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Topic: reading voltage of an N channel mosfet (Read 11944 times) previous topic - next topic

allanhurst

 Don't make the electrode +ve - it'll burn away quickly. And remember my piano wire idea.

You don't need magnet wire for this - you're not running at MHz. Solid would do.

Ordinary mild steel is not good - very lossy. Find some ferrite toroidal cores. Or just buy pre-made inductors - see previous.

Allan

mtraven

an interesting observation tonight: both the electrode holder & work piece fixture have become magnetic to the point where it will hold a paper clip or small nut.  Is this good, bad, to be expected? if bad, how might I correct this?  theories as to why it is happening?

I made a 50 turn toroidal like you suggested, seems to be doing a bit better, more testing needed though.  Tomorrow night I am going to try kerosene again with a different pump, maybe make a few more inductors & try them out.

allanhurst

I'm not surprised at the magnetisation. Might tend to hold on to the eroded particles?

Make them out of non magnetic stuff?

Allan

mtraven

#123
Jun 06, 2018, 09:14 pm Last Edit: Jun 06, 2018, 11:46 pm by mtraven
I'm not surprised at the magnetisation. Might tend to hold on to the eroded particles?

Make them out of non magnetic stuff?

Allan
I was thinking its was yanking on the electrode, but holding eroded particles would be a sound explanation for what is going on now. 

The part has to be steel, I could use an austenitic stainless steel (currently its 17-4 SS, a martensitic steel).  same goes for the fixture, but I could make it out of aluminum (does AL become magnetic?no iron so I think no?)  Materials that are electrically conductive and wont become magnetic are kinda slim, as suggestions other than what I have mentioned?

PedantEngineer

You have probably got all this already so please forgive any repetition:
The circuit impedances are quite (to very) high so you should expect some injection from gate to drain which may give you some odd readings and maybe odd behaviour. The nigh source impedance of 2M2 without anything (eg capacitor in shunt) to provide the current to charge the sample and hold of the ADC is very likely to give you a few odd readings as well. The 12V coming through the 2M2 and 250k(?) will eventually be clamped by the protection diodes on A0 so you would get a steady state voltage on A0 half a volt or so above Vdd.
Other than that, the inversion you described seems right to me, uC output low -> gate high via 10k pull up -> mosfet on -> low impedance from mosfet drain to mosfet source -> drain connected to 0V via mosfet.

allanhurst

Sorry , PE, but don't see what relevance....?

Mtraven... Ally, some stainless  (316) ( not all) , copper - anything common really apart from iron. nickel and cobalt would be fine.

Stainless is good stuff but a  pig to work - hard and tough.

Allan

mtraven

i think PE is a little behind...the current version of the circuit (designed by Allan) is sper6...have a look and see if any of that still applies.

ehhh stainless steel ain't so bad, i find turning SS easier than alum or steel sometimes....drilling can be a bit harder.  And wrt EDM, one of its main selling points is hardness of material is more or less irrelevant.  But even if I made the fixture from something that wont become magnetic (copper or brass probably), the steel part will still become magnetic and hold particles...and I know edm works on steel, so I am not sure what to do about it.

Your comment about not running in the mhz range got me thinking I should look at the pulse timing, something I have not done since the beginning.  My initial findings were alarming, though they may be a function of improper measurement...but the pulses are way longer than they should be.  Further investigation tonight....then all the other changes we've outlined.

allanhurst

#127
Jun 07, 2018, 01:38 pm Last Edit: Jun 08, 2018, 12:01 am by allanhurst
I think the latest version I sent was sper7 in post #116.

It would be interesting to prod around with an oscilloscope if you've got one.....

higher flow rate of dielectric with a low viscosity fluid could help - we've been here before!


Allan

mtraven

I think the latest version I sent was sper7 in post #116.

It would be interesting to prod around with an oscilloscpe if you've got one.....

higher flow rate of dielectric with a low viscosity fluid could help - we've been here before!


Allan
sper7 is the one with the current control correct?  I have not yet implemented that. 

I would LOVE a scope, but I dont have one :(

I dont know what had happened to my pulses, but they were way out of wack.  I changed the software so that the pulse pin is now toggled in hardware (i had been writing it high or low in an ISR())....that seems to have fixed it and changed the operation of the system for the better.

