555-based ignition coil, assistance requested / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

555-based ignition coil, assistance requested

1 2 3 4 next

Move Thread

LAN_403

Theodore

Tue Jul 14 2009, 11:33PM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Alright, tried my hand at a much lower power ignition coil.

Includes:

a 350W PC Power Supply for +12V rail (up to 16A)
a simple astable 555-timer circuit built on a breadboard
a single NTE2396 MOSFET (100v, 20A) mounted on an old Athlon aluminum heatsink w/ fan

The design is based on these two links:

Although not so much the second one. The 555 astable timing circuit is straight from the LM555 pdf, which is attached.

Also attached are some pics, one which is a labaled circuit.

Part List:
1). 350W PC Power Supply
2). LM555
3). NTE2396 MOSFET
4). .1 microFarad Tantalum timing capacitor
5). 1 microFarad cap
6). 1 microFarad cap
7). 10 Ohm 3W Resistor
8). 100Ohm 5W Resistor
9). 330Ohm 5W Resistor
10). 10k Ohm 1/4W Resistor
11). 10k Ohm Pot.
12). Athlon Aluminum Heatsink+Fan
13). Accel 8140 "Super Stock" Ignition Coil

Ok, so the behaviors:
1. First and foremost, when turned on, no output from the coil to earthground, through about 2mm of Air.
2. None of the components produce heat, except:
3. The coil itself gets pretty warm, quickly. Its hot but touchable after about 30 secs.
4. Apparently my multimeter has crapped out on me, as it wont measure Voltage but does measure Frequency.
5. That being said the PC-PS does produce some voltage, as it runs the fan.
6. Tried to check frequency of gate, and I got 60Hz ?!?
7. When I flip the off switch on the power supply I get one, quick spark from the Iggy to ground.

Based on this, I am guessing that the MOSFET is turning on and never turning off, until I kill the power, and then the change in current in the main coil produces enough voltage on the secondary for a single spark.

Can somone please point out the error of my ways, and suggest a solution, as I simply cannot figure out what I am doing wrong.

Many thanks,
Theodore

PS Note that resistor 10). and capacitor 5). arent hooked up because I was trying various different things to see if I could get an output. Normally they are connected.

]nte2396.pdf[/file]
]lm555.pdf[/file]

Antonio

Wed Jul 15 2009, 12:09AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The circuit in the second link has several problems in the connection between the 555 and the transistors with those useless diodes (some protection, maybe) and that potentiometer (?!).
The first link seems less bad. There is nothing to discharge rapidly the bases, and unmatched transistors in parallel are not reliable.

Theodore

Wed Jul 15 2009, 01:30AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

I apologize, I wasn't very clear and forgot to include important information. A quick MSPaint drawing is attached to this post. This is the design of the circuit that I am actually using. Those other two were just for reference.

Also, it has been bothering me for a long time that in most "super simple" flyback schemes the bases arent being discharged. I guess I just assumed that they discharged through the MOSFET itself, which seems perhaps foolish now. Is that what is missing from this design? A resistor to ground, from the gate pin of the mosfet?

(For easier reference, the part list:
Part List:
1). 350W PC Power Supply
2). LM555
3). NTE2396 MOSFET
4). .1 microFarad Tantalum timing capacitor
5). 1 microFarad cap
6). 1 microFarad cap
7). 10 Ohm 3W Resistor
8). 100Ohm 5W Resistor
9). 330Ohm 5W Resistor
10). 10k Ohm 1/4W Resistor
11). 10k Ohm Pot.
12). Athlon Aluminum Heatsink+Fan
13). Accel 8140 "Super Stock" Ignition Coil)

Thanks,
Theodore

ScotchTapeLord

Wed Jul 15 2009, 01:53AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

The 555 can sink the voltage on the gate of the MOSFET when it shifts to low.

