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Zener voltage limiter without voltage drop to the load

e:

drop as long as it is less than 12V and remain at 12V when the input voltag= e raises above 12V. Will the above circuit do the job?=20

in between the input and load before the zener. I cannot have that topology= as limiting resistor between input and load drops the voltage to load.=20

t.net ---

That's not a bad idea, can be expanded somewhat to something like this chea= p:

Please view in a fixed-width font such as Courier.

. . . . 8V-18V -------+----------+------+----. . | | | | . [560] | | | . | |< | | . +--------| T1 | | . | |\ | | . | | |< | . [270] +----| T2 [68] . | | |\ | . | | | | . | | | | . |+ [2.7K] +----+ . ---/ | | | . 1N4742A // \ 12V | | | + . --- zener | --- ) 12V . |- 1W | / \ ) 275R . |- | --- ) relay . | | | | coil . | | | | - . +----------+------+----+ . | . | . D . ON/OFF ---- G NUD3124 . S . | . | . | . --- . /// .

Version 4 SHEET 1 880 680 WIRE 32 -176 -192 -176 WIRE 192 -176 32 -176 WIRE 352 -176 192 -176 WIRE 496 -176 352 -176 WIRE 32 -128 32 -176 WIRE 192 -64 192 -176 WIRE 32 -16 32 -48 WIRE 128 -16 32 -16 WIRE 352 -16 352 -176 WIRE 496 0 496 -176 WIRE 32 16 32 -16 WIRE 240 32 192 32 WIRE 288 32 240 32 WIRE -192 80 -192 -176 WIRE 352 96 352 80 WIRE 496 96 496 80 WIRE 496 96 352 96 WIRE 240 128 240 32 WIRE 496 128 496 96 WIRE 32 144 32 96 WIRE 352 144 352 96 WIRE -192 272 -192 160 WIRE 32 272 32 208 WIRE 32 272 -192 272 WIRE 240 272 240 208 WIRE 240 272 32 272 WIRE 352 272 352 208 WIRE 352 272 240 272 WIRE 496 272 496 208 WIRE 496 272 352 272 WIRE -192 320 -192 272 FLAG -192 320 0 SYMBOL pnp 128 32 M180 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL pnp 288 80 M180 SYMATTR InstName Q2 SYMATTR Value 2N3906 SYMBOL res 256 224 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R1 SYMATTR Value 2700 SYMBOL res 16 -144 R0 SYMATTR InstName R2 SYMATTR Value 560 SYMBOL res 16 0 R0 SYMATTR InstName R3 SYMATTR Value 270 SYMBOL res 480 -16 R0 SYMATTR InstName R4 SYMATTR Value 68 SYMBOL res 480 112 R0 SYMATTR InstName R5 SYMATTR Value 275 SYMBOL voltage -192 64 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 0 SYMBOL zener 48 208 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value BZX84C12L SYMATTR Description Diode SYMATTR Type diode SYMBOL diode 368 208 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value MUR460 TEXT -408 168 Left 2 !.dc V1 8 18 0.1

Reply to
bloggs.fredbloggs.fred
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as it is less than 12V and remain at 12V when the input voltage raises above

12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

--
Sounds like a lot of bother rather than just going for a 5V relay and
regulating the 8->12V down to ~ 5V, like this:

.                 2N2222
.Vcc>-----+-------C   E---+--------+
.         |R1       B     |        |
.      [200R]       |     |        |
.         |         |     |        |
.         +---------+     |        |K
.         |K        | [5V COIL] [DIODE]
.      [Z6V2]       |     |        |
.         |         |     |        |
.__       |         C     |        |
.ON/OFF>--|-[2K2]-B 2N2222|        |
.         |         E     |        |
.         |         |     |        |
.GND>-----+---------+-----+--------+

The Zener's Izt and the value of R1 might have to be changed if
there's not enough drive when Vcc = 8V, but then that's just
engineering. ;)
Reply to
John Fields

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

--
Oops...

