Help with transformer

I'm experiemneting with a DC to DC converter to power the ignition on a motorcycle.

I am building the circuit below, but am stuck on the transformer. When it says 4:50 what exactly am i looking for. OK I gather this is the ratio of primary to secondary, but I'm looking for a physical transformer to use.

What sort of core should I be looking for?

Is there a standard off the peg item i can use here as I don't want to get into winding transformers?

If I do have to wind one what wire sizes will i need and how many turns?

If the transformer is used to generate the spark for ignition, you should use a transformer that is use in a car, the dutch name is Bobine. Not sure what it is in english. You could get one from the car shop .
The nature of this kind of transformer ( spark) is high voltage...a normal transformer will not work because of the high voltage.

This is a capacitor discharge ignition and this circuit is the dc to dc converter that charges the capacitor. Not the igniton coil (bobine).

I think it is a fly back transformer of some sort. The capacitor that discahrges into the ignition coil isn't shown.

The ignition coil is seperate and external to this circuit.

speedshop said:

...I am building the circuit below...

Click to expand...

Hi,
sorry, but I dont see some circuit...
K.

Maybe you can see it now:

To select a transformer, you should have an idea of the output voltage and current to achieve. Possibly, a retail SMPS transformer
can be used, e.g. from Coilcraft or other vendors.

Your presented circuit lacks of an effective current limit, because it apparently abuses the Is pin for other purposes. It must
be expected to blow the output transistor easily.

I have wound a transformer for exactly this type of application.

8Turns primary 1mm, 50 turns secondary 0.35mm
think its ETD29 Core or maybe the next size

must do it in 3 layers.

First 25T secondary, then that special polyester tape, then 7 or 8T primary, then poly tape,
then 25T sec, tape,

where the wires come out from each layer, put some insulating sleeve on them, some thin
shrink tubing will do,

ensure good insulation, ie tape covering between layers, as voltage breakdown will have consequences, especially the way a flyback works.

The core will need an airgap too, roughly 0.5mm, I use layers of transparency film for this.

maybe should do the primary with 1.2mm wire,
other option is double winding (side by side) of 0.5mm wire - this has electrical advantages, and is much easier to wind.

This one was running at 27khz resonance IIRC


Find a ready made commercial one ?? Good Luck!

As for specs ?

If you have 2 cylinders, running at 6000rpm, => 100 rps
2 firing per round => 200times / sec charging up a 1uF cap
=> charge a cap 1uF to 400V in 1/200 sec

Current control is achieved by modulating the signal on pin 3. It is linked to the pick up on the crank shaft and switches off the power supply for a period that is pre programed.

There are elements of this circuit that aren't shown.

Thanks for your replies, I'm learning all the time with this project!

The spot on the diagram shows the polarity, do the coils have to be wound in opposite directions?

The transformer I described above, all turns in SAME direction.


The primary is thick wire because 20AMP + flows.
You need a power supply decoupling capacitor of at least 3000uF, low ESR, and for
this app rated 105degC

When your SCR is triggered for a spark, the switcher should be disabled for a period.

IMO, you don't need to regulate current for this app, when the capacitor at the secondary reaches set voltage, the switcher should stop or maintain that voltage.

I puzzled with what your intention is with pin3 control..............

That is interestering. So without using the control signal on pin 3 how would I control the switch gear? As I understand the SCR on the right has to see 0 zero volts to reset auotomatically after the capacitor has discharged into ignition coil (not shown). If the power supply is suplying continuously there is a chance that the the SCR remains on all the time.

This cicuit is obviously one I have 'found' and is not quite what I want. On the full circuit there is a PIC that processes the signals from the crank pick up, its been programed to cause a schronised signal on pin3 to shut off the power supply so the SCR will reset.

If this can be achieved by other means this will be better for what i want. I've added the capacitor triggering circuit so you can see how its connected up.

A decoupling capacitor will go between the +12v supply and ground?

Your presented circuit lacks of an effective current limit, because it apparently abuses the Is pin for other purposes. It must
be expected to blow the output transistor easily.

Click to expand...

Please use the current sense, the UC3845 is a Current-mode SMPS, you also may want to create a network to filter your feedback as the output (in flyback) is always out of phase with the control.
You could use the comp pin to control your gear, by shorting the COMP to ground, it shutdown the output.
Or
You can drive high the I sense, it is not recommended but it work.

Use the datasheet
https://focus.ti.com/lit/ds/symlink/uc3845.pdf

Do as recommended mean doing it only once.