Originally posted by bobkrech
What most folks don't know is that the most rapid ignition of an ignitor occurs when the resistance of the timer circuit is equal to that of the ignitor. That because the maximum possible power is being delivered to the ignitor so it heats up in the shortest possible time. I have attached a graph of the power delivered to an ignitor as a percentage of it's resistance in the firing circuit. For example if the ignitor has a resistance of 1 ohm, optimum igintion speed occurs when the resistance of the battery, firing circuit, and wires equals 1 ohm, but anything less is ok because that means you get higher current than required but it is way about the minimum all-fire current. If the circuit resistance is greater than that of the ignitor, you might not be able to deliver the minimum all fire current if the battery is not fresh.
Um, not quite right.
{Equation 1} IgniterPower = IgniterResistance * IgniterCurrent^2,
{Equation 2} IgniterCurrent= BatteryVoltage / (IgniterResistance + ParasiticResistance),
where ParasticResitance is the internal resistance of the battery, plus the resistance of leads, connectors, switches, relays, etc.
Combining equations {1} and {2}, we get
{Equation 3} IgniterPower = BatteryVoltage^2 * IgniterResistance /(IgniterResistance+ParasiticResistance)^2
Bob K's graph is correct if you are working with a fixed parastic resistance and can vary the igniter resistance. Note that for very small igniter resistance, the numerator is small and the igniter power is also small. For very large values of igniter resistance, the denominator get huge and again the igniter power is small. The maximum igniter power will occur when the the igniter resistance matches the parasitic resistance.
However, the reverse is not true. If you are using a fixed igniter resistance, then maximum igniter power happens when the parastic resistance is zero. If parasitic resistance is increased until it matches the igniter resistance, the total current will be cut in half and the igniter power will be one-fourth.
Parasitic resistance is all bad, and should be minimized wherever possible.