Buzz Coil Condenser/capacitor

I have got most of the parts to make a buzz coil - KW point and a suitable coil, just wondering what size condenser/capacitor to use.

There seems to be very little on the net about whats inside the boxes but plenty on how to wire them to the engine, best I could find was this article . The engine has a suitable timer with facility to advance & retard the spark, it will be 6volt and uses a CM-6 10mm plug.

J

Ok first I know nothing about "buzz coils".

that said .

the capacitor/ condensor stores energy to provide the spark.. thus the bigger the better.. oh hold on this is engineering ..NOT quite that simple..

Too bigger cap. and it wont reach full voltage on charging cycle ; too small and you are wasting capacity (sorry pun). .

charge time would crudely be 5 * R *C R being the dc resistance of your charging coil. C being the value under consideration so the charge time at MAX rpm would be ?

T= 5RC

thus

C= T/(5R)..

Hope this helps

Ok a seconds research shoots all the above down (sort of).

Looks like the cap is to protect the points more that any more "critical" function..

but again too much will reduce spark.

Jason,

I would start with a 0.01micro farad and watch that the sparking across the gap is "small" it is all a little subjective I am afraid.

K

Hi

Just done a dig about and the figure I came up with is between 0.1 to 0.5 uF (micro farads), sometimes listed as 100 to 500 nF (nano farads) but this can depend on the coil being used. The only common ground is that they need to be rated at a fairly high working voltage probably 300-400V DC as a minimum. This is not so much related to the output voltage of the system but the voltage likely to be developed across the primary winding which is a function of rate of current change, when the contacts either make or break, and inductance of the winding; something like e = L x di/dt where L is inductance in Henries, di/dt rate of change of current flow and e is the generated voltage.

Sorry not to be much help but will keep my eyes open unless you get a definitive answer in the meantime.

Cheers

Martin

Why are you using a "buzz coil" as opposed to a conventional contact breaker system. I thought buzz coils (or trembler coils) went out with the model T Ford.

Russell.

Well seeing as the engine that its going to be used on is dated around the same time as a Model T the buzz coil is the ideal choice. As I said the engine also has the correct timer so that it can be advanced or retarded and a contact so the spark is not energized when its "missing" so saving the battery as it only sparks when "hitting"

I would hate to spoil the correct look of the engine by fitting contact breaker points or waste all those other parts by fitting a hall effect sensor and CDI system. This sort of thing is just as bad as people of fit spark plugs to engines that would have had ignitors just to make their lives easier.

J

Edited By JasonB on 20/07/2012 17:14:10

Thanks Martin and others, its also been said on another forum that 0.2uf is whats needed and a suitable supression capacitor from CPC suggested to me.

Another option is a condenser from something like a Briggs & stratton engine and this seems to be the type shown in teh link I posted above and also on another schematic that I have been sent so may go down that route.

You may also be interested in these two videos that show a buzz coil made using a relay rather than teh traditional points and magnetic field of teh coil, it also looks to be using a B&S type condenser. It looks a compact solution but not sure how long the relay would put up with the constant on/off buzzing?

**LINK**

**LINK**

J

Jason

If you don't want to use a relay it may be possible to use a 555 timer IC in free run, astable mode, driving something like a hexfet transistor to drive the coil. Its the sort of thing that can be used in high voltage generators which in reality is what a buzz coil system is. I might have a play around with it as I have most of the parts kicking around I think.

Cheers

Martin

PS

555 timer chip can be controlled from your current timing system by connecting this to the inhibit/reset pin.

Edited By Martin W on 20/07/2012 18:26:20

Hi Jason,

Any condensor from a small engine, ie mower, chainsaw, string trimmer etc will probably work fine and won't break the bank. The spec is not critical, as others have mentioned the condensor/capacitor is primarily there to keep the points from burning. Specs from my various small engine manuals say most small engine condensors are rated 0.4 to 0.6 micro Farads at 300 to 350 V.

As mentioned before, for use on a hot engine a condensor specifically intended for a car or small engine is a good idea, because these condensors are built to withstand heat, vibration, exposure to oil and most importantly, periodic exposure to HPM's (Hillbilly Percussion Mechanics). Normal electronics grade caps may have suitable electrical properties but are not mechanically up to the job for on-engine service.

JD

Thanks again Martin & Jeff, I think I have one of tehcondensers around somewhere if not I'll rob one off the lawn mower to see how things go. Not sure if it will be this weekend or next as the battery has not arrived yet and that Firefly is calling me again.

