Ignition Coil Driver Questions

[QMESAR]

Dear All ,
I am looking to build a circuit found on the Internet for a Ignition Coil driver ,I am looking to place a RC snubber circuit across the Coils as shown in the schematic attached.
Would it be placed correct in this manner and also what is the reason for the Diode 1N5400 in this circuit .I think it is to small as the coil current is around 8 amps and the diode is a 3A diode

Thank you so much for your advise and information

 

[duke37]

The snubber circiut is there to absorb the energy in the circuit when the fet is turned off. This limits the drain voltage. I presume that you want a large voltage output so the voltage should be limited by the spark gap.

Primary current is determined by the coil resistance and inductance. If driven with a short pulse, the current will not rise to 8A before being turned off.
Diodes can stand more than the rated current if only a short pulse is used and time is allowed for the junction to cool down. Look up the details of the 1N5400 or use a fatter diode.

Perhaps I am wrong but the 1N5400 would stop any negative voltage at the drain so the 1N4005 is surplus to requirements unless it is being used as a zener.

 

[QMESAR]

Thank you so much for your valuable info and help ,It is much appreciated.
just to clear some things the Snubbers are not part of the original schematic ,I am thinking of including it as in the past working with inductance it gave me a lot of problems with out snubber circuit,
My question is is the way I placed the RC snubber on this particular circuit correctly?

 

[duke37]

I would get rid of the snubber and put a capacitor aross the fet instead of the 1N4005.
The energy in the coil will be 0.5*I^i*L and this will be transferred to the capacitor 0.5*V*V*C. V=I*sqrt(L/C). This is how the traditional ignition worked. A dodgy capacitor would give unreliable fireing.
Any snubber with a resistor will reduce the output, however if you insist, then put it across the transformer primary.

I made a couple of capacitor discharge ingnitions which discharged a capacitor into the coil with a SCR. This could run faster than the circuit you show since it was not necessary to wait for the current in the coil to rise, just needed a high power invertor to charge the capacitor quickly.

What are you going to use this for? Electric fences put out a pulse about once a second and so give time for the victim to withdraw. This, I believe, is a legat requirement.

 

[QMESAR]

What are you going to use this for? Electric fences put out a pulse about once a second and so give time for the victim to withdraw.

I need to simulate a running engine to get a speed signal to the ECU of my Old jaguar to see if the ECU triggers the injection in a nutshell ,I just need to trigger a coil that the ECU receive the 300V + voltage from the negative of the coil at its Terminal 18 , I need 150 HZ to have 900 Rpm on the V12.

snubber with a resistor will reduce the output, however if you insist, then put it across the transformer primary.

The snubber circuit is placed over the Primary side of the Coil ,The Original circuit does not have it and then the circuit will run for 20 sec and the 555 is dead.( think it the EMI of the Coil killing the 555)
I then calculated as per some simple snubber circuit calculations examples found on the net a circuit and placed it over the Primary
side of the coil ,Now the circuit runs for hours with out loosing the 555 .It did bring some improvement,


As you will see from the scope captures the Ignition is running fine (yellow Trace) Primary voltage is fine @385V however my issue still is a two part issue
(1) the current is much to high in the Primary (blue trace)as in this scope print 14A after your help I realize what could be the problem the trigger pulse(to the FET) is at 50% which is around 3mSec that s much to long ,I will reduce that tomorrow and see what is the result on the current in the Primary.
(2) with this high current the 1N4500 runs to hot so I removed it which seems to cause me to have a large negative spike in the Primary voltage this is where my knowledge left me ,However if the pulse duty cycle brings the Current under control I will refit the 1N5400 and see if the negative spike on the primary is still there or not as shown below.


A correct operating circuit the Primary should not have the large negative going voltage
Thank you very much for your interest in my issue (problem) I will give it another go tomorrow with the info you helped me with
Regards

 

[duke37]

I do not see how the 555 can be damaged from the fet but there is an input through the 56Ω resistor. A big fat capacitor across the supply may help.

 

[Bluejets]

Think you will find any snubber there will affect the way the primary collapses as it should.

 

[QMESAR]

Thank you so much ,I will try all variants and advises ,Appreciate all your input

 

[Bluejets]

I suppose asking why would be pointless..?

 

[QMESAR]

Hi All,

I suppose asking why would be pointless..?

