BTA26-80BWRG

[Joe26]

Hello,

I have questions about a project we developed a long time ago. I will put the schematic below. We are using the BTA26-800BWRG (U8 and U9) in the schematic to parallel two inductors. The voltage applied to these two inductors is an ultrasonic signal of 600V at a frequency of 20-120 kHz. However, I don't fully understand how the TRIACs can switch, given that there is no voltage source in the loop between the two gates. Yet, the project works almost perfectly, except that sometimes, when the generator is off and the attempt is made to turn off the TRIACs, they remain switched on.

U8 & U9 (BTA26-800BWRG) & OP1 & OP2 (IL410)

Thank's in advance for your help.

Best regards,

Joe

[Peter BENSCH]

Welcome @Joe26, to the community!

Your circuit is a special application of triacs and only works because the triacs are connected antiserially (MT2 to MT2).

To understand how it works in more detail, you should take another look at the internal layer structure of triacs, e.g. in the Wikipedia article.

Put simply, when the potential at your net Self_1 is positive, you control the pn junction between MT1 and the gate of your U8, which switches it on (quadrant 3). The current via MT1, G of U8 will then take the path via gate and MT1 of your U9, which also switches it on (quadrant 1). Due to the symmetrical structure of the antiserially connected Triacs, everything is simply reversed when the polarity at Self_1 is reversed.

Does that answer your question?

Regards
/Peter

[Joe26]

Thanks for your help!

Now it's more clear about the functionality!

I just have no idea how to correct the little bug with the switch off.

Here is the schema with inductor. J200/201 is the connector for the ultrasonic transducers, which can be assimilated to capacitance.

I surch to make turn off more reliable.

Best regards,

[Peter BENSCH]

• What else is connected to US+ and US-?
• How (what) do you switch off?
• What signal is coming from L200 and L201 when switched off?

[Joe26]

We have to parallel or not the two inductance. Depend on the frequency of the US genrator.

We use the principle of resonance, it's the reason why we use variable inductances. And the voltage can rise up to 800VAC 20-120kHz.

And sometimes we can't change 1 or 2 paralelle inductances. The Triacs still passing even if the self_commut signal is at 0. Most of case when we turn off the generator, try to switch off the TRIAC, and then turn back on.

[Peter BENSCH]

I would be interested to know what precise value the voltage reaches: by 800VAC do you mean the RMS or peak value?

[Joe26]

800 VAC should be the peak value, but it's really complicated to determinate because voltage is always changing due to inconstant frequency (specificity with ultrasonics transducer). I measured with oscilloscope and save the peak.

[Peter BENSCH]

Well, I asked because I'm afraid that the triacs will be triggered by overvoltage or capacitive effects. You must not forget that triacs are actually intended for mains voltage applications, i.e. 50/60Hz, but you feed them with 20...120kHz, which leads to very high dV/dt.