NE555 Automotive Ignition Circuit

Thread Starter

Sami Alawadhi

Joined Oct 5, 2015
27
I'm creating an ignition circuit with an NE555 timer--monostable circuit--in order to manipulate the spark igniton circuit to automatically shut off after a short amount of time. The load from the ignition coil causes the timer to burn out. So, I've put a HV FET between the output of the timer and the load to prevent the timer from burning. I've attached a schematic of the circuit to show you. Disregard the timer on the far left for now.
My question is with the diode (D1) between the FET (MN1 HV) and the ignition coil. It is supposed to be a high current power diode to prevent the FET from damaging. I'm testing the circuit with a simple LED to make sure the circuit works. However, when I place a diode (any diode) between the FET and the LED, the LED doesn't emit light like it should. Why is this the case? Are there any suggestions? When I test the circuit without a diode between the FET and the LED, the LED emits light for a second after I hit the switch, as I want it to.
So, is there an issue with how I'm connecting the diode to the circuit? Is there any special way to connect it?

Thank you.
 

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MaxHeadRoom

Joined Jul 18, 2013
19,172
If switching a LED you need a series resistor, make sure the Mosfet is turned on.
I would use an IRFP460 as the fet.
Also make sure you have a large decoupling capacitor on the +ve supply.
Max.
 

Alec_t

Joined Sep 17, 2013
10,446
D1 does nothing useful connected as shown. To limit the FET drain voltage you could use something like a Schottky diode plus another (Schottky) diode connected back-to-back in series across the coil primary. The diodes would have a combined breakdown voltage below the rated FET Vds, but high enough to allow a reasonable back-emf voltage to develop across the coil primary.
BTW, a 20uS pulse width is too short to allow more than a few mA current build-up in the average primary coil.
 

Thread Starter

Sami Alawadhi

Joined Oct 5, 2015
27
If switching a LED you need a series resistor, make sure the Mosfet is turned on.
I would use an IRFP460 as the fet.
Also make sure you have a large decoupling capacitor on the +ve supply.
Max.
The FET doesn't have a switch. It's worked without a diode in between it and the LED. I think the issue is with how I'm connecting the diode. So, how should it be. Really, the diode I'm going to use as D1 is a high current diode for the ignition coil.
Since you've brought up the capacitor, what value should it be. Does it depend on the coil, FET, and diode? I've put a link for the specs of these items if that's what I need to determine the capacitance labeled "cond" connected to D1:

Diode: http://www.goldmine-elec-products.com/prodinfo.asp?number=A10621

FET: http://www.digikey.com/product-search/en?KeyWords=IXTH02N250-ND&WT.z_header=search_go

Ignition coil: http://www.summitracing.com/parts/sdk-b6a-12029-b seid=srese1&gclid=COq2kZXA2sMCFQiHaQodMRMAVw

I hope this helps. I'm just not sure what capacitance to put by D1. I have some 1 nF, 10 nF, 1000 uF, 2200 uF, and some 1 uF capacitors.
 

Thread Starter

Sami Alawadhi

Joined Oct 5, 2015
27
D1 does nothing useful connected as shown. To limit the FET drain voltage you could use something like a Schottky diode plus another (Schottky) diode connected back-to-back in series across the coil primary. The diodes would have a combined breakdown voltage below the rated FET Vds, but high enough to allow a reasonable back-emf voltage to develop across the coil primary.
BTW, a 20uS pulse width is too short to allow more than a few mA current build-up in the average primary coil.
I tried it with the Schottky diodes, but I still didn't get any light from the LED when I tested the circuit.
 

ian field

Joined Oct 27, 2012
6,540
D1 does nothing useful connected as shown. To limit the FET drain voltage you could use something like a Schottky diode plus another (Schottky) diode connected back-to-back in series across the coil primary. The diodes would have a combined breakdown voltage below the rated FET Vds, but high enough to allow a reasonable back-emf voltage to develop across the coil primary.
BTW, a 20uS pulse width is too short to allow more than a few mA current build-up in the average primary coil.
Shottky diodes typically go up to about 60V - 90V for a fair bit more money. Not the sort of thing I'd consider where back emf is concerned.

When I repaired VGA monitors for a living, I found some manufacturers used a Shottky instead of a zener to protect the MOSFET gate, the type used was rated 20V - when they fail, they make a much more certain short circuit than a zener.

Sometimes when regulation failed, the core saturated and the MOSFET punched through (short circuit all 3 ways) the charge in the reservoir capacitor invariably blew open the wire wound source resistor. On several occasions I found the Shottky protecting the gate had failed short circuit so solidly - it had actually prevented the blow up from damaging the control chip.
 
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