J

The following is how I think the circuit works. When the contact is closed we have an inductor (L) in series with a voltage source (V), plus the resistance (R) of the inductor. At the moment the contact is closed the full voltage is impressed across the inductor, and current begins to flow. If the inductor was pure, ie, no resistance, the current would increase linearly for as long as the contact was closed. In reality. because we have resistance, the current increases asymtotically to a maximum of V/R. For all practical purposes the first part of the current versus time graph is linear. When the contact is opened the current does not stop flowing. Instead it starts flowing into the capacitor. We now have a series resonant circuit. This is a second order system (two storage elements, the inductor and capacitor) and the differential equations that govern its behaviour can have an oscillatory solution. So what we get is a decaying sinuoidal current as the stored energy is moved between the inductor and capacitor. The voltage across the combination RLC can never be greater than the source voltage V. So how do we get a high voltage on the secondary coil? Simple, because the voltage at the intermediate point of capacitor and inductor can be much higher than the source voltage. It may well be hundreds of volts, and is related to the Q of the circuit, for series resonant circuits defined by:

Q=1/R*SQRT(L/C)

This voltage appears across the secondary, multiplied by the turns ratio, which is also normally large, so we can get peak voltages up into the kilovolts.

I do not think that the equation v=-L(di/dt) is directly relevant in this case. Note that there is minus sign in the equation, as the voltage tries to oppose the current change. The addition of the capacitor actually slows down the waveform after the contact is opened and limits the initial voltage across the contacts at the point at which they open. In this way it does protect the contacts from sparking, but it also plays an important role in the operation of the circuit.

Regards,

Andrew

Addendum: I must get back to playing with my experimental ignition circuit. The aim of it was to get sufficient voltage on the secondary without needing a high turns ratio in the coil. The circuit uses an avalanche MOSFET as the switch and does rely on the equation v=-L(di/dt). It's been a while since I played with it, but as I recall I got about 8kV on the secondary for 12V in and primary/secondary turns ratio of 10.

Personally I wouldn't admit to using a 555 in public, there are limits!

In ME vol 190., no., 4196. page 624, there is an artical by Jim Service called Building a "BS" Buzz Box, an ignition device for internal combustion engines. Ian S C

Hi There

I have set 4196 as the sample digital issue.

regards David

Thanks all will have a look.

J

> Personally I wouldn't admit to using a 555 in public, there are limits!

Never use a 555 when you get fit a microcontroller in there.

Or is it the other way round - I've seen chips with more power than the Lunar Module computer flashing an LED for a living

Neil

555 or 1/6th of 40106

Hi

I don't mind admitting to using a 555 timer as a basic oscillator circuit cause I believe in the KISS principle , no cracking walnuts with sledge hammers.

Andrew with regards to the primary circuit voltage on a contact breaker ignition system I would refer you to this extract. It can be seen from the oscilloscope waveforms that when the contacts break/open there is a spike on the primary winding which exceeds 300 volts, I think that this is somewhat over the 12 volts applied prior to the contacts opening. This rapidly falls back to about 40 volts during the burn time, duration of the spark burn, and then there is some ringing which will be in part due to the natural resonance of the circuit, total inductance and capacitance etc.

This 300 volt plus spike on the primary is, I assume but I am quite wiling to be corrected, the time it takes to ionise the air gap on the spark plug and the spark to develop, once the gap is fully ionised and the spark established the reactance/resistance of this path falls to a lower value limiting the secondary HT voltage to that required to maintain the spark over the fixed distance gap, this is reflected to the primary coil and hence the drop in primary volts.

This is in part why the capacitor needs to have a fairly high voltage rating. Secondly without the capacitor to absorb some of this energy then I suspect that this spike would be a great deal higher and it would maintain an arc on the contacts for a relatively long period thereby shortening their life.

Cheers

Martin

Hi

In the same article in the Technical Information Section under the heading Magnetic Inductance they say that the 300 volt spike on the primary circuit is due to (a) The number of turns in the primary winding; this in its simplest form relates to inductance, (b) The strength of magnetic flux, which is proportionate to the current flowing in the primary circuit and (c) The rate of collapse of this field which is determined by the speed of the switching path.

This is similar, if not quite the same, as inductance in henries = h, current in amps = I , rate of collapse in field units of time = t and voltage = V which I think can be represented by V = di/dt x h with or without the minus sign . As far as I am aware the capacitor is not there to form a part of a resonant circuit. With the bulk of the inductance being in the HV winding, as in this arrangement, it would generate a series resonant circuit whose impedance would be at its lowest, typically the series resistance of the coils, when at its resonant frequency. Not something one would want to try to develop several KV across .

Cheers

Martin