FETs and gate resistors - scope images

Now we need one after the diode to check how the controller supply voltage reacts…

What do you want to see on the two channels, before diode & after diode, or after diode & gate pin?

The giant electrolytic cap I'm poking around onto various pads completely at random gives an interesting change when it's placed across the diode, too, but that's not a real-world solution to anything (it's way too big). Would a smaller SMD cap in parallel with the diode accomplish the same thing? I need to figure out a way to try that...

I also swapped our known-value 10uF/16v cap for the unknown-value part that comes on a 105C, no change... I also tried the cap on the top of the board straight from the MCU pin to ground (electrically the same, just different trace lengths) with no change, and then tried the two SMD caps at the same time, one in the original location on the battery side and a second one on top at the MCU, again no change.

Also tried the big electrolytic in the same location as the small SMD cap, no change. But putting it between B+ and GND, or in parallel with the diode, did get rid of the weird overvoltage spikes.

I am assuming here that this spike thing is the cause of the mode-change issues when there's no gate resistor present, since doing anything to eliminate the spike (bypassing the diode, or adding the cap between B+/GND) also eliminated the mode-change issues. Am I at least on the right track? That the solution is something practical that eliminates the voltage spike?

Like you know I also have often this problem, just put a normal nanjg in my t08 because my test driver with some smd FETs in parallel was gone crazy…
I also tried to add caps in some different capacities(sometimes stacked half a dozen), resistors and stuff everywhere but couldn’t get a valid explanation or solution. I am happy that you try to analyze these issues:)
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If I am right that is our hypothesis:
That the controller behaves sometimes strange and what often can get wiped out with a big cap is in my eyes also a problem with the controller input voltage, because every time the FET switches it is like a short….
The idea behind the diode is that the led can’t discharge the cap behind the diode and so the controller always has enough voltage to work also in the switching phase.
A cap between the bat- and bat+ buffers this also but because it gets discharged through the led with serious amps it needs to be a big one.
Now the only reasonable guess would be that this voltage protection isn’t working as we think.
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I just want to see if that diode and cap construct works to smoothen the power supply or if the controller also sees these spikes, because I think that has to be the problem…??? So both would be interesting controller voltage and FET signal, so that we can see if there is a correlation between the switching and the power. Also the before and after diode is interesting to see if it actually works like we think it will.

What has me confused is here:

(same pic from post #18 - trigger here is on the rising edge of CH1 (gate signal) at 1.00v, that's close to the FET's threshold voltage)

I can (mostly) understand the spike on CH2 after the PWM turns off. What I don't get is how the gate voltage rises to ~6v, and stays there, while the supply to the MCU is still down at battery voltage. If they tracked together more closely it wouldn't be much of a mystery. The ~6v being generated when it turns off, sure. Removing the load, it releases stored energy in the cap, and voltage spikes. But it's doing a constant, rock steady 6v output to the gate pin while input to the MCU is still down at ~3.9v.

These spikes always are generated when switching an inductance, all these switching and the diode+cap leads to the high voltage. Every spike gets over the diode to charge the cap at a higher voltage but never gets discharged down to the supply voltage again. You have to imagine there are a lot of these spikes and the controller don’t use much power so the voltage don’t breaks in.
Like someone said its a boost circuit.
Ch2 is not input of mcu because there is a diode between your measuring point and the controller and the diode prevents current flowing in that direction. If you measure the controller voltage I am sure we will see the 6V supply…

Maybe we have a overvoltage problem and no undervoltage problem? Do these problems also occur on Zener modded nanjgs?

I haven't looked at any of this on a zener-modded driver yet, I don't have one built at the moment...

Do you have a prediction as to what the scope will show with a 10uF SMD cap in parallel with the polarity diode?

The parallel cap sees the voltage spike on both legs so it shouldn’t get so charged up like the stock one. Only the diode forward voltage should be between the two legs. I guess that compensates the spike and stabilize because there is then a cap in both directions: one from controller+ to GND and one from controller+ to V. So the “negative” side of the cap is the V side….
But this are only unfunded fantasies….

Have you never used a Zener modded FET driver? Never thought about that but now I need to go the the soldering iron….and test some things.

