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Zener kinda turns on

P

panfilero

Jan 1, 1970
0
I was making a circuit that when you power it up with 5V a 5V LED indicatorturns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

But... it's not coming on with 5V...

I spiced it, and it doesn't work in simulation either...

if you look at the image on the link

I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a5V supply (as it should be) and the circuit works at 12V... but not at 5V....

link is below

http://vampiresquidlabs.com/cms/temp/


much thanks!
 
G

George Herold

Jan 1, 1970
0
I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12Vindicator goes on.

But... it's not coming on with 5V...

I spiced it, and it doesn't work in simulation either...

if you look at the image on the link

I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LEDlight up... I'm basically measuring a little over 1V at the gate.

I think I can remedy this problem by changing R2 to a lower value... like1k maybe, but why is D1 turning on at all?  Is it leakage current that'scausing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

link is below

http://vampiresquidlabs.com/cms/temp/

much thanks!

That 12V zener is not doing very much...

But look at an I-V curve for the 5.6V zener, I'd guess there's still
some current flow at 5.0V. If the FET gate is at 1 V then you have
~1uA of current flowing through the zener.

George H.
 
R

Robert Baer

Jan 1, 1970
0
panfilero said:
I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

But... it's not coming on with 5V...

I spiced it, and it doesn't work in simulation either...

if you look at the image on the link

I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

link is below

http://vampiresquidlabs.com/cms/temp/


much thanks!
Err...it takes about 4V to turn on a "typical" LED, so use of a 5V
(or even 3V) zener will guarantee no light from the LED.
 
P

Phil Allison

Jan 1, 1970
0
"Robert Baer"
Err...it takes about 4V to turn on a "typical" LED, so use of a 5V (or
even 3V) zener will guarantee no light from the LED.



** D4 is the 5V LED, D3 is the 12V LED.

The dopey OP has failed to allow for leakage current in the 5.6 V zener and
is trolling us as usual.


.... Phil
 
M

mike

Jan 1, 1970
0
A wee bit overkill...

http://www.analog-innovations.com/SED/Panfilero_LED_5V_12V_Question_SED.pdf

but Perfectionist 'R' Us>:-}

...Jim Thompson

Here' in the trailer park, we'd have just painted the 5V power plug red
and the 12V one green.

Over in town, they'd have used a dual-color LED, put a zener in series
with the red one and called it green and orange.

Or use rectangular LEDS and call it a bar graph. Probably works better
for the color-blind.

I've hosted a bunch of design reviews.
Seen a lot of "solutions" that were WAY out of proportion to the root
problem being addressed. Often comes under the heading
"dimininishing returns". ;-)


KISS 'R' US...
 
G

George Herold

Jan 1, 1970
0

OK I'm most likely just slow this morning, but how does that turn off
the one LED when the transistor turns on? And doesn't it turn on the
first LED at ~ 2volts or so. How about if you put a second LED in the
leg without the transistor? Then it would turn on at ~4 volts and
turn off with current through the transistor.

I like Jan's circuit.

George H.
 
J

Jamie

Jan 1, 1970
0
George said:
OK I'm most likely just slow this morning, but how does that turn off
the one LED when the transistor turns on? And doesn't it turn on the
first LED at ~ 2volts or so. How about if you put a second LED in the
leg without the transistor? Then it would turn on at ~4 volts and
turn off with current through the transistor.

I like Jan's circuit.

George H.
Different color LEDs have different voltages. So the one with the
transistor in line will operate a lower voltage led, this will then
remove forward current from the higher voltage LED..

THat is the way I see it, could be doing something different, who
knows.

Also, if you put a diode in series with one LED, and then T-bone
that node to another LED and then to a switch (transistor for example),
that switch should cause a voltage drop to where the in series diode LED
node will lose the required voltage..

Just a thought..
Jamie
 
G

George Herold

Jan 1, 1970
0
If it were a BJT, the +12V LED wouldn't ever turn off.

Ahh thanks,
There's no-way I'm playing crossword puzzle component design with
J.L.
He'll kick my butt every time. I'm a 'class A' brute force kinda
guy.

George H.
 
