BJT behaviour at ridiculously low current levels

The Supertex LND150 is a 500 volt depletion fet. A source resistor will program the current, and they can apparently be strung in series without problems.

Three of them would do. They are around 30 cents each.

A spin on Piglet's circuit might to the blinker.

It might, but Piglet's circuit excels at low power. This is not what you want to discharge a capacitor. Just power a 3V3-12V zener through your DMOS at the highest possible current limited by its thermal SOA and power any blinker from that zener. Plenty of free milliwatts to burn.

Best regards, Piotr

Yeah, and a 'current-limit diode' is also a depletion FET. With a zener limit specified (I'm not sure about LND150). Multiples only share the voltage with such a matching zener specification.

A resistor would work, too, but the alarm blinkie would change frequency dramatically.

Does anyone actually sell surface mount LND150s at the moment? All the usual suppliers seem to be out of stock for at least several months.

John

Mouser has LND250s, basically the same part.

A current sink is a great way to discharge a power supply. No long exponential tail.

In series with an LED, brigntness is constant until it hits bottom and winks out. My HV blinker would be similar, constant blink rate all the way down.

I'd probably be happier with a gate bias string to force the voltages to equalize. Of course the resistors would have to be physically quite large to take the voltage.

Cheers

Phil Hobbs

Thank you - John

TO-92s are good for the soul.

Cheers

Phil Hobbs

(Who just designed one into a supply-dictated board spin.) :(

Maybe I should create a composite SOT-23/TO-92 footprint "just in case". John

Rarely! But we're using a TO-92 LM35 to measure the temp of a CPU cooler that we're using as a mosfet heat sink. We'll glue it to the side of the cooler and plug it into three little pin sockets.

Are TO-92s easier to get than surfmount parts? At least it's an option.

Somebody told me (was it you?) that they are OK in series. One or two may avalanche somewhere above 500 volts and one will actually regulate the current. I'm talking 50 or 100 uA in my case.

We have 2500 in stock. I could spare a few to anyone who wants to play with them.

Just put them in parallel on the schematic.

Some of them.

Cheers

Phil Hobbs

I've never used them above about 250V.

Cheers

Phil Hobbs

I tested one. It avalanches at 620 volts, no damage at low current.

The gate zeners at about +-40, again no damage.

You are very welcome, always pleased when a dirt cheap descrete circuit still beats the world. It is not my design, the complementary series multivibrator has been around since the 1960s.

piglet

My preference would be to skip the depletion fets and just use dropper resistors - cheaper, more reliable, and best of all the non constant blink rate gives a good visual confirmation that the HV capacitor is actually discharging.

Blinking could be as simple as a diac or bjt used as negistor. Spice won't model E-B breakdown and reverse beta so can't simulate negistor operation but breadboarding would be very quick.

piglet

This is how it looks now. L1 and L3 pump the flyback core, L3 boosts the voltage of the core reset pulse to 600mV+, even at 1.2V_IN, R5 steals that pulse (or not), Q3 detects if there was the pulse, C3/Q4 is a zero-power monostable (assuming there are no pulses from Q3) and M1 makes that information accessible to the external world. It is shocking how simple it is and how well it works.

Best regards, Piotr

Version 4 SHEET 1 2136 1236 WIRE 416 -176 176 -176 WIRE 720 -176 416 -176 WIRE 1264 -176 720 -176 WIRE 1648 -176 1264 -176 WIRE 720 -128 720 -176 WIRE 416 -80 416 -176 WIRE 720 48 720 -48 WIRE 960 48 720 48 WIRE 960 64 960 48 WIRE 720 96 720 48 WIRE 416 144 416 0 WIRE 656 144 416 144 WIRE 960 160 960 144 WIRE 416 224 416 144 WIRE 720 272 720 192 WIRE 960 320 960 224 WIRE 960 320 784 320 WIRE 960 384 960 320 WIRE 176 528 176 -176 WIRE 416 576 416 288 WIRE 528 576 416 576 WIRE 2080 608 1904 608 WIRE 416 624 416 576 WIRE 528 624 528 576 WIRE 720 640 720 368 WIRE 2080 656 2080 608 WIRE 1904 736 1904 608 WIRE 176 752 176 608 WIRE 416 752 416 704 WIRE 416 752 176 752 WIRE 720 752 720 720 WIRE 720 752 416 752 WIRE 960 752 960 464 WIRE 960 752 720 752 WIRE 1264 768 1264 -176 WIRE 2080 768 2080 736 WIRE 176 784 176 752 WIRE 1648 816 1648 -176 WIRE 1264 864 1264 848 WIRE 1392 864 1392 784 WIRE 1392 864 1264 864 WIRE 1472 864 1392 864 WIRE 1584 864 1552 864 WIRE 1264 880 1264 864 WIRE 416 896 272 896 WIRE 1392 896 1392 864 WIRE 272 928 272 896 WIRE 528 928 528 704 WIRE 1056 928 528 928 WIRE 1200 928 1136 928 WIRE 1648 928 1648 912 WIRE 1776 928 1648 928 WIRE 1904 928 1904 816 WIRE 1648 944 1648 928 WIRE 1392 976 1392 960 WIRE 1072 992 1072 976 WIRE 416 1008 416 976 WIRE 1776 1008 1776 928 WIRE 1856 1008 1776 1008 WIRE 1264 1024 1264 976 WIRE 1120 1040 1120 976 WIRE 1648 1040 1648 1024 WIRE 1904 1040 1904 1024 WIRE 1120 1136 1120 1120 FLAG 176 784 0 FLAG 416 1008 0 FLAG 272 1040 0 FLAG 1264 1024 0 FLAG 1392 784 V_OUT FLAG 1648 1040 0 FLAG 1392 976 0 FLAG 1072 992 0 FLAG 1120 1136 0 FLAG 2080 768 0 FLAG 1904 1040 0 SYMBOL voltage 176 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 3 SYMBOL pnp 784 368 R180 SYMATTR InstName Q1 SYMATTR Value BC557C SYMBOL npn 656 96 R0 SYMATTR InstName Q2 SYMATTR Value BC547C SYMBOL res 704 -144 R0 SYMATTR InstName R1 SYMATTR Value 10Meg SYMBOL res 944 368 R0 SYMATTR InstName R2 SYMATTR Value {R_OSC} SYMBOL res 400 -96 R0 SYMATTR InstName R3 SYMATTR Value {R_OSC} SYMBOL cap 400 224 R0 SYMATTR InstName C1 SYMATTR Value {C_OSC} SYMBOL cap 944 160 R0 SYMATTR InstName C2 SYMATTR Value {C_OSC} SYMBOL res 704 624 R0 SYMATTR InstName R4 SYMATTR Value 1m SYMBOL ind2 400 608 R0 WINDOW 0 -42 23 Left 2 WINDOW 3 -102 64 Left 2 SYMATTR InstName L1 SYMATTR Value {L_PRI} SYMATTR Type ind SYMATTR SpiceLine Cpar=0 SYMBOL ind2 400 880 R0 WINDOW 3 -55 -7 Left 2 SYMATTR Value {L_SENSE} SYMATTR InstName L2 SYMATTR Type ind SYMBOL ind2 544 720 R180 WINDOW 0 -41 86 Left 2 WINDOW 3 -111 59 Left 2 SYMATTR InstName L3 SYMATTR Value {L_SEC} SYMATTR Type ind SYMATTR SpiceLine Cpar=0 SYMBOL res 256 912 R0 SYMATTR InstName R5 SYMATTR Value 33 SYMBOL ind2 944 48 R0 SYMATTR InstName L4 SYMATTR Value {L_PRI} SYMATTR Type ind SYMBOL npn 1200 880 R0 SYMATTR InstName Q3 SYMATTR Value BC547C SYMBOL res 1248 752 R0 SYMATTR InstName R6 SYMATTR Value 10Meg SYMBOL pnp 1584 912 M180 SYMATTR InstName Q4 SYMATTR Value BC557C SYMBOL res 1568 848 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 1Meg SYMBOL res 1632 928 R0 WINDOW 0 -56 28 Left 2 WINDOW 3 -91 53 Left 2 SYMATTR InstName R8 SYMATTR Value 10Meg SYMBOL cap 1376 896 R0 SYMATTR InstName C3 SYMATTR Value 33n SYMBOL sw 1040 928 R270 SYMATTR InstName S1 SYMATTR Value MYSW SYMBOL voltage 1120 1024 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 1 300m 10n 10n 300m 1) SYMBOL res 1888 720 R0 SYMATTR InstName R10 SYMATTR Value 1000 SYMBOL voltage 2080 640 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 12 SYMBOL nmos 1856 928 R0 SYMATTR InstName M1 SYMATTR Value BSS123 TEXT 216 -368 Left 2 !.param C_OSC=470p TEXT 216 -344 Left 2 !.param R_OSC=100Meg TEXT 560 -424 Left 2 !.param AL=5000n TEXT 560 -400 Left 2 !.param N_PRI=26 TEXT 560 -376 Left 2 !.param N_SEC={N_PRI*4} TEXT 560 -352 Left 2 !.param N_SENSE=8 TEXT 560 -320 Left 2 !.param L_PRI={N_PRI*N_PRI*AL} TEXT 560 -296 Left 2 !.param L_SEC={N_SEC*N_SEC*AL} TEXT 560 -272 Left 2 !.param L_SENSE={N_SENSE*N_SENSE*AL} TEXT 240 800 Left 2 !K L1 L2 L3 L4 0.995 TEXT 144 1048 Left 2 !.tran 1 TEXT 912 -376 Left 2 ;PRI/SEC: twisted 6x26 DNE0.2mm @ 10mm OD ring F938. TEXT 912 -352 Left 2 ;SENSE: TIW!!! TEXT 1376 1048 Left 2 !.ic V(V_OUT)=0 TEXT 1008 1192 Left 2 !.model MYSW SW(Ron=1m Roff=1G Vt=.5 Vh=-.4)