Spark ignition-radio interference.

[Peter Coombe]

Seeking to fly old sparkies with radio assist.Is it possible ? Have tried PCM,PPM on 35meg and even 27 AM and FM.Have tried resistor in HT lead,wrapping same in foil,positioning rx as far from engine as possible,rx aerial at different angles,mixing and matching Sanwa,Fleet,JR and Futaba,all to no avail.I have been told that 2.4 may be the answer.Reluctant to venture into 2.4 if I can avoid it.

Edited By Peter Coombe on 26/03/2010 11:35:32

[Doug Ireland]

Hi Peter, is your "Old Sparkie" the type that has contact breaker points on the engine and an ignition coil? If so, I don't think you can avoid the EMI as the "noise" is coming from the points and not the spark-plug.

[Brian Parker]

With 2.4Ghz and CDI ignition I have tried to induce a malfunction by placing both an unshielded coil (running at 250volt primary) and a CDI circuit board in direct contact with the receiver, the servos and the cabling in turn.
 The system performed perfectly, (confirmed by a scope trace).

2.4Ghz  plus CDI is the way to go.

 However, before you spend your money, and if not already fitted, you might like to try a capacitor across the CB contacts plus another  from the switch side of the coil to the engine ground.

I take it that you are using a separate battery for the Ignition and that it is electrically isolated from the RC.

[Peter Coombe]

Specifically,we are trying to operate Ohlsson and Frog engines,all of 1940s vintage.Standard coil and contact breaker points,a gizmo from US in place of condenser(Larry Davies item) and seperate three cell battery.A micro switch in circuit,mechanically operated from a "throttle " servo,serves as kill switch.

This exercise was to a degree,inspired,or at least accelerated, by an article in recent BMFA News by Andy Brough.It is likely to be of fairly short duration.These old engines have no silencers and even the small 23 and 29s are pretty noisy so it is the intention to only operate them at our isolated private site and vintage meetings where  they are accepted.Please enlarge re capacitor across points and what is meant by electrically isolated from RC.

[Brian Parker]

The ‘condenser’ is actually a capacitor and your ‘gizmo’ is intended to do the same job (I assume).
 When the contacts ‘break’ the current still tries to flow and causes arcing. When a capacitor is fitted across the contacts it receives a charge as the contacts break thus reducing the arcing and also ensures a rapid collapse of the coil magnetic field (better spark). When fully charged the capacitor discharges through the primary windings.

The collapsing magnetic field causes lines of force to cut through both the primary and secondary windings, the induced primary voltage could reach 100 volts, the secondary voltage gives your spark and is several thousand volts.

The arcing, the magnetic forces and the induced voltages are your enemy.

The ignition circuit should be completely separate from the RC circuit with its own battery ie. completely isolated from the RC system.

All cables need to be as short as possible. Keep everything close to the engine and screen it all off, foil is OK.

 Your fitted micro switch servo control needs to preserve this isolation. An opto switch is the safest.

A Capacitor of .1uf will be OK provided it is rated above the primary induced voltage, ie. 250volts.
What exactly is your ‘gizmo’?

 Are you using ‘ModelAero’ type coils?

 

A bit lengthy, hope it helps.

[Peter Coombe]

Have at last found instruction leaflet for above mentioned gizmo.It is a small sealed,purpose made item from Larry Davidson-US specialist in vintage ignition engines.A precis: "Bolt of Lightning Trigger"....a transistor switch used to carry the heavy current that the points normally do.The points now switch the transistors "base" to ground and the transistor switches the heavy coil current,causing the spark to be produced at the HT terminal.Points now switch a tiny current,there is no arcing at the points,point cleanliness and condition is far less critical".The coils we use are of course miniature aero specific type.Will try the capacitor and also experiment with a metal mesh screen to "fence off" the engine from radio.Thanks for your advice.

[Brian Parker]

Peter,

So your gizmo has ‘transistorised’ the points.
 This will in fact reduce arcing as stated by Larry.  The circuit may already contain a capacitor but no harm to try an additional capacitor in parallel.

 Make sure all the 0volt cables are to a common connection to maintain a low impedance 'earth'.

 Most of these vintage coils are designed for 3 volt operation. ie. two 'dry' cells. Three fully charged Ni-MH cells are in excess of this voltage. It will give you fat spark at the plug but will  increase the electromagnet field (and interference) and also increase the coil loading.

 It might be worth just trying a couple of alkaline cells, this may solve the interference problem.

 You can also reduce the Ni-MH voltage with a zener diode across the supply (but check the current rating). Also make sure the cells have a large enough capacity to prevent voltage fluctuation.

 A simple regulator can be constructed by using a 317 type regulator IC to reduce the voltage to 3volts but you will need to increase the cell count or use a Lipo.

 I would not advise using rectifier diodes to reduce the voltage because of lack of regulation when the cells start to discharge with use.

 You could of course try it without the transistorised unit, if you have not already done so, just to check that this is not the cause of your problem.

Good luck

Edited By Brian Parker on 28/03/2010 15:43:24

[Peter Coombe]

Brian,

Thanks for the information.Off to Maplins.Will report back in due course.Or give up and screw in a good reliable four stroke!