AW: Re: Aw: [EVGRAY] Attachments out of order

Hello Ole, I expressed myself a little unhappily with a heartbeat, I meant a full cycle. 1/4 cycle to load and the next 1/4 cycle to unload. I've built an extra circuit with a scr to test the LC circuit. Unfortunately on the anode side with the load and cathode directly on minus pol. The problem is that there is no resistance at the cathode which limits the current so that the capacitor can even charge. I had thought just to put a capacitor in parallel between the cathode and winding so that the stored current charges the capacitor and turns off the thyristor, perhaps the thyristor shuts off sooner. The circuit should be passive but if you have an idea!?! Greetings Sven


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-------- Ursprüngliche Nachricht --------
Von: "[email protected] [EVGRAY]" <[email protected]> 
Datum:18.02.2018  18:00  (GMT+01:00) 
An: [email protected] 
Betreff: Re: Aw: [EVGRAY] Attachments out of order 

Hi Sven,

"If we look at 1 Hertz this is a circle and we have 1/4 time to load the diode plug and 1/4 time to discharge it."
It is 1/4 of a cycle or period no matter the frequency.

"So I was loading and unloading properly every cycle. OK."
The D3 or D4 diode was conducting as it had the around 0.8V or so across it. First when the voltage has decreased to zero there will be no current through it. This means the choke L1 is working in continuous mode and the current through it never stops during the cycle. The energy isn't used fast enough or its value is just too big.

"I would like to force the thyristors to switch off earlier."
Standard thyristors are trigger devices that can't be turned off. They latch after being turned on. The current has to stop entering them to reset. If trying to stop the current through a coil a flyback voltage is generated. The energy has to leave the coil which is done by the flyback voltage. It's better to let the energy stored go to the load or else it has to be dissipated in a snubber to prevent the flyback voltage from destroying something. At least some over-voltage protection has to remove the energy from the SCR and anything else it can smash if disrupting the current too fast. Actually SCRs triggers by avalanche breakdown which just starts the current once again when the tension gets too high.

"If you could not install a class B commutation because I would switch an inductance anyway, you could switch a capacitor parallel to the cathode and the inductor. So that one triggers only a short pulse but then should be huge."
This could be done to divert the current around the SCR to force it into off state. A transistor could also be used but then the transistor could be used as a substitute for the SCR. But if cutting off the current during conduction the semiconductors have to be protected from the generated kickback from any inductance in the current path. The voltage must increase to the value that preserves the current even if this means punching through some insulation. Compare it to stopping a running flywheel. If stopping it by grabbing it the momentum has to continue even if it means breaking your arm. This makes a protection network is very important.

Regards
Ole




---In [email protected], <s.friedrich@...> wrote :

Hi Ole,

I have read this message at evgray and have not received any email. It is complicated here.

Hector has a nice way to explain it complicated.

Ok 1/240 second is half a half-wave at 60Hz.

If we look at 1 Hertz this is a circle and we have 1/4 time to load the diode plug and 1/4 time to discharge it.

This applies in each case to the positive half-wave and the negative half-wave.

At 50Hz everything is slower luckily.

We are talking about 1/200 second for the discharge and 1/200 second for the charge of the diode plug.

That means I have 5 milli seconds to load and 5 milli seconds to discharge it would be better in 4 milli seconds.

So I was loading and unloading properly every cycle. OK.

I would like to force the thyristors to switch off earlier.

If you could not install a class B commutation because I would switch an inductance anyway, you could switch a capacitor parallel to the cathode and the inductor. So that one triggers only a short pulse but then should be huge.

regards

Sven

Hi Sven,

When turning off the thyristor with current through the coil a flyback voltage is made. The flyback pulse (kick back) has to go somewhere as else it destroys something. If making a switching converter transistors are preferable as switches. Of course it is possible to use the slow thyristors. This was done before power transistors got the size to do that job. For higher power and low frequency the thyristor may still be a choice.

Some old TV-sets used thyristors for different purposes. That was in the overlap of tube and transistor ages. I haven't really seen how that works but just heard that it was hell to fault seek. Thyristors are better for AC current as this makes them turn off at zero current and turn on anywhere on the half cycle.

For a passive circuit the current would have to pass a capacitor which cuts off the current when fully charged.

Regards
Ole


 

---In [email protected], <s.friedrich@...> wrote :

 Hello Ole, I expressed myself a little unhappily with a heartbeat, I meant a full cycle. 1/4 cycle to load and the next 1/4 cycle to unload. I've built an extra circuit with a scr to test the LC circuit. Unfortunately on the anode side with the load and cathode directly on minus pol. The problem is that there is no resistance at the cathode which limits the current so that the capacitor can even charge. I had thought just to put a capacitor in parallel between the cathode and winding so that the stored current charges the capacitor and turns off the thyristor, perhaps the thyristor shuts off sooner. The circuit should be passive but if you have an idea!?! Greetings Sven
 

 

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