Donald Smith Devices too good to be true

[mr.clean]

It looks like this guy has it all down to an art. He builds 900V solid state IGBT 'sparkgap' modules and connects them in series for the desired breakover voltage. He states that the effective resistance is quite a bit lower than a typical sparkgap as well (if the circuit is built well), so that would mean less power loss in the sparkgap.
SIDAC / IGBT SPARK GAP - by Terry Fritz - PDF File

Also, another cool feature he describes is that if you use a potentiometer for R4 in his circuit, you can adjust the 'spark' firing on time.
His circuit might just be the cat's meow.

thats cool, and actually i have IGBTs i want to use, that would be killer if i could do away with the spark and still have sharp on/off

[ewizard]

Yes the SIDAC's are interesting and looking at mouser.com they are surpisingly cheap even for some high current and fairly high voltage units. Lots of them under a dollar each :
SIDAC | Mouser Electronics, Inc.

Probably find some for even less elsewhere. I wonder how the SIDAC's compare in use to the GDT's?

[level]

Clarification on attraction and repulsion effect on tesla coil

I did some more testing again today regarding the attraction and repulsion effect I was noticing yesterday when I brought the neon bulb lead close to the tesla coil secondary coil, or when I touched the wire right to the coil. I have to correct what I said previously. It seems the pushing away of the neon bulb lead is due to the arcing itself. It seems the arcing creates a bit of an outward pressure, and tends to keep the wire pushed away from the coil a bit. Since the arcing extends out further towards the top of the secondary, I notice it pushing away the wire more in the upper portion of the tesla coil. However if I push the neon bulb lead in so that it makes contact with the secondary winding wire, I then notice that the wire is kind of stuck in place a bit to the coil. Since the current flow seems to be stronger towards the bottom of the secondary coil in my arrangement, it feels like it tends to stick a bit more to the coil in that area. So it seems the repulsion is caused when there is arcing between the coil and the neon bulb lead, when it is not actually touching the coil, and the attraction or sticking to the coil happens when the neon bulb lead makes contact with the coil wire. So, maybe not quite so mysterious, but still not sure why when the neon bulb lead touches the secondary it tends to stick to it a bit, but it is a bit easier to picture why arcing (a plasma) might create a bit of pressure that tends to push the wire away a bit from the coil.


The arcing gets really bright towards the bottom of the secondary even though I am only driving the secondary with about 15 to 20 watts, so it might not be a good idea to look directly at the really bright arcing too much without some eye protection.

[level]

Something like this would move the switching point for the mosfet?

Yes, I think that should allow you to adjust the gate bias down even though you increase the supply voltage, which if adjusted right should prevent the FET staying on all the time.

[level]

I wonder how the SIDAC's compare in use to the GDT's?

Not sure. I personally haven't tried them. One problem with SIDACs is once they are triggered into conduction, they will keep conducting even as the voltage across them is falling until their minimum holding current level is reached, so SIDACs by themselves might not give a very sharp shut off. If you want a sharp shutoff, you would have to have another element that suddenly breaks the current flow through the SIDACs at your desired cutoff interval. However, if the pulse capacitor can discharge fast enough (needs a very low resistance discharge path), then you might still get a fairly sharp discharge pulse. One advantage of an IGBT is you can get some with a very low Collector to Emitter on resistance, so gives a very low resistance discharge path for the pulse capacitor. Not sure if any SIDACS are available with comparible low conduction resistance...

[janost]

Yes, I think that should allow you to adjust the gate bias down even though you increase the supply voltage, which if adjusted right should prevent the FET staying on all the time.

Yes, it worked

It now runs on 6V@390mA.
I dont think I can run it on 12V without a heatsink.
The mosfet heats up in about 10min.

On the secondary there is 175V

here is a scope shot of the voltagedrop over a 0.3ohm resistor in series with the battery
Here you see that it actually does not draw 390mA and there is a deep valley in every cycle as the diode on gate feeds back to the battery.

The scope says 62mV average.
That would make 206mA.

[janost]

I managed to get the current down to 160mA by adjusting the pot.
Also there is some switching noise I need to find out where it comes from.

I noticed that the current does not drop to zero when the mosfet is off.
Could it be that it does not turn completely off?

[level]

I managed to get the current down to 160mA by adjusting the pot.
Also there is some switching noise I need to find out where it comes from.

I noticed that the current does not drop to zero when the mosfet is off.
Could it be that it does not turn completely off?

Since it is the gate voltage that turns on the FET, you can monitor the gate voltage with your scope to see what kind of waveform you have there. You want to see the gate voltage going a bit negative on negative peaks or at least going down close to 0 volts on the gate. You have to watch that the negative and positive peaks on the gate don't come too close or exceed the gate's maximum negative and positive voltage ratings though.

