FAQ
TL;DR: In this HV sensor, one path breaks at ~75 V while the LED path triggers near ~28 V—so LEDs fire first. Expert note: “The 3 DIAC path will never get to 75V.” [Elektroda, Earl Albin, post #21665238]
Why it matters: If the low‑threshold path always triggers, adding parts for “more current” won’t work without rethinking thresholds; this FAQ shows how, for hobbyists and engineers diagnosing DIAC‑LED sensors.
Quick Facts
- R1 = 2 MΩ, R2 = 1 MΩ; the design targets a high‑impedance HV sensor front end. [Elektroda, George Capka, post #21665229]
- Each DIAC fires around 26 V; three in series sum near 75–78 V. [Elektroda, George Capka, post #21665237]
- Competing paths: ~28 V path (1 DIAC + 3 LEDs) triggers before the ~75 V path (3 DIACs). [Elektroda, Earl Albin, post #21665238]
- Requested headroom: about +25% more current capability. [Elektroda, George Capka, post #21665237]
- Lowering divider resistance increases current, but may keep DIACs latched and defeat the sensor’s intent. [Elektroda, Earl Albin, post #21665234]
What’s actually happening in this DIAC–LED HV sensor?
Two threshold paths compete. The LED path has one DIAC plus LEDs (~28 V), so it triggers first. The three‑DIAC path needs ~75 V and never reaches breakover while the lower path conducts. Result: LEDs light and the higher‑voltage path stays idle. [Elektroda, Earl Albin, post #21665238]
Can I increase current without changing R1 (2 MΩ) or R2 (1 MΩ)?
Within this divider‑driven topology, the forum advice was clear: more current comes from lower resistance. Keeping R1/R2 fixed preserves the very low source current, so added loads will starve the DIACs or relight cyclically. “You get what you get” unless you redesign the front end. [Elektroda, Earl Albin, post #21665240]
Why might high resistances make the DIACs chatter or switch off?
If the charging path is too resistive, the DIAC’s holding current isn’t met. It turns off after breakover and the node recharges, creating relaxation‑oscillator behavior and unstable indication. That’s classic DIAC operation when current is limited. [Elektroda, Earl Albin, post #21665230]
What are the actual resistor values and purpose here?
R1 is 2 MΩ and R2 is 1 MΩ. The high values form a high‑impedance divider to sense HV while limiting current into the DIAC/LED network. This preserves safety margin but severely limits drive for added components. [Elektroda, George Capka, post #21665229]
What is a DIAC, in plain terms?
A DIAC is a bidirectional trigger diode that stays off until its breakover voltage, then conducts and latches until current drops below a holding value. Typical breakover is about 30 V. [“DIAC”]
What current increase did the OP want, and what were the DIAC thresholds?
They asked for roughly a 25% increase in current capability. Each DIAC had a 26 V firing voltage, so three devices imply about 78 V for that branch. [Elektroda, George Capka, post #21665237]
Why do the LEDs always light first in this schematic?
Because the LED branch includes one DIAC plus LED drops, it reaches about 28 V and fires earlier than the three‑DIAC branch at ~75 V. Early conduction clamps the node, blocking the higher‑threshold path. [Elektroda, Earl Albin, post #21665238]
Could adding more LEDs or parts boost current capability?
Adding parts raises load current. With a 2 MΩ/1 MΩ divider, available current is tiny. Extra load risks dropping below DIAC holding current, causing flicker or oscillation instead of more drive. [Elektroda, Earl Albin, post #21665230]
Is there a general spec for DIAC breakover I can use for estimates?
Typical DIAC breakover is 30–40 V per device. That statistic helps budget thresholds and decide path priority before you add or move components. [“DIAC: The bidirectional diode”]
I want help but don’t want to reveal the full design. Any advice from the thread?
State measurable targets (e.g., +25% current), thresholds, and constraints (R1/R2 fixed). That enabled replies without IP disclosure and kept discussion productive. [Elektroda, George Capka, post #21665241]
How do I debug competing breakover paths in three quick steps?
- List each path’s total breakover: count DIACs and LED drops; compute thresholds.
- Measure node voltage rise with the divider; confirm which path triggers first.
- Remove or raise the lower threshold path to test if the higher path can fire cleanly. [Elektroda, Earl Albin, post #21665238]
What if my DIACs say 26 V but someone mentioned 24 V in the thread?
The OP clarified their DIACs fire at 26 V. Any earlier 24 V mention referred to allowing pass above about 24 V in that drawing. Use the actual device spec for design math. [Elektroda, George Capka, post #21665237]
A reply said, “lower the two resistor values.” Why might that defeat the purpose?
Lowering R1/R2 increases current, but it can also keep DIACs latched on, changing a threshold sensor into a continuously conducting path. As one expert put it, “That’s what Engineering is.” [Elektroda, Earl Albin, post #21665234]
What if the community can’t see my schematic?
You’ll get little actionable help. One user noted the image only said “CAN ANYONE HELP ME PLEASE.” Always re‑post a clear diagram to speed diagnosis. [Elektroda, Frank Bushnell, post #21665243]
Do typical DIACs support higher current loads directly?
DIACs are triggers, not drivers. They supply a sharp pulse after breakover; you still need a load path that honors holding current and your divider’s limits. Consider external drivers only after threshold conflicts are resolved. [“Tutorial on DIAC”]