G

George Herold

Jan 1, 1970
0
//dl.dropboxusercontent.com/u/53724080/Optos/LED_5_12s.JPG[/url]
A & B waste current, conducting 2x LED current at 12V. C is wishful, let's see some numbers.
I really like "A" for its elegant symmetry.
This takes 6 parts, but it's power efficient, as is Jan's circuit, with a little
tweaking.
https://dl.dropboxusercontent.com/u/53724080/SED/LEDs_5_12.JPG
That works, dunno why you need a FET though.
If it were a BJT, the +12V LED wouldn't ever turn off.

It might not turn off anyway, given Vgs(abs max) is 8V.

I kinda like Jan's idea, here is an "over easy" mod of it.

http://www.speff.com/LED_thing.jpg- Hide quoted text -

- Show quoted text -

Nice, reminds me of a more spendy LED indicator circuit that I use
for control loops, I drive two leds to show when the 'control
parameter' is above or below the set point. Both turn off at 'zero'
error.

George H.
 
F

Fred Abse

Jan 1, 1970
0
Change your zener for a 7.5V one (which being an avalanche one will be
much 'stiffer'), lose R3, and as you said make R2 10K to be safer and
you're done.

You don't need D2.

Only downside is deadband between 8V and 9V, where neither LED lights.

This simulation shows (Monte-Carlo'd for 5% tolerance zeners and 1%
resistors):


Version 4
SHEET 1 880 704
WIRE -384 48 -672 48
WIRE -96 48 -384 48
WIRE 112 48 -16 48
WIRE 304 48 112 48
WIRE -384 128 -384 48
WIRE 112 208 112 48
WIRE 304 256 304 48
WIRE -672 288 -672 48
WIRE -384 288 -384 192
WIRE -64 288 -384 288
WIRE 64 288 -64 288
WIRE -384 352 -384 288
WIRE -64 400 -64 288
WIRE -384 464 -384 432
WIRE -672 624 -672 368
WIRE -384 624 -384 528
WIRE -384 624 -672 624
WIRE -192 624 -384 624
WIRE -64 624 -64 480
WIRE -64 624 -192 624
WIRE 112 624 112 304
WIRE 112 624 -64 624
WIRE 304 624 304 320
WIRE 304 624 112 624
WIRE -192 688 -192 624
FLAG -192 688 0
SYMBOL LED 288 256 R0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
SYMBOL nmos 64 208 R0
SYMATTR InstName M1
SYMATTR Value AO6408
SYMBOL res 0 32 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value {mc,2.2k,tolR}
SYMBOL res -80 384 R0
SYMATTR InstName R2
SYMATTR Value {mc,10k,tolR}
SYMBOL LED -400 464 R0
SYMATTR InstName D3
SYMATTR Value NSPW500BS
SYMBOL res -400 336 R0
SYMATTR InstName R4
SYMATTR Value {mc,1.2k,tolR}
SYMBOL zener -368 192 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D4
SYMATTR Value BZX84C7V5
SYMBOL voltage -672 272 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
TEXT -440 656 Left 2 !.dc V1 0 12 10m
TEXT 352 392 Left 2 !.MODEL BZX84C12V D\n+ IS=2.6665E-18\n+ N=.82284\n+ RS=.51617\n+ IKF=11.760E-3\n+ CJO=48.079E-12\n+ M=.32805\n+ VJ=.62172\n+ ISR=47.692E-12\n+ BV={mc,12.119,tolZ}\n+ IBV=.54544\n+ TT=825.22E-9
TEXT 352 40 Left 2 !.MODEL BZX84C7V5 D\n+ IS=2.6665E-18\n+ N=.82284\n+ RS=.51617\n+ IKF=11.760E-3\n+ CJO=63.513E-12\n+ M=.33559\n+ VJ=.66795\n+ ISR=1.1222E-9\n+ BV={mc,7.6329,tolZ}\n+ IBV=.94329\n+ TT=2.7411E-6
TEXT -24 656 Left 2 !.param tolZ=.05 ; +/- 5% zener tolerance
TEXT -24 688 Left 2 !.step param X 0 20 1 ;dummy
TEXT -24 672 Left 2 !.param tolR=.01 ;resistor tolerance
TEXT -256 8 Left 2 ;plot I(D1), I(D3)
 
F

Fred Abse

Jan 1, 1970
0
Full listing at...


Sorry! Looks like a sloppy solution :-(

Not my solution, just an MC analysis of Fred Bartoli's solution.

Not as elegant, or precise as yours, but cheap.
 
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