[janost]

Since it is the gate voltage that turns on the FET, you can monitor the gate voltage with your scope to see what kind of waveform you have there. You want to see the gate voltage going a bit negative on negative peaks or at least going down close to 0 volts on the gate. You have to watch that the negative and positive peaks on the gate don't come too close or exceed the gate's maximum negative and positive voltage ratings though.

The gate goes to 0volt and it goes negative aswell so it could be the AC-coupling on the scope that shows an error.

But there are 2 pulses on the gate when it turns off and that gives the switching noise.
Dont know where they are coming from?

Otherwise it swithes fast, almost squarewave with rounded tops.
That is why i can run without a heatsink.

I did try the circuit with a 2N3055 and it would not oscillate with any bias setting.

I also parallelled my ignitioncoil with it and it works with a currentdraw of 400mA

I have to make some sort of heatsink before I go 12volt on this.

[level]

The gate goes to 0volt and it goes negative aswell so it could be the AC-coupling on the scope that shows an error.

But there are 2 pulses on the gate when it turns off and that gives the switching noise.
Dont know where they are coming from?

Otherwise it swithes fast, almost squarewave with rounded tops.
That is why i can run without a heatsink.

I did try the circuit with a 2N3055 and it would not oscillate with any bias setting.

I also parallelled my ignitioncoil with it and it works with a currentdraw of 400mA

I have to make some sort of heatsink before I go 12volt on this.

Hmm, you may be able to reduce noise on the gate with a 0.1 uF cap (you might have to try some different values) from the gate to ground. Also if you maybe put a 10 or 22 ohm resistor or there abouts in series in the gate lead it might help a bit, but you would just have to try some values to see if it helps at all.

[janost]

Hmm, you may be able to reduce noise on the gate with a 0.1 uF cap (you might have to try some different values) from the gate to ground. Also if you maybe put a 10 or 22 ohm resistor or there abouts in series in the gate lead it might help a bit, but you would just have to try some values to see if it helps at all.

Why does the true-RMS DMM say that the current drawn is 390mA and I measure it with a scope over a dropresistor to 160mA?

Is it because it cant handle the waveform?

[level]

Why does the true-RMS DMM say that the current drawn is 390mA and I measure it with a scope over a dropresistor to 160mA?
Is it because it cant handle the waveform?

Not sure. They should give close readings as long as they are both set to read RMS values. Also, if your scope probe is set to x10, make sure the scope is also set to x10 to match the probe.

[janost]

Not sure. They should give close readings as long as they are both set to read RMS values. Also, if your scope probe is set to x10, make sure the scope is also set to x10 to match the probe.

Yes they are.

I've been thinking that the meter can only show correct if the current/voltage is 50Hz and the waveform is a true sine?

For a digital scope it is easier as it just adds up its storage buffer and divides by the number of samples?

[level]

Yes they are.
I've been thinking that the meter can only show correct if the current/voltage is 50Hz and the waveform is a true sine?
For a digital scope it is easier as it just adds up its storage buffer and divides by the number of samples?

If it is an actual true RMS meter, then it shouldn't matter what the waveform shape is. As for the maximum frequency it can read accurately to, that depends on the meter's specs. The specs should list a maximum frequency for making current and voltage RMS readings. Many true RMS meters will read RMS readings accurately to at least 1 kHz, but it depends on the exact make and model you have. Some true RMS meters can read RMS to a much higher frequency. You probably already know this, but if your true RMS meter has a frequency counter feature, be careful not to mix up the frequency counter max frequency for the max frequency for RMS readings. These are different specs.

As a test for comparison, you could put 6VDC or whatever across a resistor, and then compare the results with the true RMS meter and scope to see how close they are.

[janost]

If it is an actual true RMS meter, then it shouldn't matter what the waveform shape is. As for the maximum frequency it can read accurately to, that depends on the meter's specs. The specs should list a maximum frequency for making current and voltage RMS readings. Many true RMS meters will read RMS readings accurately to at least 1 kHz, but it depends on the exact make and model you have. Some true RMS meters can read RMS to a much higher frequency. You probably already know this, but if your true RMS meter has a frequency counter feature, be careful not to mix up the frequency counter max frequency for the max frequency for RMS readings. These are different specs.

As a test for comparison, you could put 6VDC or whatever across a resistor, and then compare the results with the true RMS meter and scope to see how close they are.

It does have a frequency meter function that works up to 40KHz.
It still shows wrong. Chinese junk.

On the other hand the scope shows correct.
And that is also Chinese junk