Atmel rates the maximum Voltage for the tiny at 5.5V.

Yes, I've built lots of them, I just don't have one put together right now. Have plenty of parts on hand to build them.

Torchlite SVD7 MTG2 built for DayLighter:

Another 10uF/16v cap placed at D1, with the diode piggybacked:

CH1 = gate, CH2 = VBAT

CH1 = gate, CH2 = MCU's Vcc, pin 8:

The weird behavior of some drivers without a gate resistor, where the MCU seems to shut down right after changing modes, is likely due to the overvoltage to the MCU from that spike. It's doing exactly what it seems like, it's shutting itself down! It sees that ~6v and says 'whoa dude, I need a little nap - wake me up when shit gets back to normal.'

So how did adding the stupid gate resistor cover up this issue - just by reducing the load the MCU has to drive, therefore reducing the current draw through the diode, which reduced the tendency to create the voltage spike?

So by putting in the cap, the load resistor can go away?

Correct, the 100-130 ohm gate resistor no longer needed.

Also, due to the different circuit layout of the zener-mod drivers, those don't suffer the same voltage spike, so don't need any changes. Unless it's one of the zener-specific boards that are designed to keep a polarity protection diode. I don't know if any are, I haven't looked at them that closely. But a normal driver converted with a resistor in place of the diode, and the zener in parallel with the cap, isn't going to be able to generate these spikes. Those drivers likely never needed the gate resistor in the first place.

Do you have a East-92 to compare? As I said earlier in post #987 here, the diode is likely being used to help with this problem. I could certainly be wrong, but I suspect that the diode may work better. Once the cap is charged/discharged it maxes out. Bigger caps will have diminishing returns & high costs. The diode will continue conducting to remove our stray voltage until Vf is reached.

Good work so far.

The resistor softens the turn-off. The sharp turn-off is what causes the spike. The purpose of the diode as I understand it is give the spike a path to escape through rather than shoving itself where we don’t want it.

I'm posting stuff all over the place, these two were in the Oshpark thread.

Stock 105C hardware, CH1 on Vdd (MCU pin 6), CH2 on B+ before the diode:

Battery voltage was well below 4v for that pic.

This is a 20DD, no gate resistor, but otherwise using the original parts in the original spots (without the add-on capacitor across the diode). CH1 is after the diode (MCU pin 8 EIGHT, stupid auto-smiley crap), CH2 is before the diode (B+). Battery voltage was around 4.08v no-load.

Look at that beautiful flat line for CH1... if only it weren't way up there at 6 volts. 6 volts!!

I have one east-092 but it's one of the crap versions. I have dead stuff I can scavenge parts from, where should the diode go - between LED+ and LED-?

It seems that voltage is not always accurately depicted. In your trace with the Stock 105C hardware Vdd is shown >4v, this is not accurate if B+ is <4v. Actual Vdd will be much lower, the combined effects of the protection diode, B+ drop under load, and the ATtiny13A’s hardware should put Vdd closer to 3.0v I’d estimate?

Yes, but the anode and cathode will be backwards compared to the LED.

If I use the scope probes to check the battery voltage, it matches what my DVOM says. And when I add the capacitor in parallel with the diode, the gate voltage no longer goes above battery voltage - the 'boost circuit' is effectively disabled (or at least severely impaired). Shorting across the diode also gets rid of the overvoltage on all 3 spots - gate, Vcc, & B+.

edit: see post #14 for a comparison of diode normal vs. diode shorted, gate voltage drops to battery voltage just by shorting the diode

For clarity I am referring to the trace shown in post #35. I realize that you have it in quotes, but there is no ’boost circuit’. The ATtiny is completely incapable of providing an output higher than it’s input. Vdd is the enable pin on a 7135, the 7135 definitely does not increase the voltage of that pin beyond what the ATtiny puts there. The trace clearly show that the voltage is constant across the pulse, it is not a spike. The battery voltage is clearly shown as lower than the Vdd voltage. All I’m saying is that that is definitely incorrect. Where did you attach your ground clips?

EDIT: see dave_’s post #47 & #50 in this thread. He explains clearly why what I wrote in this post is wrong. (in other words there does appear to be an unwanted boost circuit created by our component layout)