I don't think higher flow rate would help, higher pressure might, but more pressure tends to move the electrode around....that leaves a thinner DE, I still intend to go back to kerosene, I just keep getting caught up with other stuff.

This machine has consistency worked better right when I start it and then over a few hours, just fades out.  I now think this is the result of the magnetism.  I am not clear as to how it is magnetizing, I thought you needed a current carrying conductor wrapped around a core in order to magnetize?  When it shat out on me last night, I moved the part off center and started a new hole, it seemed to work right away despite the magnetism and it has run reasonably well for several hours...the parts are still magnetized, but not as much....I don't know what this all means, any theories are welcome.


allanhurst

You've effectively got a single turn ( psu+electronics+spark gap+work) coil - which ,with a current flowing, generates a magnetic field. This will tend to magnetise anything nearby.

Basic physics..

Try smacking it with a hammer once in a while !

( not quite silly - this is how the 'coherer' worked... look it up)

Allan

PedantEngineer

I was thinking its was yanking on the electrode, but holding eroded particles would be a sound explanation for what is going on now. 

The part has to be steel, I could use an austenitic stainless steel (currently its 17-4 SS, a martensitic steel).  same goes for the fixture, but I could make it out of aluminum (does AL become magnetic?no iron so I think no?)  Materials that are electrically conductive and wont become magnetic are kinda slim, as suggestions other than what I have mentioned?
Oooops. Clearly my last post is way out of date. I'll run and catch up just as quick as I can :-)

I must laugh though, you are discussing metallurgy and the phase states of stainless steel but you are not sure if Al is ferromagnetic! LOL

I'm not having a go at you, it is just funny to have uncommon knowledge of metallurgy as you do but not know that the only elements that exhibit ferromagnetic properties are iron, cobalt and nickel.

This thread has become far more interesting than it was and several of you appear to know what you are talking about and in impressive detail, so time for me to go read through it all and immerse myself in some satisfying technical discussion from some savvy chaps.

Apologies again for my totally irrelevant post.

allanhurst

PE - glad you're enjoying watching the normal slow and frustrating process of development !

Ferromagnetism - don't forget the rare earths, from which modern high energy magnets are made . But they're not common.

We await further revelations, Mtraven...

Allan

mtraven

I must laugh though, you are discussing metallurgy and the phase states of stainless steel but you are not sure if Al is ferromagnetic! LOL

I'm not having a go at you, it is just funny to have uncommon knowledge of metallurgy as you do but not know that the only elements that exhibit ferromagnetic properties are iron, cobalt and nickel.
I don't think either of us thought AL was ferromagnetic, I know I did, and I am sure Allan did to.... I was more asking about the implications of it being paramagnetic.


You've effectively got a single turn ( psu+electronics+spark gap+work) coil - which ,with a current flowing, generates a magnetic field. This will tend to magnetise anything nearby.

Try smacking it with a hammer once in a while !

I guess I just didn't recognize that it was a single turn.  And in reading about de-magnetizing , physical abuse (ie hammer strike) was one of the ways, so it doesn't seem silly to me, it is however, impractical as a strike hard enough to accomplish this, would knock the part out of alignment.

So I am not done with this project, but I have to build a deck now, so there is unlikely to be any updates in the next week-10days but I will be sure to update you all as soon as I get back to it.

PedantEngineer

The part has to be steel, I could use an austenitic stainless steel (currently its 17-4 SS, a martensitic steel).  same goes for the fixture, but I could make it out of aluminum (does AL become magnetic?no iron so I think no?)  Materials that are electrically conductive and wont become magnetic are kinda slim, as suggestions other than what I have mentioned?
Not to pick hairs here but "does AL become magnetic?no iron so I think no?" is not a question about Al being paramagnetic as per your clarification in #132.

mtraven

Not to pick hairs here but "does AL become magnetic?no iron so I think no?" is not a question about Al being paramagnetic as per your clarification in #132.
hence the need for clarification.

look man, I am farmer & machinist, I am doing my best. If I knew all this shit, I wouldn't be on here seeking help.   If you have no useful advice, there is no need to hear from you.  Thanks for following.

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