Version 4
SHEET 1 880 680
WIRE -112 32 -224 32
WIRE 192 32 -112 32
WIRE 368 32 288 32
WIRE 528 32 368 32
WIRE -112 64 -112 32
WIRE 368 112 368 32
WIRE -112 176 -112 144
WIRE 240 176 240 96
WIRE 240 176 -112 176
WIRE 528 208 528 32
WIRE -112 224 -112 176
WIRE -224 240 -224 32
WIRE 240 304 240 176
WIRE 368 320 368 192
WIRE 48 352 -16 352
WIRE 176 352 128 352
WIRE -16 400 -16 352
WIRE -224 528 -224 320
WIRE -112 528 -112 288
WIRE -112 528 -224 528
WIRE -16 528 -16 480
WIRE -16 528 -112 528
WIRE 240 528 240 400
WIRE 240 528 -16 528
WIRE 368 528 368 400
WIRE 368 528 240 528
WIRE 528 528 528 272
WIRE 528 528 368 528
WIRE -224 608 -224 528
FLAG -224 608 0
SYMBOL npn 192 96 R270
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL npn 176 304 R0
SYMATTR InstName Q2
SYMATTR Value 2N2222
SYMBOL res -128 48 R0
SYMATTR InstName R1
SYMATTR Value 300
SYMBOL zener -96 288 R180
WINDOW 0 -38 33 Left 2
WINDOW 3 -134 66 Left 2
SYMATTR InstName D1
SYMATTR Value BZX84C6V2L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL ind 352 96 R0
SYMATTR InstName L1
SYMATTR Value 1
SYMBOL diode 544 272 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL voltage -224 224 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(7.5 12 0 1s)
SYMATTR InstName V1
SYMBOL voltage -16 384 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(0 5 0 10u 10u .1 .2 4.5)
SYMATTR InstName V2
SYMBOL res 144 336 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 2200
SYMBOL res 352 304 R0
SYMATTR InstName R3
SYMATTR Value 50
TEXT -200 560 Left 2 !.tran 0 2 0 1u startup uic
Reply to
John Fields

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

--
On the de-energize side, perhaps, but trying to energize the relay by
dumping energy into its coil from another inductor is going to be
problematic, I think.
Reply to
John Fields

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

Not really, you just need a few hundred mH but at very modest current. Could be done with a cap as well but I try to avoid electrolytics wherever I can. Or use a simple form of a bosst switcher, that avoids the large inductance.

[...] 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

Sure, regulating down is easier. Can also be done with the depletion mode FET.

But it comes with a dissipation penalty. Might make sense to go to a 9V coil in this case. 9V is one of the standard coil voltages.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

Am I the only one who thinks wasting 1.6W in R1 constitutes a design flaw?

Reply to
mike

Yep. You're off by an order of magnitude.

--
Using Opera's revolutionary email client: http://www.opera.com/mail/
Reply to
David Eather

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

--
Probably.

When the relay's on, If Vcc = 12V and the cathode of the Zener is at
6.2V, then the voltage across R1 will be Vcc - 6.2V = 5.8V, and the
power it'll dissipate will be 5.8V²/200R = 0.168 watts

When it's off, with essentially 12V across R1, it'll dissipate about
12V²/200R = 0.72 watts.
Reply to
John Fields

alid&gt;

85160849573218

y shouldn&#39;t drop as long as it is less than 12V and remain at 12V when = the input voltage raises above 12V. Will the above circuit do the job?

which can

tage source

miting resistor in between the input and load before the zener. I cannot ha= ve that topology as limiting resistor between input and load drops the volt= age to load.

e energized when

uot; that produces a

12V relays

y and

if

s just

flaw?

t

out

But the 2N2222 is pushing 1.4W.

Reply to
bloggs.fredbloggs.fred

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

Can't argue with your math. But I will challenge your interpretation of the specification.

"Input voltage varies from 8-18V."

18^2/200 = 1.62W

If the voltage were limited to 12V this whole thread would be (even more) superfluous.

Reply to
mike

Joerg&lt; snipped-for-privacy@>> &gt;&gt;

formatting link

shouldn&#39;t drop as long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

which can

source

limiting resistor in between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

energized when

that produces a

relays

--
If Vcc is 12V and the voltage into the relay is about 5V, then the
drop across the transistor will be about 7V at 100 mA, which is 700mW.

A 2N2222 is rated for 625mW, so a little judicious trimming here and
there should make the circuit realizable. 

If not, the topology is still valid and there _are_  alternatives to
the 2N2222.
Reply to
John Fields

long as it is less than 12V and remain at 12V when the input voltage raises above 12V. Will the above circuit do the job?

between the input and load before the zener. I cannot have that topology as limiting resistor between input and load drops the voltage to load.

--
Aarghhh...

