The Paschen's law doesn't really apply in EDM. The gap is only ~0.001 inch and it is in oil. The electrodes form a basic capacitance that will break down when the electric field reaches a certain intensity, the break down is somewhat due to the microscopic carbon and metal contaminates in it. And even using Paschen's, 0.001 inch is pretty close to what that law states.

They do this in industry with as little as 50VDC, even though no one on the forums will believe me. What I'm building is actually 95VDC @ 15Amps. After the arc starts the current and gap voltage drops to 35 to 25VDC. Depending on the electrode size.

I I forgot you were doing edm. The 'sparks' are metal vapor and droplets erupting from the surface as current flowing through point contacts of the edm electrode and work piece. Not exactly a spark gap. Just visible light from molten metal with no gap.

 

The Paschen's law doesn't really apply in EDM. The gap is only ~0.001 inch and it is in oil. The electrodes form a basic capacitance that will break down when the electric field reaches a certain intensity, the break down is somewhat due to the microscopic carbon and metal contaminates in it. And even using Paschen's, 0.001 inch is pretty close to what that law states.

They do this in industry with as little as 50VDC, even though no one on the forums will believe me. What I'm building is actually 95VDC @ 15Amps. After the arc starts the current and gap voltage drops to 35 to 25VDC. Depending on the electrode size.

I daresay it would have made matters simpler had you chosen to share that info initially! IMO there are no 'dumb questions', info-barren questions, however, tend to elicit less than helpful responses!

With constructive intent
HP

 

I've only seen one run a couple of times. Never realized the current was so high. How long is the pulse?

Depending on what is being 'burned'(the term for the cutting) from around 25uSec to 100uSec on time, with an off time of around 25uSec to 50uSec. But during the on time there may be up to around 50,000 actual sparks. The gap surfaces are in the microscopic level very rough/jagged and the spark jumps from high point to high point across the surfaces. The off time allows the dielectric oil to flush the particles out of the gap so the next round of sparking can start up.

Although I'm only using 15Amps for a hobby machine, that is high for a hobby machine and low for an industrial machine. Just happened to luck into a transformer that high of amperage. But even the commercial machines use around 80VDC to 100VDC open circuit voltage.

 

I daresay it would have made matters simpler had you chosen to share that info initially! IMO there are no 'dumb questions', info-barren questions, however, tend to beget less than helpful responses!

With constructive intent
HP

About the only questions I ask are involving this EDM project. Just assumed everyone looking knew my intentions, when I mentioned spark gaps and electrodes. And I am dumb at electronics. I can and have built and follow schematics, but this is my first design project. I know what I want and need to do for this to work, just don't know how to go about it. They do it in industry but getting access to schematics is almost impossible.

 

I think you could measure current on the ground side to determine when the spark initiated. Before spark no current, when spark starts, current also starts. Since it's a changing current you could use a simple CT or a hall sensor or you could even measure the voltage drop over a trace or part of a wire.

Alternatively (assuming it's really 100V like in your drawing) you could trigger on the rapidly decreasing voltage over the spark gap when the spark starts.

 

About the only questions I ask are involving this EDM project. Just assumed everyone looking knew my intentions, when I mentioned spark gaps and electrodes. And I am dumb at electronics. I can and have built and follow schematics, but this is my first design project. I know what I want and need to do for this to work, just don't know how to go about it. They do it in industry but getting access to schematics is almost impossible.

Hey no problems! Apologies if the tone of my post seemed rude --- As a new member, I wasn't aware of your focus on EDM (based upon context I'm assuming EDM="electrical discharge machining"??). Again, thanks for your civil reply to my (admittedly) curt post...

Best regards
HP

 

Your diode idea should work. Maybe something like this:

 

Thank you for that Ron! I went back and looked at the thesis the idea came from, and they actually used two diodes in series not one for the measuring point. I really wish I'd have gotten into understanding electronics earlier in life. I know what needs to happen in my project but not how to use components to make it work.

 

Hey no problems! Apologies if the tone of my post seemed rude --- As a new member, I wasn't aware of your focus on EDM (based upon context I'm assuming EDM="electrical discharge machining"??). Again, thanks for your civil reply to my (admittedly) curt post...

