open, and needs to be pulled up. So im driving it with a PNP transistor which switches the P channel gate to GND. Now for some reason I am getting a Max voltage of 10.6 over the load(coil) in simulation. I am quite new to mosfets and was under the impression this should switch the gate to open fully and give a 12v over the load, but no matter how I move my config it does not happen.
Here is the circuit I am using for the constant GND and the +12v pulses:
PLEASE NOTE I HAVE NOT ADDED IN THE SNUBBER PROTECTION CCT OVER THE "SIMULATION COIL", the circuit is just to give an idea.
______________________________________________________________
Is there any reason why this is like this? Anybody maybe know? like I said I am fairly new to mosfets, and would really appreciate help if possible?
3. Last question, is it better to use a proper RC Snubber over the coil in the above mentioned circuits, or will a simple Flywheel diode do the trick in protecting from back EMF? are there any advantages in using a proper snubber cct?
Many thanks in advance
Cheers