BJT behaviour at ridiculously low current levels

Just a supplement: the interesting part is IMO discharging the caps through the flyback chokes. It makes the pulses at no additional energy cost (beyond what has already been transferred through the 100M resistors), so they are both signal and power at the same time. Unfortunately, the amount of energy stored in that 470p capacitor charged to 1V is so tiny that the autotransformer is not able to boost the voltage (merely ~150mV at the low V_IN end) to open Q3. This is a constant energy regime: if you add more turns to L3, the voltage reaches some saturation point and the reset pulses are getting longer instead, as L and C both go up. This saturation point is ~400mV, too low for an NPN to notice (assuming zero power detection; you can't afford a proper PNP long tailed pair). A 400mV EPAD would cut the mustard, but another solution is to double the amount of energy deposited to the core. Hence the dual-pumping idea utilising L1 and L4. With that it works even down to 1.1V V_IN. Balancing these available picojoules is sort of obscene. :)

Best regards, Piotr

Reply to
Piotr Wyderski
Loading thread data ...

Yabbut, at 1400V at least one and probably two of them will be continuously avalanching during normal operation, unless the drain curves are squishy enough to take up the unit-to-unit spread in I_DSS. (Maybe they are.) That would take a lot of testing to confirm as safe.

The avalanche voltage increases with temperature, so there'd be no automatic sharing of dissipation.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

If two avalanche at about 650 volts each, that runs the third one as the current limiter, at roughly 100 volts. Seems like it just works.

At 100 uA, the two that avalanche dissipate maybe 65 mW each. The linear one is necessarily less.

Now I need a good blinker circuit.

Reply to
jlarkin

The discharge would be a slow exponential and the blink rate would approach zero towards the end. Neither sounds good to me.

Reply to
jlarkin

Provided that there's no long-term damage mechanism. It's going to be in that state for a long long time.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

On Wednesday, 25 May 2022 at 06:41:00 UTC-7, Phil Hobbs wrote: ...

...

The Art of Electronics - The X Chapters. depicts that configuration and variants with a discussion in section 3.x.6.5

The simple configuration with just source resistors can have steps in the current as the voltage is varied as each device in turn avalanches.

kw

Reply to
ke...

The simple version, 3x57a with LND150s would be fine for a 1400 volt LED blinker. And there are some diacs that would work for the periodic cap discharge into the LED. 9 or so parts total.

Reply to
John Larkin

Why wouldn't you just use a 1.7kV+ rated DMOS?

Best regards, Piotr

Reply to
Piotr Wyderski

Had a part in mind? If it's enhancement mode, it will need something to bias it up.

We do have the LND150s in stock, SOT23 package.

Reply to
jlarkin

Mouser has this IXTA1N170DHV. More expensive that LND150, but I am not sure if a safety circuit is the best place for savings and IIRC your business is of the high-added-value/low volume profile anyway. Just saying, it's your decision.

I use the IXTA08N100D2HV for the same purpose (have a blinker too, and a buzzer specifically tuned to emulate a fighter jet warning system:

formatting link
just couldn't resist), but my cap voltage is under 900V, 650V typical.

Best regards, Piotr

Reply to
Piotr Wyderski

The 1400v supply is low current and doesn't store lethal energy, but someone could damage parts by poking around while there's still voltage. The linear discharge with blinker helps with that, and is cool too. Sometimes we do things just because they are cool.

That is an interesting part, but Mouser has four for $20 each. That's another factor these days, availability and price.

We also use DN2530, a 300 v depl fet in SOT89. It good for discharging bigger caps.

In a couple of boxes with gigantic caps, we used a depletion fet + LED indicator and a discharge resistor + pushbutton inside, so our techs are warned and didn't have to wait minutes to poke around.

Reply to
jlarkin

Oh, in that case never mind. My comment was entirely safety-driven.

So do I. :)

I use it as a resettable fuse. The short-circit current is limited and the part just stops being hot when the short-circuit is removed. The carbon-based PTCs wear off.

Best regards, Piotr

Reply to
Piotr Wyderski

Ixys makes a self-protecting SSR, CPC1540. Off leakage is picoamps.

If you drive the input full-time, it becomes a fuse.

Reply to
jlarkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.