Question about zener diode circuit - Page 1
Forum section: electronics / beginners

author: laejf
author_rank: Regular Contributor
timestamp: September 19, 2022, 09:05:34 am
content: Hi,I'm wondering if anyone can help me understand the circuit below. It says the LED turns on when Vcc has reached 45V. Why is that?

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 19, 2022, 11:07:00 am
content: Dza 43V opens at 43 V. LED lights at 2V across it. 43+2 is 45 V.Everything except Dza, Dzb is a current source for a LED.Dzb is a protection from overvoltage for current source and LED.

author: laejf
author_rank: Regular Contributor
timestamp: September 19, 2022, 11:19:59 am
content: So they don't mean Vcc is 45V since it should be higher?

author: strawberry
author_rank: Super Contributor
timestamp: September 19, 2022, 12:47:14 pm
content: as long as R1 Dza Dzb can surviveVCC 75V max limited by R1 1W0.03A * 75V = 2.25Wwith JFET could be x3 more efficient

author: mikerj
author_rank: Super Contributor
timestamp: September 19, 2022, 01:14:54 pm
content: Quote from: laejf on September 19, 2022, 11:19:59 amVcc is obviously variable, this circuit is designed to maintain a constant LED current over a range of input voltage. However the LED will not light until at least 45v is present on Vcc. The the constant current circuit will not be operating until Dzc starts conducting at around Vcc=48v.

author: laejf
author_rank: Regular Contributor
timestamp: September 19, 2022, 01:18:57 pm
content: How did you arrive at 48V?

author: laejf
author_rank: Regular Contributor
timestamp: September 19, 2022, 01:19:34 pm
content: I would also like to scale this to work at a Vcc of 18V. Any idea how to do that?

author: ledtester
author_rank: Super Contributor
timestamp: September 19, 2022, 03:07:56 pm
content: Perhaps this Falstad simulation is of some value:https://tinyurl.com/2eck2b5eCircuit also saved as a text file below which you can load usinghttps://falstad.com/circuit/circuitjs.htmlPlay around with the Vcc slider on the right hand side. LED current is plotted on the bottom. You'll see the current is limited to around 8.5mA which is reached at Vcc = 60V.

author: strawberry
author_rank: Super Contributor
timestamp: September 19, 2022, 04:00:34 pm
content: simpler circuit

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2022, 05:01:03 pm
content: Quote from: strawberry on September 19, 2022, 04:00:34 pmBoth are non-functional:The Zener diodes are not connected as Zeners (I presume "ZD" means Zeners, but I may be wrong). In fact, I don't know why they're there at all.The first circuit is dangerous, there's a very big risk that the emitter-follower will oscillate at several MHz.The second will probably work, but the 1N4148 are very "soft" and will only have forward voltage of aound 0.4 V, but with a wide spread.

author: ledtester
author_rank: Super Contributor
timestamp: September 19, 2022, 06:13:27 pm
content: Quote from: strawberry on September 19, 2022, 04:00:34 pmAbout the RHS circuit...The formula for I isn't correct -- it should be I = (Vz-0.7)/R where Vz is the Zener voltage of the LHS Zener. Often you'll see this circuit where the that Zener has been replaced by two regular diodes in series (forward biased). This effectively makes Vz = 1.4V and so the formula I=0.7/R holds.There's more commentary on that circuit here:https://www.eevblog.com/forum/beginners/npn-constant-current-source-with-different-transistors/

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2022, 07:53:30 pm
content: Quote from: strawberry on September 19, 2022, 04:00:34 pmWell, you've very quietly (almost secretly) corrected the ZD orientation and replaced the 1N4148s with a Zener (it's normal and polite here to insert an "EDIT" note to say you've changed something).But their function is still a mystery. Why are they there?I attach a circuit that actually works. It will also work with just one LED, but transistor power dissipation will increase. For low-current LEDs (say, 20 mA), that's not an issue. No Zeners in the collector needed.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 19, 2022, 08:38:56 pm
content: Quote from: laejf on September 19, 2022, 01:19:34 pmUse 15-16VDC rated Zeners at Vza.

author: strawberry
author_rank: Super Contributor
timestamp: September 19, 2022, 10:43:37 pm
content: better circuit

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2022, 11:06:12 pm
content: Quote from: strawberry on September 19, 2022, 10:43:37 pmYes, much better.

author: laejf
author_rank: Regular Contributor
timestamp: September 20, 2022, 06:51:19 am
content: Quote from: Vovk_Z on September 19, 2022, 08:38:56 pmI've been trying all sorts of values on all the zeners and resistors in LTspice, but I cannot get it to work.

author: strawberry
author_rank: Super Contributor
timestamp: September 20, 2022, 08:14:11 am
content: because Dzc is 5.1V

author: mikerj
author_rank: Super Contributor
timestamp: September 20, 2022, 08:28:17 am
content: Quote from: laejf on September 19, 2022, 01:18:57 pm(V)Dza+(V)Dzb = 43+5.1 = 48.1. In practice Zener's always start conducting below their rated voltage so there won't be a precisely defined threshold where the circuit moves into constant current.

author: laejf
author_rank: Regular Contributor
timestamp: September 20, 2022, 08:44:30 am
content: Quote from: mikerj on September 20, 2022, 08:28:17 amI'm guessing you mean Dzc. Dzb is 10V. and can you ignore the voltage drop across R2?

author: laejf
author_rank: Regular Contributor
timestamp: September 20, 2022, 08:47:54 am
content: I've played around with different values with imported models of BZT52C diodes and a LED of 3V and got the result below. I added some 'ripple' on the input voltage and the current through the LED seems to stay constant.Any feedback on this?

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: September 20, 2022, 09:11:59 am
content: Quote from: laejf on September 19, 2022, 01:18:57 pmWhat cannot be ignored is the voltage drop across R1, which is approximately 9v. So the value would be close to 57v.

author: MrAl
author_rank: Super Contributor
timestamp: September 20, 2022, 12:30:12 pm
content: Quote from: laejf on September 19, 2022, 09:05:34 amHi,Looks to me like somebody had a sale on zener diodes (ha ha).No current flows before roughly 45 or so volts because DZA drops 43 volts.The actual turn on voltage may vary somewhat though because with 45v in the most even current would be around 2ma which isnt that much for LEDs unless they are high brightness.

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2022, 02:38:13 pm
content: Quote from: laejf on September 19, 2022, 09:05:34 amYou have to read the text correctly:"The LEDfirst begins to lightwhen the positive supply rail voltage exceeds 45 V." (my emphasis)This means that the LED lights a teensy-weensy bit when the voltage is higher than VDZA+ VLED. Probably almost imperceptible.The transistor starts conducting when supply voltage is over 43 V; VCE(sat)is negligible.

author: strawberry
author_rank: Super Contributor
timestamp: September 20, 2022, 04:12:12 pm
content: works

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2022, 06:34:52 pm
content: works.But makes the Zener sales guy unhappy. Sim trace is LED current vs a supply voltage sweep of 0...20 V.