Inductive proximity for ignition.-

[Iampappabear]

I am considering using a small inductive proximity switch on the Farm Boy engine I am currently building. If I were to do this, would I need any additional amplification to drive an ignition coil?

Thanks in advance.

Colin

 

[BaronJ]

I am considering using a small inductive proximity switch on the Farm Boy engine I am currently building. If I were to do this, would I need any additional amplification to drive an ignition coil?

Thanks in advance.

Colin

Hi Colin,

Possibly, some proximity switches require a voltage supply and will happily trigger a transistor. They often have a tell tale led on them. Others are simply a coil of wire wound around a magnet or just an iron core. This type do need some signal conditioning.

 

[Iampappabear]

The style I have has the indicator light and requires 12-24 VDC to operate, my concern is if there will be enough potential to fire the coil.

Colin

 

[BaronJ]

The style I have has the indicator light and requires 12-24 VDC to operate, my concern is if there will be enough potential to fire the coil.

Colin

Hi Colin,

Unlikely ! A coil requires both a voltage and quite a high current !
Basically the coil is just a pulse transformer.

To find out, put a small resistor in series with the sensor output and measure the voltage. Say 100 Ohms. If you get the same voltage as the one that you are using to power the sensor, try 10 Ohms. The idea is to find out if there is enough current available to do what you want. On the other hand you might find that there is little voltage across 100 Ohms.

The rule is 1 Ohm, 1volt equals 1 amp. So 10 volts 10 Ohms would be 1 amp. 10 volts 100 Ohms = 0.1 amp and pro rata.

You will also need a catch diode, connected the right way round across the coil/sensor output to suppress any spike that could destroy the electronics inside the sensor.

 

[Iampappabear]

Baron J, Looking seriously at using the prox in conjunction with a solid state relay. Can you give me any more insight into how to select and wire in the catch diode?

Colin

 

[BaronJ]

Baron J, Looking seriously at using the prox in conjunction with a solid state relay. Can you give me any more insight into how to select and wire in the catch diode?

Colin

If you are using a SSR you shouldn't need one !

However, the positive end of the diode should go to the most positive point.

 

[Iampappabear]

Based upon your comments about needing high current, my thoughts behind the SSR were to use it to amplify the current sufficiently to fire the coil properly. Does this make sense?

Regards

Colin

 

[BaronJ]

Hi Colin,

Yes it should work OK ! The only thought that crosses my mind is the SSR fast enough. It should be, since it is basically a transistor driven Triac. I would stick a catch diode across the coil, just in case. The coil back emf can be several hundred volts. A 1N4007 can handle 1Kv. They only cost pence.

As an aside the circuit I posted would work as well ! You would need a FET that can handle the current at whatever voltage you use, and the FET has the catch diode built in.

 

[bluejets]

Transistor assist ignition don't use a diode on the coil.
The diode is built into the transistor if you use the correct device.
Used the MJ10012 for many years.

 

[DKGrimm]

Colin and BaronJ,

Just read your recent exchanges on proximity switches driving ignition coils. I worked with solid state ignition switches and with coil ignition systems a number of years ago. I thought I might jump in with my own thoughts from that experience. The main point I wanted to make is that an ignition coil is not just a pulse transformer; it actually stores the spark energy in the coil as a magnetic field from a relatively high current that will be flowing in the primary when the switch (or points) opens.

At that moment, the stored energy in the magnetic field is released, and two main things happen: Both primary and secondary voltage spike very high to make the plug flash over, and most of the stored energy dumps into the spark. In addition to that, bursts of damped oscillation occur before the spark fires, and again when it ends. You can see all this with an oscilloscope.

The semiconductor switch that carries and interrupts the coil current must be able to handle the voltages that come with this event (think hundreds of volts). It will also see a big spike of power dissipation because the voltage spike building while a lot of the coil current hasn't finished shutting off. Snubbing or suppressing these transients with a diode is not a good idea because you don't want to steal any of the energy away from the spark itself.

I don't have any part numbers or information on semiconductor switches available today; all my work with coil switches was done so long ago that my favorite parts are long obsolete.

A couple of working papers I wrote (mostly to myself) several years ago can be found at www.dkgsite.com under the section called Magneto Working Papers if you are interested. There are discussions about magnetic theory and few oscilloscope pictures of spark waveforms, amongst other things.

Don

 

[BaronJ]

Hi Don,

I've just been and had a nosy around your web site, not read your papers yet, but will read them ! So particular thanks for your post.

Many years ago I too was involved in similar work designing and manufacturing ignition systems for speedway bikes, essentially using Ni-Cad batteries and SCR's to fire a coil, relying on the back EMF to turn the SCR off. The systems were simple and worked well, at least until the battery went flat, which was usually in the middle of the second heat.

Happy days stood in the middle of the track shouting your favorite rider on.

 

[bluejets]

Colin and BaronJ,

Just read your recent exchanges on proximity switches driving ignition coils. I worked with solid state ignition switches and with coil ignition systems a number of years ago. I thought I might jump in with my own thoughts from that experience. The main point I wanted to make is that an ignition coil is not just a pulse transformer; it actually stores the spark energy in the coil as a magnetic field from a relatively high current that will be flowing in the primary when the switch (or points) opens.

At that moment, the stored energy in the magnetic field is released, and two main things happen: Both primary and secondary voltage spike very high to make the plug flash over, and most of the stored energy dumps into the spark. In addition to that, bursts of damped oscillation occur before the spark fires, and again when it ends. You can see all this with an oscilloscope.

The semiconductor switch that carries and interrupts the coil current must be able to handle the voltages that come with this event (think hundreds of volts). It will also see a big spike of power dissipation because the voltage spike building while a lot of the coil current hasn't finished shutting off. Snubbing or suppressing these transients with a diode is not a good idea because you don't want to steal any of the energy away from the spark itself.

I don't have any part numbers or information on semiconductor switches available today; all my work with coil switches was done so long ago that my favorite parts are long obsolete.

A couple of working papers I wrote (mostly to myself) several years ago can be found at www.dkgsite.com under the section called Magneto Working Papers if you are interested. There are discussions about magnetic theory and few oscilloscope pictures of spark waveforms, amongst other things.

Don

.....which was why I referred to the switching transistor above in # 9