Ok another electronics post by yours truly - Figgie
Who is the audience?: Specifically ANYONE that is running any type of standalone and running a coil in inductance mode and the dwell time is UNKNOWN. This will not cover how to convert the time into angular degree.
and you guys thought that math was only usefull in school.
Well got some bad news for you guys. You are about to need it for this
I went through this excersice once before helping some guys with a BMW out
So grab a comfy chair, your favorite drinks and be prepared for a brain overload
remember we have to make assumptions
for this example we are going to use 14 vdc as that is what the alternator output when it is turning.
Using some Bosch coils for this example (I got these numbers directly from Bosch Motorsports Australia).
Bosch coil 0 221 504 410
Peak primary current 7.5A
Primary Inductance 3.7mH
Primary resistance 0.5ohm
Secondary Inductance 38H
Seconradary Resistance 13.3 K
Energy 70mJ
Clamping voltage 350
Voltage gradiant 1.1kV/micro sec
We test the coil at 100Hz
First the simplictic but rough version
First assume the current has already reached the anticipated 7.5 amps, we need to work out how much voltage is actually available across the inductive part of the primary winding. That available voltage is what actually is responsible for creating the ramp up of current.
Let's assume the battery voltage is 14.0 volts (from the alternator).
Let's also assume the voltage drop across the ignition module is 1.5 volts with 7.5 amps flowing through it.
We can work out the voltage drop across the resistive part of the primary winding from Ohms law. E=IxR E= 7.5A x 0.5 Ohms = 3.75volts.
So we start out with 14.0V but lose 3.75v and 1.5v. That leaves 8.75 volts to ramp up the current.
Dwell time = max current x primary inductance / available voltage
Dwell = 7.5 x 3.7 (mH) divided by 8.75 volts
Dwell = 27.75 / 8.75
Dwell = 3.17 (mS)
To do this more accurately the exact battery voltage will need to be known, the voltage drop across the ignition module will need to be known, plus any additional voltage drops in the wiring. All of these may change slightly with temperature. But 3.0 to 3.5mS will probably get you in the ballpark for that particular coil.
the hard version , aka more precision.
Assume fixed 1.5v drop across igniter, then Imax=(14v-1.5)/0.5=25A
t=(3.7mh/0.5)(log(1/(1-(7.5amps/25))))=2.64msec
this last one uses this equation
http://www.allaboutcircuits.com/vol_1/chpt_16/4.html
questions, comments, concerns, bitches, moans and complaints.
You know where to reach me
edit #1: forgot to put the Bosch coil 0 221 504 410
Who is the audience?: Specifically ANYONE that is running any type of standalone and running a coil in inductance mode and the dwell time is UNKNOWN. This will not cover how to convert the time into angular degree.
and you guys thought that math was only usefull in school.
Well got some bad news for you guys. You are about to need it for this
I went through this excersice once before helping some guys with a BMW out
So grab a comfy chair, your favorite drinks and be prepared for a brain overload
remember we have to make assumptions
for this example we are going to use 14 vdc as that is what the alternator output when it is turning.
Using some Bosch coils for this example (I got these numbers directly from Bosch Motorsports Australia).
Bosch coil 0 221 504 410
Peak primary current 7.5A
Primary Inductance 3.7mH
Primary resistance 0.5ohm
Secondary Inductance 38H
Seconradary Resistance 13.3 K
Energy 70mJ
Clamping voltage 350
Voltage gradiant 1.1kV/micro sec
We test the coil at 100Hz
First the simplictic but rough version
First assume the current has already reached the anticipated 7.5 amps, we need to work out how much voltage is actually available across the inductive part of the primary winding. That available voltage is what actually is responsible for creating the ramp up of current.
Let's assume the battery voltage is 14.0 volts (from the alternator).
Let's also assume the voltage drop across the ignition module is 1.5 volts with 7.5 amps flowing through it.
We can work out the voltage drop across the resistive part of the primary winding from Ohms law. E=IxR E= 7.5A x 0.5 Ohms = 3.75volts.
So we start out with 14.0V but lose 3.75v and 1.5v. That leaves 8.75 volts to ramp up the current.
Dwell time = max current x primary inductance / available voltage
Dwell = 7.5 x 3.7 (mH) divided by 8.75 volts
Dwell = 27.75 / 8.75
Dwell = 3.17 (mS)
To do this more accurately the exact battery voltage will need to be known, the voltage drop across the ignition module will need to be known, plus any additional voltage drops in the wiring. All of these may change slightly with temperature. But 3.0 to 3.5mS will probably get you in the ballpark for that particular coil.
the hard version , aka more precision.
Assume fixed 1.5v drop across igniter, then Imax=(14v-1.5)/0.5=25A
t=(3.7mh/0.5)(log(1/(1-(7.5amps/25))))=2.64msec
this last one uses this equation
http://www.allaboutcircuits.com/vol_1/chpt_16/4.html
questions, comments, concerns, bitches, moans and complaints.
You know where to reach me
edit #1: forgot to put the Bosch coil 0 221 504 410
