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I have been working on a strobe for vehicle use, this can be setup to trigger at a preset shaft angle so engine problems can be traced and viewed, I'm also thinking of adding a slowmo function so you can see something rotating quickly in slomo to pick up problems such as non rotating vlave stems, worn rockers and the like.
Obviously such a strobe is a commonplace device in garages, and has been a VERY common project over the decades - but making a 'slightly' adjustable one is a bit different
Ignition strobes were really common, however electronic engine controls have kinda made them less popular, but you still see them, every accesory shop has them.
Being able to look at anything at a certain engine position regardless of engine speed has many uses, also handy for industry.
This machine was designed to examine contactors, but I dont use it for that I use it with the free running osc for looking at dodgy couplings, pumps and the like.
On the diac or tigger diode situation : they are now back in style as most CFL circuits use one to start the ballast oscillator. When the bulb no longer lights you can pry these open and get a nice little collection of parts.
I forgot to mention in this thread that the main reason the dimmer circuit works with only 'one' wire (no neutral) is that the load looks like a very low resistance compared to the trigger circuit, so BEFORE the triac turns on almost all of the line voltage appears across the trigger circuit, giving it plenty of power to operate on. Once the triac turns on, it shorts out the trigger circuit so it no longer gets much voltage, but then it does not need power anymore because AFTER the triac is turned on it stays on and only turns off when the line current goes through zero. So this makes it very clear why the neutral wire is not needed.
If the firing angle is adjusted too long however, the sine line voltage again approaches zero (greater than 90 degrees) so there is not enough line voltage to trigger the triac, so the cheapie circuits wont be able to trigger the triac. That and the fact that they need perhaps 30v to overcome the diac voltage means some firing angles wout be possible.
But the main reason it works at all is because of the former paragraph, the triac stays on once triggered and so does not need power anymore after that to keep it turned on for the duration of the half cycle.
On the diac or tigger diode situation : they are now back in style as most CFL circuits use one to start the ballast oscillator. When the bulb no longer lights you can pry these open and get a nice little collection of parts.
Yes it is true, some of them use them or at least they did at one time. Supposedly to start the tube, but i've never looked into the circuits that deeply to find out why they wanted to use a diac. Not all of them do it that way though, and the more modern ones may not use them anymore, but of course i can not say for sure what ones *still* use them.
It is probably used as a sort of one shot, to get high voltage for a short time and then that's it.
Nigel and Al: I have personally only taken apart two, but a collection of circuits posted by another agreed with my experience. The trigger diode, a DB3 in my case, starts the Osc by pulsing on one of the transistors, otherwise it just sits there doing nothing.
Well considering that the tubes take a higher initial voltage to start them, i'd think the diac would be used to stop the high output after a short time. So in other words when first turned on the diac would not conduct and the high output would appear across the tube, then after some cap charges up the diac fires and turns the transistor on and that stops the high output and it goes to the normal output for that tube, and it's a one shot deal because the diac never turns off until the power is removed.
I don't think that is the diac's purpose as when the tube strikes it will pull down the voltage. The oscillator consists of two HV NPNs driven by a three winding saturable core toroid. When power is first applied both transistors are off so nothing happens. The diac is connected to a cap charged from V+ and the base of the lower transistor ,when it trips this starts conducting until the core saturates and the upper transistor turns on. This makes on Osc that switches at a particular current level, exactly what you want in a ballast circuit. The striking voltage is generated by putting caps across the tube to create a series resonant circuit with the main ballast inductor. There is also a PTC thermistor in series with the heater elements to speed up the light output.
I still don't get the purpose of a diac for triggering a triac. I know it conducts when the breakover voltage is reached, but what's it's purpose in the circuit? Is it just to get a clean turn-on or does it affect firing angle?
I still don't get the purpose of a diac for triggering a triac. I know it conducts when the breakover voltage is reached, but what's it's purpose in the circuit? Is it just to get a clean turn-on or does it affect firing angle?
It's to give a time delay - triggering the triac so long after zero-crossing.
The capacitor starts charging up, via the potentiometer (making it adjustable), once it reaches the diac trigger point it then fires the triac. The higher value the potentiometer is set to, the longer it takes, and the dimmer the lamp.
I don't really know - I've never tried it, or seen a dimmer without something similar - the firing of the diac discharges the capacitor as well as triggering the triac.
I suspect one reason for it is to swamp the changes in triggering point of the triac?.
It's all pretty 'old hat' though now, as such crude dimmers are well in the past
I don't think that is the diac's purpose as when the tube strikes it will pull down the voltage. The oscillator consists of two HV NPNs driven by a three winding saturable core toroid. When power is first applied both transistors are off so nothing happens. The diac is connected to a cap charged from V+ and the base of the lower transistor ,when it trips this starts conducting until the core saturates and the upper transistor turns on. This makes on Osc that switches at a particular current level, exactly what you want in a ballast circuit. The striking voltage is generated by putting caps across the tube to create a series resonant circuit with the main ballast inductor. There is also a PTC thermistor in series with the heater elements to speed up the light output.
So the diac starts things going then, but there is a delay from when power is first applied to the time the diac fires. Why such a delay is needed if the tube can start itself. It seems that the diac will cause a lower voltage on the tube. So the diac 'allows' the high voltage but only for s certain time maybe in case the tube does not start properly i prevent damage to the rest of the circuit.
Again though, i have not studied this in detail yet.
Nigel: I have a circuit written down on paper. I was hoping I would not have to get involved with uploading something, but it seems I will because Al is totally confused.
Al: the transistors are in a totem pole arrangement , neither has any DC bias, they are driven by the transformer thru a 10 ohm base resistor and a 1 ohm emitter R. ( Don't know why they have both.) Without the trigger diode this arrangement will never start oscillating. The output of the Osc drives the transformer primary and the main ballast inductor in series, with these in series with the lamp. ( There is also a diode back to the startup cap to keep it discharged. The startup delay is just a byproduct of this arrangement.)( The designers don't care if the tube starts or not.)
On Throb's question: the amount of current needed to trigger the triac is far larger than that which is used to charge up the timing cap. In theory you could put a resistor in series with the gate and have it trigger when the voltage reached a certain point but this works very poorly. The diac is an open circuit until the trigger voltage is reached, making things much easier.
The typical diac only discharges the cap by 5V, leaving 25-30V to be drained back through the resistor into the triac. With short on times some voltage will remain which will delay the next half cycle, which will then affect the next, making this circuit hard to adjust for low light output.
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