I misread read 8->16V as 8->12V

MFC...
Reply to
John Fields

My guess is its automotive app and input is 13.8 nominal-all other values are transient. All the circuits get blown to bits by a 60V load dump- except the TIP41C ckt of course.

Reply to
bloggs.fredbloggs.fred

I have an entry for the silly circuits contest.

Tie the relay to +VCC and thru a N-channel FET to ground. Diode across the relay. Drive the fet with the PWM output of a single-chip microcontroller. Use the A/D in the uC to control the pwm duty factor to set the relay current as a function of VCC. Can do the math or a simple lookup table. It's math, so no feedback required. Need a third pin to turn it on/off.

That'll do as well as any of the other circuits that try to make a 12V relay work at 8V. And should you decide that running a relay at rated voltage is a good idea, you can substitute a

Reply to
mike

On Wednesday, July 11, 2012 12:24:56 AM UTC-4, mike wrote: snipped immaturity...

You're just not getting it. The 8-18V range in the OP was a giveaway that t= his is an automotive application. The 8V "used" to be the standard for temp= orary system low voltage during cold crank starting through which a circuit= was supposed to work without disruption for so many seconds (10?), and the= 18V was the upper limit of continuous operation due to a failed voltage re= gulator. It is not necessary that that the circuit be able to pull-in a rel= ay during the 8V cold crank, but it is necessary that the relay not drop ou= t if it is already pulled-in. There is no chance an automotive relay will d= rop-out at 8V, even at the highest temperature extreme.

Reply to
bloggs.fredbloggs.fred

Lighten up! I posted a simple, cheap, elegant solution to the problem as presented..OK, OK the thermistor one was silly.

is an automotive application.

The OP never returned, so we don't know his intentions. YOU jumped to the conclusion that it's an automotive application. YOU jumped to the conclusion that an automotive relay was being used. You're berating me for taking a cautious approach to an ill-defined problem.

Based on the admittedly small sample of 4 12V automotive relays I have in my hand, 275 ohms is about 4x too high for coil resistance.

One of the recurrent themes in newsgroups is lack of empathy. People act more like angry traffic cops than educators. They make unwarranted assumptions and proclaim solutions with authority!

I try to put myself in the mindset of the person asking the question. When a question looks like it's about the first thing a newbie thought of as a solution, the answer to the question they didn't ask might be more important than a proclamation of a "shoot from the hip" answer to a wrongly interpreted question.

Most newsgroup questions are not about solving a problem. The problem is rarely stated. More often it's about tweaking some difficult part of an already determined solution. Many times, I've seen engineers throw money and time at some difficult implementation when just talking to the guy in the next cubicle and repartitioning the system, would make the problem just go away.

Many times, I've seen recommendations that, while useful in the hands of a skilled practitioner with the right equipment, would put a newbie in harm's way.

I don't have a solution for the newsgroup problem. I've tried posting detailed info to focus my question only to have people completely ignore it and start firing irrelevant info from the hip. But I digress...

So, If I'm building one or ten systems where the consequences of failure are small, I wouldn't have any problem characterizing ten 12V relays and using them at 8V. If it's a hundred thousand automotive brake control systems, that's entirely another matter.

In this case, we just don't know. Conservative recommendations are in order.

It would be very interesting to attend one of your design reviews.

Reply to
mike

that this is an automotive application.

problem.

I already linked to a line of Panasonic compact automotive relays that are in this 1/2W coil power category..

If you really want a simple fix then change to a line of relays that can handle continuous 18V. Tyco can . end of problem.

Reply to
bloggs.fredbloggs.fred

as long as it is less than 12V and remain at 12V when the input voltage rai= ses above 12V. Will the above circuit do the job?=20

tween the input and load before the zener. I cannot have that topology as l= imiting resistor between input and load drops the voltage to load.=20

My apologies for being away after posting. Its an automotive application bu= t the relay is not automotive grade and this unit is going to be part of la= rger infotainment system. The relay manufacturer says the relay can withsta= nd sustained 14V. Coil Pick up voltage is 9.5V. The relay has 275 ohm coil= .

Thanks bhav

Reply to
bhavanireddy

The question would be: Does it have to pull in at 8V? That's a voltage that you normally only see during engine start or when a battery is seriously depleted. Normally nothing should come on during those times.

You could use the trick like with the depletion mode FET to limit it to

12-13V. It provides the least in dissipation which is always a concern in cars. But for the rest of the electronics be careful with that 18V spec. Load dump spikes in cars can easily reach 60V. Sometimes more. 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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