How UC3845 Control Pin work?

[amidis86]

Hello, i have a dc cdi hv circuit, it using UC3845 as oscillator, but the circuit not complete, i need to understand how hv circuit work in dc-cdi.

Should i used same signal that trigger scr?

 

[Rleo6965]

Firts time to see this kind of circuit for CDI.

Pin3 over current sense input use to shutdown the High Voltage Inverter Circuit for 1uF 300V cap. Trigger circuit should be connected to gate of scr or IGNITION.

CDI circuit was not complete.

 

[amidis86]

Rleo6965 do you know any idea how to make dc-cdi with analog system. Nowadays all used microcontroller to trigger the scr..

Here ac-cdi analog type :

i plan to convert ac input with high voltage circuit.. but i not understand how the h.v circuit function..

 

[duke37]

Both of the circuits operate in the same way. A capacitor is charged up to a high voltage, about +300V. When a positive pulse goes to the gate of the scr, the scr fires and pulls the capacitor down to earth. This gives a 300V negative pulse at the primary of the ignition coil which will be transformed up by about 20 times to give 6kV at the plug.

The first circuit uses a switch mode boost convertor, the second seems to get the same voltage from an alternator. How does this work before the engine has started?

There are differences in the minor parts, possibly to reduce radio interference or to limit voltage spikes or to aid scr commutation to improve reliability.

 

[Rleo6965]

You must find a real working analog cdi circuit to be able for you to build one successfully. Specially if your only new in electronics.

I've able to build multiple spark cdi from the internet. But it have lot of problem such as CDI will not work if Battery voltage less than 12V. Frequent frying of power mosfet and misfiring.

I have almost modify the whole circuit and component to be able to successfully build it using the same functional block design.
My CDI now still ran even at battery voltage of +8V and current consumption of 2 amp which able me to removed heatsink for power mosfet.
I still use my car original distributor with magnetic pickup to trigger the multiple spark cdi.

If you can find one. We will give you advice if you need help.

If you want a Transistor type ignition system. I can can give you the circuit diagram. It's for contact point car ignition.

 

[amidis86]

Thank duke and Rleo6965, ok, here is dc-cdi that fully function with Yamaha LC135 system.

But, it is possible if i want to remove PIC controller and run the circuit with only pickup signal and maybe simple transistor circuit to trigger scr and uc3845? Example like this :

 

[CocaCola]

How does this work before the engine has started?

On older motorcycle's the initial generation of electricity, as well as turning the engine over is generally done with the kickstarter, or on older mopeds a peddle start... On newer motorcycles less the kickstarter (assuming no fuel injectors or fancy electronic ignition) you can 'pushstart' them and dump the clutch but that's never any fun to be in that situation

On new motorcycles like mine you are SOL without a battery to pressurize the fuel injectors and get the electronics up and running...

 

[Rleo6965]

Very good design of scr type cdi. The uc3845 have over voltage pin2 and over current protection pin3 and also turn off signal from trigger circuit to turn off uc3845 pin1 when high voltage capacitor was discharging.

Hope that uc8345 inverter circuit still run with weak battery of +11V. Remove the diode D5 1N4004 from +12V supply might help.

Yes. You can install another trigger circuit in replaced with the microcontroller. Are you going also to install this dc cdi to a motorcyle? What trigger device you will be using?

Remember , you still need the ferrite transformer for converting +12V into high voltage for your cdi.

 

[amidis86]

Hello, so sorry for late reply, i'm tracing my original dc-cdi circuit, and here it is:

I think to use arduino atmega328 to replace P89LPC915, it is possible to generate oscillator signal to BLUE arrow, and pulser to RED arrow. The yellow arrow will be signal RPM to control both oscillator and pulser to control transformer and scr?

 

[Rleo6965]

It's very hard to interpret your diagram. It just like component arrangement in pcb.

I can give you advise how to add those required circuit. But you need to convert it first to schematic diagram.

 

[mostafa_gordy]

where is circuit image ??
please post circuit schematics?
regards

 

[KrisBlueNZ]

Pin 3 on the UC3845 is not an overcurrent sense input. It is intended to monitor the MOSFET current and should be connected to the source of the switching MOSFET (which must be returned to 0V via a current sense resistor) via an R-C delay circuit as shown in some of the diagrams posted. Without this connection, the current-mode control of the UC3845 will not operate in the intended way. Likewise, pin 2 is not an overvoltage sense input; it allows the UC3845 to regulate the output voltage of the boost converter.

The UC3845 is being used simply as a high-voltage generator. It charges the capacitor, which is then discharged into the coil, as explained by duke37. There is nothing special about the UC3845.

Someone queried the idea of using the alternator to power a circuit before the engine is running. I expect the alternator will generate voltage in response to the cranking of the engine (whether this comes from a foot-powered starter lever or a starter motor).

I was going to write a lot more about CDI, dwell time, etc but I have little experience here. I will watch this thread, though.

 

[mostafa_gordy]

whats substitute for UC3845 ?

 

[KrisBlueNZ]

I don't think there are any substitutes for the UC3845 (apart from UC1845 and 2845 which are the same but in different temperature grades). It's still readily available AFAIK.