Best regards
HP

No problems on my side. I'm not as articulate as I wish I could be with electronics. Spent my whole life working with my hands as a skilled tradesman. No one in my circle, even the electricians, had any real knowlege or desire to discuss electronics.

Yes it is an electrical discharge machine that I'm working towards. Have had the mechanical parts done for a long time, just want a more sophisticated electronics than what is being used in the hobby community currently.

 

I think you could measure current on the ground side to determine when the spark initiated. Before spark no current, when spark starts, current also starts. Since it's a changing current you could use a simple CT or a hall sensor or you could even measure the voltage drop over a trace or part of a wire.

Alternatively (assuming it's really 100V like in your drawing) you could trigger on the rapidly decreasing voltage over the spark gap when the spark starts.

Thanks Praondevou, didn't even think of a current transformer. The problem with a current shunt is finding one with a small enough resistance and still able to withstand the amperage. I'll have to read up on CT's. I've been to focused on voltage comparators for what I need.

 

Thank you for that Ron! I went back and looked at the thesis the idea came from, and they actually used two diodes in series not one for the measuring point. I really wish I'd have gotten into understanding electronics earlier in life. I know what needs to happen in my project but not how to use components to make it work.

That's how it is supposed to work. We get to learn about EDM and you get to learn about electronics.
Two diodes makes the circuit easier in a way. Now a single diode can be used for the voltage reference for the comparator - about .7 volts and 2 diodes for the current sense - about 1.4 volts.

 

What limits the current? Is it the oil or does the circuit do it?
In other words, how do you know 15 amps will be enough to still get 25 or 30 volts across the gap?

 

Here is a couple of pages of the method used by the old Elox pulse EDM sinker.
Max.

 

I want to use the voltage on the ground side of the gap to start a timer, to control the length of the sparking time. If I measure the circuit voltage at the other side of the electrode, That won't tell me when the sparking starts. Just the voltage level. The voltage level is measured there also, with a window comparator as feedback to the servo that controls the electrode gap size. The servo voltage is adjustable for gap working conditions from 35 to 25VDC.

In that case, you can possibly use an inductive pick up similar to the old timing lights used to set engine timing with. It could be as simple as a wire wrapped around a torroid place over the ground wire.

I thought I finally caught a break finding that thesis with their circuit, that they claim worked, but guess not.

I might have been a little too quick to dismiss this circuit. I would need to see the actual paper and the author's reasoning. Still, I think there would be a better way to accomplish what you're trying to do.

 

On the Elox there is three conditions monitored, Open arc, Shorted arc and Cutting.
The servo circuit monitors the -ve arc voltage for all three conditions and continually retracts or advances the electrode in order to maintain a cutting arc.
Max.

 

Max makes a good point. All of those things are important to know and could be sensed on the high side since you have to go there anyway.
Maybe sensed like this:
Cutting pulse on and 100 volts = open
Cutting pulse on and less than ~ 10 volts = shorted
Cutting pulse on and negative transition = start cutting voltage and time.
You might still want a shunt in the low side or a high side current sensor to see average cutting current.

 

The Elox uses a simple comparitor circuit on the input to the servo drive, it monitors the -70v arc directly via a resistive divider on a 2n3645.
Max.

 

Lucky for us they make chips for all that now days.

 

On the Elox there is three conditions monitored, Open arc, Shorted arc and Cutting.

If that is the case, then I would assume the goal is to stay in some window (Goldilocks zone) of ideal voltage. So the electrode moves harder into a workpiece if voltage is too high (no load), and backs away from the work piece if voltage is too low (shorted), and approaches again at a slower speed? Would that be the correct logic?

 

What limits the current? Is it the oil or does the circuit do it?
In other words, how do you know 15 amps will be enough to still get 25 or 30 volts across the gap?

I'm just assuming. The window comparator for the gap voltage will allow some adjustment to the gap distance thus the voltage. From running these machines I'm pretty sure 15A is more than enough, at least for the small stuff I'll be doing.

Current limiting is going to come after it is up and working. By different power resistors in the rectifier section. Not energy efficient but it worked that way in a lot of the old machines.