No do not think so ,it is /was just a matter of 1 thing at a time ,

@duke37
Adjusting the FET switch ON pulse to 2mSec (as per car manual dwell time for Idle which is the frequency of my circuit) I am at 8A as per vehicle spec ,Thanks a million for mentioning the short pulse that made me see some light
I then refit the 1N5400 and it is running cool no heat problems also as mentioned
The secondary is also running fine having a 5mm gap a nice strong spark jumps over the gap
However I am still a bit concerned some times I see some strong negative going pulse on the signal as in the attachment on the FET Trigger signal and also on the current clamp trace ,I have no understanding why that is and that makes me afraid hooking up the ECU to this circuit could possibly damage my ECU even more .

@Bluejets,
With the snubber and also with out the snubber I do not notice a difference on the operation however I believe you this could possibly be an issue with the snubber when this circuit must drive the Ignition in a running car where there are various different load conditions on the engine and the Ignition must be able to supply enough spark under all load conditions which is not the case with me I only want to simulate a engine speed signal to my ECU to get it to trigger the injection system to see if it is alive.

Now I would like to ask why ?

I would think that it could damp the collapse of the primary current under switch OFF as it has a Time constant delay in charging up
however I am not an electronics specialist ,and the real reason would be something to learn.

The below scope print is without the snubber in the circuit in

Thank again to all the help and advise from you guys ,much appreciated

 

[WHONOES]

Would it not be safer for you and your ECU to generate 300V from a transformer supply and switch that on and off.

 

[QMESAR]

Thank you for your reply ,It sounds like a good idea ,can you show how to connect that suggestion as said before I am not an electrical person .

 

[WHONOES]

I will see what I can come up with.

Edit: Do you have any small transformers to hand?

 

[QMESAR]

Hi
Thank you very much ,I can get small transformers locally no problem if I only know what and how to use it
much appreciated

 

[WHONOES]

A transformer with either a single 12V or two 6V secondaries and a single 230V or two 115V primaries. A transformer of 3VA or so will be plenty.
I will have to test possible ideas here and then pass them on.
I assume that you can build and test circuits.

Edit: Don't buy anything yet.

 

[QMESAR]

Hi
Thank you so much yes, I can build and test circuits that is no problem ,I appreciate your help and efforts very much, Also the Primary Ignition Voltage does not need to be 300V,a 200V pulse at the negative terminal during switch of period is more than enough .
wow this would be really nice

 

[WHONOES]

So, is it a negative going pulse With Respect (WR) to 0V (ground) that you require or is the pulse positive WR to ground.
Have been trying out running a small transformer backwards plus a bridge rec' and smoothing cap' and can achieve over 400V no problem. Am using a MOSfet and driving it with my Sig' Gen. Was interested to see what sort of frequency would give best results. On my set up it is about 11.1KHz.
If 200V is adequate, things will be a little easier.
I found a 0.5VA transformer in my box of bits with 8-0-8 Primary and 230V secondary that works well.

Edit: Do you know what the input impedance of the port on your ECU is?

 

[QMESAR]

Wow this sounds very good ,
To trigger the ECU it is a Positive Pulse WR ground . this is how the pulse look ,The problem is there is no diagrams or specs for Automotive ECU's ,I have no idea of the impedance, is the maybe a way I can measure it ? I appreciate your help and effort ,this is amazing ! I hope the image helps

Ps I only need max 1KHZ which is 6000 Engine RPM so what you have is more than enough.if this can be variable to simulate engine speed range will be great. but a fix 300HZ will be ok for testing (1500Rpm)
Million Times tank you again

 

[WHONOES]

I've tried a set up with a 1.5VA 7-0-7 transformer with 2 X 115V primaries. I ran it backwards i.e. use the secondaries as the input and the primaries as the output. For the drive, I used a 40106 running as an oscillator. I use this as it gives a 50 - 50 mark space. The output drives a 2 transistor "totem pole" arrangement to then drive a MOSET which drives the 7-0-7 end of the transformer connected in series and then to a 12V supply.
The 2 115V windings are also connected in series. A bridge rectifier made up from 4 X 1N4007 diodes is connected to this end of the transformer with a 2.2μ Polyprop' capacitor for the smoothing / storage. A 10MΩ resistor was placed across the cap to discharge it when power was switched off.
The oscillator was set to run in the range of about 500Hz to 19KHz.
When tried, max output was generated at about 560Hz. A handy feature was that the output volts could be varied by adjusting the drive frequency.
A circuit to pulse the 200+ volts would also be required.
If you want to try this, build the 200V generator first and get that going then we can move onto the pulse circuit.
If you need a diagram for the generator let me know and I will pass one on.

 

[QMESAR]

Hi
This sound very good Thanks a million for your efforts I would appreciate if you can pass me on the schematic,I will then get the components and build the circuit ,
I can not believe the help I receive from you ,I am so happy thank you again
Have nice time