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Back EMF blowing 555ic. How to stop it??

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user_88

Member
Do you have an oscilloscope?

It would be interesting to observe the voltage waveform at the collector of the NPN transistor.
You are probably getting a voltage spike at this point, which drives a reverse current back through the IC ..... through leads 4 or 8.

Try putting a diode in series with the ignition coil... between the + supply and the ignition coil .... The diode specification should handle several amps ... and have a sufficient voltage operating level so that it is not damaged.

... not taking any responsibility for additional damaged ICs .... just a guess at this point.
 

spitso

Member
i have access to my school oscilloscope but im not running the risk of damaging it.
I allready added the diode but problems still occur. Instead of the igniton coil im using a transformer (not that it matters alot because they are pretty much the same thing).
The transformer puts out about 400-500ish volts AC which i have then put through a series of voltage multipliers which changes it to about 4Kv DC. This is where the sparks occur.

any other ideas?
 

ericgibbs

Well-Known Member
Most Helpful Member
i have access to my school oscilloscope but im not running the risk of damaging it.
I allready added the diode but problems still occur. Instead of the igniton coil im using a transformer (not that it matters alot because they are pretty much the same thing).
The transformer puts out about 400-500ish volts AC which i have then put through a series of voltage multipliers which changes it to about 4Kv DC. This is where the sparks occur.

any other ideas?
hi,
Very wise, dont scope EHT voltages with a standard scope probe.:eek:

Can you post your circuit, showing any decoupling you have on the pcb.?
 

spitso

Member
attached is the schematic, pretty obvious what it is.
Try to get as highest voltage as possible outta transformer seconday, tried winding my own apparently thats easier way to go ...but i had no luck.
Have you got any ideas on how to make a transformer that would step up voltage to somewhere from 9v to 250-600v?
Aswell as protecting circuitry from HV kickback?
 

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user_88

Member
If you can find a capacitor ... something like 400V rating ....1 uF .... try placing this between the NPN collector and circuit ground.
The idea is to ground out some of the higher frequency components of the voltage spike that occurs when the transistor is turned off.
....not absolutely sure about the specifications or value necessary for the cap ... you will just have to try something and see if there is any improvement.
 

Boncuk

New Member
Hi spitso,

why don't you use a power MosFet instead of the ancient electronic work horse 2N3055? (I use that one for a quick replacement of my electric shaver charger.)

If you use an IRFP460 with a 1N4004 or higher voltage (1N4007) connected between source and drain you won't have back EMF on the timer circuit. Use 470Ω for a gate resistor.

Secondly, you can generate high voltage (20 to 35KV) using an ignition coil (made for electronic ignition systens with a resistance of the primary winding of 0.7Ω) with it's primary winding connected to terminal 1 of the ignition coil and the drain and terminal 15 connected to +12V.

Why bother with a selfmade HV transformer which might spark inside without you knowing it caused by one twisted (and uninsulated) turn already?

Give the ignition coil a break every once in a while using short pulses (duty cycle 30% or less) to prevent overheating. Using 1KHz clock frequency corresponds to a 6-cylinder engine running at 20,000 rpm.

Only racing engines run at this high speed and cooling of the ignition coil is mandatory in that case (racing engines therefor use multiple ignition coils)

So my suggestion: do not exceed 100Hz for a safe and reliable function of the circuit, still meaning 6,000 rpm for a single cylinder two-stroke engine, which takes off like a scared rabbit at this rpm. :D

Boncuk
 

Nigel Goodwin

Super Moderator
Most Helpful Member
This exact question has already been answered recently, it's essential to power the 555 circuit and the coil independently, running separate wires back to the battery.
 

spitso

Member
Thanks Boncuk, yea i agree and forgot to mention about the change of transistor to at first an irf540 but the circuit blow it so now im using a tip3055 (off top of my head, im in bed and not getting out hahah).
I will put diode between source and drain obviously reversed bias correct?

Ignition coils are too large, i am making a tazer so needs to be handheld. As soon as i find or possibly make transformer i will play with pots to find best settings then replace them with fixed resistors.

any links for making of a transformer?

thank-you for help
 

BrownOut

Banned
A clamping resistor across the transformer primary should do the trick. But that will affect the output voltage.
 

Boncuk

New Member
Hi spitso,

making a transformer yourself requires close watch from purchasing the enamelled copper wire to finally put one winding next to the other, as I recall from the time as radio amateur (ham).

The most critical point is the enamel, which will crack and leave an uninsulated piece of wire if not treated properly.

Purchasing enamelled copper wire at any shop and observing the employee using techniques like grandma uses to coil wool for the next sock to knit is certainly the worst method. (Just slay him/her if he/she does it that way. :D )

With every turn the copper is being twisted 180 degrees and the enamal laquer can't stand that stress, breaking and leaving a blank copper wire. (good for a perfect short)

This is the proper way to make a transformer:

Get the seller wind the desired amount of copper wire off the spool directly to another one (which you should have prepared already when purchasing the wire) without any angular change of turns. (This can be done easily using the body you have provided and a chuck of any drill)

When winding the transformer you must keep the same order not to break the enamel. Hand winding the transformer means turning the core underneath the copper wire you have in use. (Turning the wire is absolutely deadly!)

I wound a 1.5KV transformer by hand for an RF-power amplifier without any trouble, but it took me about two days to get it done.

You might use a drill rotating at minimum speed to get one winding next to the other and layer on layer that way. (Don't forget an insulating plastic sheet between layers.)

Boncuk
 

tytower

Banned
Thats a lot of work but I agree to keep the bending to a minimum.
I have always wound a layer then painted on some varnish and while wet wind the next layer and so on
 

bountyhunter

Well-Known Member
Hi all

I have built a simple 555 oscillator (http://www.geocities.com/capecanaveral/lab/5322/coildrv.htm)
to run an audio transformer to a voltage doubler. The Ic's dont really last when im playing with the variable frequency. im 99% sure the problem is back emf.
Is there away to stop this? (not emf itself cause thats nearly impossible, but to divert it away from the IC.

Thankyou
You need to put a good electrolytic cap right at the 555 power pin 8 to ground. I also recommend using a small series resistor like 51 Ohms in series with the IC power pins (4 and 8) and a 14V Zener diode from pin 8 to ground to protect the IC from spikes.
 
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spitso

Member
ok thank-you:)

Also in an atempt to find a transformer which could produce around 250-600v off 9v i opened up a compact flurosent light globe and removed the transformer inside of it. The problem is when i tested all 4 legs with multimeter set on continuty i only recieved a conection between 2 legs??? does this seem right?
 
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