Why is my Falstad circuit not properly working?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 07:04:51 pm
content: A bit of a followup to the following idea (essentially converting a multimeter from using a 9v battery to 2s lithium cells)Quote from: LooseJunkHater on September 12, 2023, 02:28:23 pmCircuit in question:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWK0BMBmAnADiwdgShpgGy6TYohq4hIKS0CmAtGGAFDUNgaWvEMUCYiH7coUdgHMQAFjAjmGEfMVYGG9gCVwKLKIXc9B3CI1yGWaLIkboCdgDM5C0cpeLDVNOHvJIKOwA7rr6YuC8BgKSaDYAXowAdowATszEftzQuGCyaARYPHpg8CgsNIIgGUiQ7HERlK5ForL6lAnJadWc8UmpzD4MGNBoZawQMJCylGjZPhPsCo2RTZHENu19aYPgtrAcMs3rDSDqtuwpJ+HN45QM9LWXowzhzwZgd7Rw0lQoDMdvM6aS5CTzRN7hbhkThkX4vNgzP4tfQTGKwm4Iq6tXaaAAOWP07mY2J2tXxEOxeU+pPY5KRr3pKBQNhp+KpUQY7OYTJZ51k5Cq-ORpzgwsG0DM-nQJXgsrllECOlw9LOype7nMQpKtmQDkuWFGgtOhsx3CcIpe2INfBJjUysECMmt4A+xsaxk0IWdA0ozuOtRkaqiICD5lqIXWL0NkaN4bdRpj-uCckN4W9XlqYFwNmE8NdudEJvAsuQ2AQMrlsogWkYAGcAJa1gAuAENEgBjRjaWjGW60V19zXiMP2ZMIUyiY4x9I2AO0aL8edhtGWVNeb3HVGPJfvSgFXcSMDqeA9EAdfr6LI5PIFIqFUrlCS+yT1N6uCltM9bZhYRbERHcCIbzHJsnQDIeeyQBwIQQl4-LwtEtRAAAttached image shows the purpose of each part. 8.4v is derived from an XL6007 boost converter, which receives power from a standard 5v USB port. The 2s battery will have a BMS.Problem: In falstad, I'm wanting the LED circled inorangeto light up when 8v+ is reached. However, I can't get the LED to light; what am I doing wrong?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 07:08:07 pm
content: (In case the image isn't clear enough, everything within the red circle will not exist in the IRL circuit; it just allows me to simulate a charging battery within Falstad, because AFAIK, there are no constant-current "components" in Falstad)

author: Terry Bites
author_rank: Super Contributor
timestamp: September 19, 2023, 07:45:15 pm
content: Erm.. Your using Falstad?

author: pqass
author_rank: Super Contributor
timestamp: September 19, 2023, 07:50:16 pm
content: The reverse-breakdown of the zener is defined as 8V. Add LED Vf of 1.7V giving 9.7V where it will begin to light.But you'll need a couple of mA through the zener, 170R, and 500R, so you're looking at 11.4V.Why don't you use LM431 + pot to fine tune instead.http://www.reuk.co.uk/wordpress/storage/tl431-battery-voltage-monitor/

author: ledtester
author_rank: Super Contributor
timestamp: September 19, 2023, 09:13:34 pm
content: There are a lot of "low battery indicator" circuits which use a zener + a transistor. The transistor does two things: 1) it isolates the zener current from the LED current; 2) it amplifies the zener current so the turn on happens more or less when the zener starts to conduct instead of being a soft turn on.Here are some discussions I found:https://electronics.stackexchange.com/a/589274https://electronics.stackexchange.com/a/131014And here are some other ideas:https://www.homemade-circuits.com/low-battery-indicator-circuit-using-two/

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2023, 09:34:46 pm
content: Quote from: Terry Bites on September 19, 2023, 07:45:15 pmWell, it has nice colours, and it's Oktoberfest (locally "Wiesn") time over here. Fits!

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 20, 2023, 01:24:10 am
content: Quote from: pqass on September 19, 2023, 07:50:16 pmI don't have a lot of knowledge/experience on zener diodes yet, so go easy on me. If the forward voltage (the voltage needed to turn on the LED?) for the LED is 3v, and I want the LED to turn on at 8v, should I actually be using an 8v zener diode or a 5v zener diode?Quote from: pqass on September 19, 2023, 07:50:16 pmIn this specific case, I'd rather the circuit be simple. I don't need the LED trigger voltage to be precise.

author: pqass
author_rank: Super Contributor
timestamp: September 20, 2023, 03:35:23 am
content: Quote from: LooseJunkHater on September 20, 2023, 01:24:10 amIdeally, a 5V zener + 3V LED should work but...Diodes (red to white LEDs to reverse-biased zeners) aren't precise devices; they're a little mushy. That is, as the current ramps up they start to turn on below their documented voltage drop and can go beyond with higher current. Stacking a real world LED and zener just makes it more imprecise. Also, finding an LED+zener combination whose combined voltage drops matches your chosen turn on voltage is difficult. Modern LEDs are also more efficient and visible at very low current.It's far easier to use a cheap LM431+resistor divider as it has a sharp turn on characteristic, closer to an ideal zener.FYI, primer on real world zeners:

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 20, 2023, 04:24:52 am
content: As others have mentioned, you just need to change the zener to a lower voltage but it's going to be a soft turn on/off. Kind of like this in Falstad:

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 20, 2023, 03:32:54 pm
content: Thank you @Kim Christensen and @pqass for the knowledge!Watching @pqass video, I never had heard of the TLV431 (only the TL431). I know it's off-topic, but are the main differences between the two being that:TLV431 = lower power consumption, vref of 1.25v, output voltage between 1.25-6vTL431 = vref of 2.5v, output voltage between 2.5v-36vOr is there more "important" details to know about the two?_____________________To also get back onto topic, I guess I'll very likely be using the TL431/TLV431 for my IRL circuit, thanks!