Quote from: Groundloop on May 13, 2012, 03:48:29 PM
TK,

I did measure 0,09 Ampere BIAS current @ 13,75 Volt when the circuit was in DC mode (not oscillating).
MileHigh did say that this was too high. What can the reason be for that relative high current? The DC input
voltage to my circuit was 24 Volt. I did calculate that in order to get that current the BIAS voltage input must
"see" a resistance of 91 Ohm.

GL.

Thanks for the PF50 data sheet. I didn't realize there was such a thing.

But Ainslie is still using PG50s? It doesn't really matter I guess, because I've already confirmed that it works even with IRF830as -- much cheaper, lower Rdss -- and makes a fine negative power product with them too. And she herself has said that the type of mosfet doesn't matter.... and yet she found time to criticise me when I wasn't using them.

I don't know what the explanation is for the high bias draw during the gate HI signal but as you can see from my videos Tar Baby experiences this too (the loading is happening on both the HI and LO portions of the FG signal, just not as much on the HI part.)
Perhaps it is due to the Zener body diodes. I haven't had time to look at the issue in detail yet.

Right now I'm out in the backyard observing this massive sunspot that seems to be ready to emit a flare. It's the biggest most complex sunspot  I've ever seen and it even looks like there is some vorticity to it. I'm trying to get a photo, and attempting to perfect my solar photo technique for the upcoming Transit of Venus in early June.

GL, I'm not quite following that last diagram. In the mosfet, the zener diode is between the drain and the source, and as far as I can tell the gate drive input shouldn't be passing any measurable DC current at all to the rest of the circuit. It's as if there's a small capacitor (the gate capacitance)  just after your 50R in the +13 volt gate voltage input. When that cap is full the DC stops flowing, and the mosfet turns on from the electric field ( field effect transistor ) in the cap. Only AC should be able to pass power across this cap to the rest of the circuit.

But I'm still trying to figure out the role of the _other_ mosfet's Zener in all of this.

What happens if you pull out the Q2 mosfet and repeat the Q1 bias current test?

Quote from: TinselKoala on May 13, 2012, 05:12:51 PM
GL, I'm not quite following that last diagram. In the mosfet, the zener diode is between the drain and the source, and as far as I can tell the gate drive input shouldn't be passing any measurable DC current at all to the rest of the circuit. It's as if there's a small capacitor (the gate capacitance)  just after your 50R in the +13 volt gate voltage input. When that cap is full the DC stops flowing, and the mosfet turns on from the electric field ( field effect transistor ) in the cap. Only AC should be able to pass power across this cap to the rest of the circuit.

But I'm still trying to figure out the role of the _other_ mosfet's Zener in all of this.

What happens if you pull out the Q2 mosfet and repeat the Q1 bias current test?

TK,

Hard to see yes. There is a current path through the Q1 internal diode. (I referring to my circuit drawing Q1).
My Q2 is fully on conducting current. So the drawing represent what is happening. If I pull Q2 then the 1,6 Ohm
RdsOn will disappear and the bias current must go another route. Have not theoretical looked at that yet.
I do not have the test rig up and running right now so I can't test this. I think I see a paradox in the posted
drawing. The 24VDC is "seeing" a RdsOn of 1,6 Ohm, but the 13,75VDC bias current is "seeing" a RdsOn
of 30,39 Ohm at the same time. Is that even possible?

GL.

You can do a quick and dirty mosfet test using a DMM with a diode forward voltage check function.

Use a bit of wire to short all the mosfet pins together briefly. Set the meter to the diode fwd voltage function (diode check), then touch ONLY the pins in the following order.

Touch the positive lead from the DMM to the mosfet Source pin and the Negative lead to the mosfet drain. You should read the fwd voltage drop of the Zener, around 0.5 volts. If you read OPEN or SHORT the mosfet is bad.

Now reverse the DMM leads touching the positive to the Drain and the negative to the Source. The mosfet should read OPEN, very very high resistance, on my Fluke 83 this is ".OL". If it reads SHORT or a very low voltage drop the mosfet is _probably_ bad.
-- IF and only if it reads SHORT or a low drop on this part of the test, leave the Positive DMM lead on the Drain and touch the negative DMM lead to the GATE pin briefly. Then go back to the Source with the DMM lead. If it still reads SHORT the mosfet is bad.

Now, leaving the NEGATIVE lead of the DMM on the source pin, briefly touch the POSITIVE lead of the DMM to the GATE, then go QUICKLY back to the Drain. The DMM should read a low voltage drop, my PG50s read 0.002 volts here (but it will increase fast as the gate charge leaks away). If it reads OPEN or a consistent high voltage drop, the mosfet is probably bad.

Now set your DMM to the resistance measurement range and look from the Gate to the other pins, using both polarities.
There should NEVER be continuity or anything except "infinite" resistance between the gate and either other pin when measured by the DMM in this way. If the gate-to-drain or gate-to-source resistance is anything other than OPEN, the mosfet is bad.

OPEN of course means open circuit, the opposite of SHORT circuit.

This is probably old news, but it's Sunday, a slow news day.

Quote from: TinselKoala on May 13, 2012, 05:36:24 PM
You can do a quick and dirty mosfet test using a DMM with a diode forward voltage check function.

Use a bit of wire to short all the mosfet pins together briefly. Set the meter to the diode fwd voltage function (diode check), then touch ONLY the pins in the following order.

Touch the positive lead from the DMM to the mosfet Source pin and the Negative lead to the mosfet drain. You should read the fwd voltage drop of the Zener, around 0.5 volts. If you read OPEN or SHORT the mosfet is bad.

Now reverse the DMM leads touching the positive to the Drain and the negative to the Source. The mosfet should read OPEN, very very high resistance, on my Fluke 83 this is ".OL". If it reads SHORT or a very low voltage drop the mosfet is _probably_ bad.
-- IF and only if it reads SHORT or a low drop on this part of the test, leave the Positive DMM lead on the Drain and touch the negative DMM lead to the GATE pin briefly. Then go back to the Source with the DMM lead. If it still reads SHORT the mosfet is bad.

Now, leaving the NEGATIVE lead of the DMM on the source pin, briefly touch the POSITIVE lead of the DMM to the GATE, then go QUICKLY back to the Drain. The DMM should read a low voltage drop, my PG50s read 0.002 volts here (but it will increase fast as the gate charge leaks away). If it reads OPEN or a consistent high voltage drop, the mosfet is probably bad.

Now set your DMM to the resistance measurement range and look from the Gate to the other pins, using both polarities.
There should NEVER be continuity or anything except "infinite" resistance between the gate and either other pin when measured by the DMM in this way. If the gate-to-drain or gate-to-source resistance is anything other than OPEN, the mosfet is bad.

OPEN of course means open circuit, the opposite of SHORT circuit.

This is probably old news, but it's Sunday, a slow news day.

TK,

My MOSFET transistors are brand new. I did put two new ones in the circuit before testing the bias current.
The circuit operates in accordance with my theoretical calculations. I also did test the MOSFET with
my Ohm meter. Looks OK to me. When you have the time could you check what bias current you get
for your setup when the circuit is in DC mode?

GL.