Velleman ignition amplifier K2543 making audible ticking noise

[dieselnutjob]

Hi
I installed a pair of Velleman ignition amplifiers in my 1970s Peugeot. These kits work by using the existing points (contact breakers) to fire a TIP 162 transistor. The transistor then earths the coil instead of the points. This reduces the load on the points from several amps to a few milliamps and so reduces points wear and increases reliability.

The circuit diagram is here

although mine don't have the 680R and LED.

The circuits are working well and the engine runs well, however in order to have a stealth installation, and to keep the circuits away from engine heat I ran wires inside the passenger compartment and put the circuits in a box underneath the dashboard.

The problem is that the circuits are making an annoying ticking noise which sounds like a couple of cicadas in the car. Both circuits do it (my car needs two because it has two breakers and two coils) so I don't think it's a component or assembly fault.

What could cause a circuit board with no moving parts to make an audible noise? what can I do to stop it?

 

[duke37]

If you have a noise, then you have a moving part!
The circuit provides a high current pulse which can move wires due to the formation of a magnetic field. You do not say what your box is made from but a steel box will couple to the wires more than an aluminium box.

 

[dieselnutjob]

It's a plastic box. Each circuit is screwed to a big heatsink and the heatsink is screwed to the box. So the PCB isn't connected directly to the box at all.

Maybe I should just fill the box with potting compound then?

Picture of the box before closed and hidden in the dashboard

 

[duke37]

Do not pot the box, there are several resistors dissipating quite a bit of power. These will overheat if they are not cooled sufficiently.
Can you locate the source of the noise, say by using a hearing tube? I would use plastic, the circuit could bite if you use metal.

 

[Resqueline]

My theory is that there are large current loops in the design, and that the capacitor discharges through the transistor as it turns on.
I would try to move the capacitor to the ignition coil (which is the part it is really supposed to interact with) and see if it helps.

 

[dieselnutjob]

The only thing is that the way that they have put the capacitor (across the transistor) does mimic having the condensor across the points that the normal mechanic system has.

That said, I could move the capacitor off of the PCB and put it into the engine compartment whilst still keeping it across the transistor.

Do you think that might quieten it down?

 

[Resqueline]

Imho the old way of putting it across the points was also "wrong", I think it should have been across the coil, always.
One possible reason for putting it across the points is that being a grounded metal case type it would have been "risky" to put it on the "hot" side of the coil.
Move it off the PCB and onto the coil. Don't keep anything across the transistor.
Yes, I believe a high capacitor discharge current spike is the only possible source for a sound in that circuit.
I might be wrong but I believe it's worth a try.

 

[(*steve*)]

...And capacitors can certainly make a sound when discharged rapidly.

 

[dieselnutjob]

actually I was thinking that it doesn't really make a difference from a functionality point of view whether the capacitor is across the coil or across the switch.
If it is across the coil then it is directly coupled to the coil in a resonant circuit, whereas if it is across the switch it is coupled to the coil via the car battery, so with a 12V DC offset.
The DC offset is irrelevant to the capacitor so as far as the high frequency switch pulse the capacitor will act in the same way.

Just moving the capacitor out of the cabin may have a positive impact on audible noise, in fact I might be able to use the existing metal can condensors instead and just not bother with them.

Moving the capacitor to being across the coil would have the additional benefit of reducing electrical noise in the wiring of the car and across the battery which will probably give less interferance to the radio. It doesn't have a radio yet but the previous similar car I had wasn't good in that respect.