Understanding electricity in the TPU.

[gyulasun]

Hi wattsup,

I have just found this link and you may consider this normally OFF power JFET too: SJEP170R550    It has about 0.5 Ohm ON resistance when conducts and 1700V drain -source breakdown voltage. Here is the link I noticed it:
http://www.diyaudio.com/forums/vendors-bazaar/163776-zhou-fangs-sales-thread-parts-you-always-wanted-but-hard-get-they-here.html  and the data sheet is here:
http://www.semisouth.com/products/uploads/DS_SJEP170R550_rev1.3.pdf

Also on that DIYAUDIO forum they mention the SJEP120R100, also normally OFF JFET, with 0.1 Ohm resistance and 1200V breakdown voltage, data sheet is here:
http://www.semisouth.com/products/uploads/DS_SJEP120R100_rev1%202.pdf

The guy uses paypal and orders above $50 are free shipping from Singapore.  Delivery time 7-8 working days.

The DIYAUDIO guys in the forum consider FET parameters from audio power amplifier output stage points of view, you wish to use them for switching. From this respect the input and output capacitances and ON channel resistance what counts, g_M transconductance not so important.

rgds,  Gyula

[wattsup]

@gyulasun

Thanks for your information. SemiSouth said they will ship ASAP so I think for now I will wait till these arrive for initial testing.

Incredible that these JFETs are used in audio. Must be high end audio systems. SM was an audio guy also and he did express the fact that his audio produced pure signals, so this just may be the way to go.

I will start very carefully because the prices for these are rather extreme. I will surely need your help when the time comes to make sure I set these up correctly for testing.

I just got notice that my order for a good 200 standard JFETs has arrived at the post office. I will use these in parallel and do some testing while I wait for the SemiSouth JFETs.

My main way of pulsing is using my FG which is a HP 8111A. It goes up to 22mhz, 16.5 volts but very very low amperage which is a good thing.  A copy of the manual is available on my ftp OU site located here;
http://purco.qc.ca/ftp/Equipment%20Manuals/

The FG goes to the gate but I then use my power supply on the source/drain line to pulse the required power into coils, etc.

So I can't really put the positive of the FG on the source and the negative in the gate, since the positive is connected to my power supply. Man why is EE so complicated. Can't it work like regular plumbing. lol Just jokin.

Thanks again for looking out for these JFETs.

wattsup

[wattsup]

This is neato.
http://www.firstwatt.com/j2.html

[gyulasun]

...
I will start very carefully because the prices for these are rather extreme. I will surely need your help when the time comes to make sure I set these up correctly for testing.

The most important thing to see perhaps is power dissipation in the switch (i.e. in the JFET, MOSFET etc):  if you have a coil you place in series with the drain (or source) electrode of a FET, you HAVE TO consider the possible maximum current drawn from your power supply when the FET is switched ON. 
How can you figure out the maximum current? 
First by simple Ohm's Law.  You have a coil with ,say, 1 Ohm DC wire resistance and you use ,say, 12V DC supply. When your FET is ON, and suppose it has a 0.1 Ohm ON resistance, the current will be 12V/(1+0.1)=10.9 Amper. (I neglected your power supply inner resistance which also in series with the coil and the FET and I considered it as zero.)

Now what power dissipates in this FET? Still using a static state (you do not pulse the input yet and the FET is ON continuously) the voltage drop between its drain and source electrodes is 10.9A*0.1Ohm=1.09V and the heat dissipation in the FET will be 1.09²/0.1=0.118W This 118mW will heat the FET, no real need for a heat sink to the FET body in case of a TO220 or similar casing.
Of course when you use AC pulse control for the FET switch, the current flowing via the coil (and the FET) will depend on the duty cycle and the frequency so power dissipation surely decreases with respect to the above static case, I used the example to show that the FET you happen to use must be able to handle the 11 Amper in the given circuit parameters.

Now you understand that in case your FET has a 10 Ohm ON resistance instead of the 0.1 Ohm, then the maximum current will be 12/(10+1)=1.09 Amper, using the same 1 Ohm coil and the 12V supply voltage. But the FET dissipation INCREASES because the voltage drop across the drain-source will be 1.09A*10Ohm=10.9V so the dissipation in the FET will be 10.9²/10=11.88W  now this is a serious heat that would quickly make the FET tost soon, a decent heat sink is strongly needed. BUT the best to avoid using a FET switch with 10 Ohm ON resistance in this example circuit I used with the 1 Ohm coil.

And now, after the current handling consideration, there comes the drain-source maximum voltage consideration for the FET. Because the coil is in series with the switch, the switch-off instant imposes the voltage spike across the switch so you have to use at least a 300-400V FET or even higher, not ruin it at the first switch-ON.
The induced voltage when the current is switched off in a coil depends on how suddenly you switched it OFF, how much current was flowing in the coil and what self inductance the coil has. V_i=L*dI/dt  where V_i is the induced voltage, L is the coil inductance in Henry, I is the current change and t is the time under which the switch-off happens. 
If you have L=100uH I=1A t=1us then V_i=0.0001*(1/0.000001)=100V spike.  If you have a faster switch with ,say, 100ns OFF time, the spike willl be 1000V!  AND YOU WILL HAVE a fast switch because your HP generator and either a MOSFET or the JFET is able to switch ON or OFF around 100ns or even quicker.

Now it is obvious you may wish to start any such switching test with the smallest power supply voltage possible, say, starting with maybe 2-3V DC instead of the 12V or whatever, to keep the current first at a low value, thus everything within safe parameters.

I just got notice that my order for a good 200 standard JFETs has arrived at the post office. I will use these in parallel and do some testing while I wait for the SemiSouth JFETs.

Now you may have some info how to NOT toast them.  When a lot of them is in parallel, maybe it is then not the current that ruins them but their 25-30V drain-source maximum voltage... 
Though you have to consider current too because if 10 of them in parallel have ,say, the 10 Ohm ON resistance and you use them in the above example with the 1 Ohm coil, then the 1 Amper current from the 12V supply will drive 100mA through in each, (10 times 100mA=1A) and these small signal jfets cannot handle 100mA but maybe 30-40mA maximum each, depends on their type of course.

...
My main way of pulsing is using my FG which is a HP 8111A. It goes up to 22mhz, 16.5 volts but very very low amperage which is a good thing.  A copy of the manual is available on my ftp OU site located here;
http://purco.qc.ca/ftp/Equipment%20Manuals/

The FG goes to the gate but I then use my power supply on the source/drain line to pulse the required power into coils, etc.

So I can't really put the positive of the FG on the source and the negative in the gate, since the positive is connected to my power supply. Man why is EE so complicated. Can't it work like regular plumbing. lol Just jokin.
...

You have a very useful function generator indeed. It can give out negative or positive or symmetrical polarity waveforms which are very useful. For your N channel JFETs the negative polarity is the one needed because all the waveform is UNDER the zero voltage line.
In case of the FG the output voltage is defined with respect to the BNC socket 'outer metal ring', let's name its metal cylinder as the zero voltage point. And the BNC socket's middle point in the center of the cylinder metal body gives out the polarity (and amplitude) with respect to the zero point.
I mention these so that you can think of them correctly when you mention polarity in case of a power supply. What you sound to call positive at the FG, it is the FG's BNC output metal cylinder, the zero voltage or ground level point, ok?  And you can connect it to the source electrode of the JFET (or MOSFET), no problem if your power supply's negative polarity is also connected to it.  And the FG's center pin of its BNC output can go directly to the gate electrode of the JFET (or MOSFET) because the polarity of this pin can be chosen from the FG's front panel button, Output Mode Selection shown in Page 41 in the PDF file  ( http://purco.qc.ca/ftp/Equipment%20Manuals/hp-8111a/08111-90002.pdf ).  (The Complement mode simply means the 'phase' of the output pulse is changed: where there was zero voltage, it changed to max amplitude and vice versa.)

And your power supply's positive output can go to the drain electrode of the FET or MOSFET (via a coil), its polarity has nothing to do with the FG's output polarity, you know that.

rgds,  Gyula

[wattsup]

@gyulasun

Thanks again for your teaching. I will keep these posts handy for continuous referencing. I never used those sym buttons before.

Today I replaced my el cheapo JFET and used three PN4416A in parallel, with a NPN on the positive and on the negative sides of the coil circuit. I put the negative of my FG to the base of the JFET and the positive of my FG to the base of the 2 NPNs. Passed my power supply through the circuit and bingo, when I put my two scope probes on the source and drain of the JFET, I can now see them pulsing in opposite directions so this tells me the pulsing is now working.

So I tried pulsing my new toroid coil with the moving inner blotch wall and sent the outer coils to my dioded capacitor tank and this is not giving me what I was expecting but it is showing me something else. This type of coil will require a very new outer winding method that I will do today and report back soon. The one coil outer winding is not working. But the pulsing is working and the 3 JFETs are holding their own very well. Hallelujah, so I can still play with this on other coil builds while I wait for the bigger JFETs.

But from what I have seen, any moving blotch wall will require many outer coils of less winds so the travel can be more complete over each outer coil. I will show this effect soon.

wattsup