It seems you may have mixed up the +12V on the MOSFET and the ground attached to the coil. If you tried this then there would be a short resulting from the current passing right through the MOSFET's antiparallel diode from source to drain and across the coil... but then, turning it off should do nothing. I'd try grounding the third pin of the MOSFET and putting 12V into the + terminal of the coil.

EDIT: I've found that a 10K resistor from gate (1) to source (3) is suitable if the 555 doesn't discharge the gate enough

Dr. Dark Current

Wed Jul 15 2009, 06:41AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think the transistor is not turning off. If you're sure you have connected it up right, it's probably fried.

Theodore

Wed Jul 15 2009, 09:17PM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

I put a 10k, 1/4W resistor from ground to gate. No luck.

Double checked my setup (should be +12V -> Inductive load -> MOSFET Drain (Pin 3)). Had it wrong. Switched em. No luck.

Must have blown the FET, I figured. I cant know for sure because I destroyed my multimeter. Switched to a brand new FET, hooked it up correctly (as far as I can tell). Turned it on... Nothing.

I have no idea what is going on. The output of the Iggy will travel to earth ground (copper rod in ground), right?

Also, I think I destroyed my multimeter by hooking it directly up to the 12V leads on the PC-PS. It has a max of 400mA, the PC-PS puts out 17A on that rail.

I guess I will go drop another $50 on a multimeter.

EDIT: Upon opening the multimeter I noticed a fuse and a convienently placed replacement fuse. Swapped em and its up and running again.
Going to try and figure out what the hell is wrong, but I could totally use more input and suggestions.

-Theodore

Sulaiman

Thu Jul 16 2009, 12:22PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

The voltage rating for your transistor is way too low,
by transformer action, if you want 20 kV output the primary voltage will be
20 kV x (Np/Ns) which from memory is about 20kV / 50 = 400V.

Due to leakage inductance etc add another 100V for 'safety'
So I'd use a transistor rated for 600V or more.

Check the data sheet for whichever transistor you intend using to ensure it's Avalanche rated.
(If the word Avalanche isn't in the datasheet then it isn't rated)

You may consider igbt transistors that are designed for this operation.

You can check for a 'blown' transistor with a bulb & battery/psu
since it will usually be a short-circuit from Source to Drain.
If the power supply is 'beefy' enough the transistor may have gone short-circuit
then blow open-circuit.
Connect Gate to Source, a good transistor will 'look' like a diode from Source to Drain.

In this kind of circuit with no protection,
when the transistor 'blows' it will often take the 555 with it.

Dr. Dark Current

Thu Jul 16 2009, 01:10PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, a MOSFET can't be destroyed by overvoltage (as long as you don't overheat it) so I'd say something else is wrong in the circuit if the OP gets no output at all.

Theodore

Tue Jul 21 2009, 09:03PM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Still no luck.

I redid the 555-timer circuit on the breadboard, using the simple astable design here:

(A little less than half way down page.)

I also hooked my multimeter up in current mode in series with the +12V, i.e.:

+12V rail -> multimeter -> + on ignition coil

When the PC-PS is turned on it is reading about 6.5A of current going to the ignition coil, which slowly and steadily drops by 10mA/1s increments the longer it is left on (only left on for about 5 sec though).

I also switched to a new MOSFET (same model, new unit), and am getting the exact same behavior.

I am going to build a 555 test circuit after I pick up some LEDs.

In the mean time, anyone have any ideas? Should I use a <10k resistor from gate -> ground?

Thanks,
Theodore

PS EDIT: Tested it without pin 3 of the 555 hooked up to the gate, no current was detected. I guess that means the FET is ok, and the problem is with the 555 or the gate not discharging properly. Need to build the tester!

Antonio

Wed Jul 22 2009, 12:30AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

You can easily see with a voltmeter if the 555 is oscillating. Put a capacitor in series with the meter and look at the voltage between pin 3 and ground, with the meter measuring AC voltage.

1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob