Re: undervoltage lockout for flyback powersupply input

On Mon, 03 Dec 2007 04:13:36 GMT, Jamie Morken <jmorken@xxxxxxx>
wrote:

Eeyore wrote:

Jamie wrote:

Jamie Morken wrote:
I have a flyback powersupply that operates from about 20V to 450V+ (AC
or DC input) and I would like to reduce the operation to 75V to 450V,
what is a possible circuit to use on its input to keep it from starting
up until this threshold is reached?

For simplicity reasons, Get your self a DIAC rated around 75 volts
connect one end to the cathode of D34 and the other end will be
the V+ supply for R5 instead of connecting R5 directly to V+.

Neat idea actually Jamie. 30-40V diacs are more commonly available though
(I've never met a 75V diac) so perhaps 2 of those in series.

http://uk.farnell.com/jsp/search/browse.jsp?N=500004+1000013&Ntk=gensearch_001&Ntt=diac&Ntx=

Graham


Thanks for the ideas guys, also is there a way to shut off the circuit
when the voltage drops below 75V? I think the DIAC and comparator
circuits will only prevent startup until 75V, but won't disable the
circuit if the voltage drops below 75V.

---
This ought to turn it on at 75V on the way up and turn it off at 75V
on the way down. View in Courier:

..FROM +V>---+----------------+
.. |5%,2W |1%
.. [150K] [7.5M]
.. | |
.. +----------------|-----+ TO Q3 BASE
.. | | | |
.. | | 5 |
.. +--[909K]--+-----|---4|-\LT1716 C
.. | 1% | | | >1----[100K]--B 2N3904
.. |K |1% +---3|+/ 5% E
.. [LN4040] [100K] |1% 2 |
.. |10V | [100K] | |
.. | | | | |
..GND>-------+----------+-----+-----+-----------------+

At least, according to this:

Version 4
*** 1 1276 680
WIRE -32 32 -432 32
WIRE 528 32 -32 32
WIRE 576 32 528 32
WIRE -32 64 -32 32
WIRE 528 64 528 32
WIRE 96 176 -336 176
WIRE 528 208 528 144
WIRE 528 208 352 208
WIRE 96 224 96 176
WIRE -32 240 -32 144
WIRE 64 240 -32 240
WIRE 528 240 528 208
WIRE 640 240 528 240
WIRE 176 256 128 256
WIRE 288 256 256 256
WIRE -336 272 -336 176
WIRE -288 272 -336 272
WIRE -160 272 -208 272
WIRE 64 272 -160 272
WIRE 528 304 528 240
WIRE -160 336 -160 272
WIRE -32 336 -32 240
WIRE -432 352 -432 32
WIRE -336 352 -336 272
WIRE 528 384 528 368
WIRE -432 480 -432 432
WIRE -336 480 -336 432
WIRE -336 480 -432 480
WIRE -160 480 -160 416
WIRE -160 480 -336 480
WIRE -32 480 -32 416
WIRE -32 480 -160 480
WIRE 96 480 96 288
WIRE 96 480 -32 480
WIRE 352 480 352 304
WIRE 352 480 96 480
WIRE 528 480 528 448
WIRE 528 480 352 480
WIRE -432 544 -432 480
FLAG -432 544 0
FLAG 576 32 V+
SYMBOL Comparators\\LT1716 96 256 R0
WINDOW 0 33 -64 Left 0
WINDOW 3 20 -36 Left 0
SYMATTR InstName U1
SYMBOL res -48 48 R0
SYMATTR InstName R1
SYMATTR Value 7.5e6
SYMBOL res -48 320 R0
SYMATTR InstName R2
SYMATTR Value 1e5
SYMBOL voltage -336 336 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL res -176 320 R0
SYMATTR InstName R3
SYMATTR Value 1e5
SYMBOL res -192 256 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 9.09e5
SYMBOL voltage -432 336 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 450 0 1s 1s 1s 4s)
SYMBOL res 512 48 R0
SYMATTR InstName R7
SYMATTR Value 1e6
SYMBOL zener 544 368 R180
WINDOW 0 46 32 Left 0
WINDOW 3 32 -1 Left 0
SYMATTR InstName D1
SYMATTR Value BZX84C10L
SYMBOL zener 544 448 R180
WINDOW 0 47 30 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value BZX84C10L
SYMBOL npn 288 208 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res 272 240 R90
WINDOW 0 75 61 VBottom 0
WINDOW 3 83 58 VTop 0
SYMATTR InstName R5
SYMATTR Value 100k
TEXT -402 514 Left 0 !.tran 5
TEXT 640 240 Left 0 ;>TO Q3 BASE
TEXT 600 104 Left 0 ;YOUR R5
TEXT 576 376 Left 0 ;YOUR D11


--
JF
.