FAQ
TL;DR: Build a mains undervoltage/restore controller with a 5‑minute delay and an 11 V hysteresis window; “The difference between 180 and 191 is 11v.” Use a 555 timer, a zener string, and a divider to hit your thresholds. [Elektroda, Peter White, post #21672693]
Why it matters: It prevents short-cycling and nuisance starts in HVAC/refrigeration when line voltage sags or after blackouts.
Quick Facts
- Target thresholds: OFF below 180 V, ON above 191 V; application: air conditioning and refrigeration. [Elektroda, DJS Lambs Marketing, post #21672687]
- Simple approach: op-amp comparator with hysteresis; scale mains by ~100× (e.g., 180 V → 1.8 V). [Elektroda, Rohit Dubla, post #21672688]
- 555-based solution works with extra parts for thresholding and timing; 5‑minute delay is feasible. [Elektroda, Peter White, post #21672690]
- Hysteresis window here is 11 V between cut-out and cut-in. [Elektroda, Peter White, post #21672693]
- Safety first: lethal voltages—get qualified oversight before commissioning. [Elektroda, Richard Gabric, post #21672695]
What problem does this circuit actually solve?
It guards equipment from low mains and rapid restarts. It cuts power when voltage drops under 180 V and only restores above 191 V, then adds a 5‑minute delay. That protects compressor-based loads like air conditioners and refrigerators from short-cycling and brownout stress. [Elektroda, DJS Lambs Marketing, post #21672685]
How does a hysteresis comparator help here?
A hysteresis comparator changes state at two different thresholds. In this case, it turns OFF below 180 V and turns ON only after the line rises above 191 V. Scaling the mains down lets common op-amps handle sensing safely. “You need an hysteresis based comparator.” [Elektroda, Rohit Dubla, post #21672688]
Can I do this with a 555 timer, or do I need an op‑amp?
You can use a 555. Add front-end thresholding so pins 2 and 6 see a scaled, hysteretic signal, then use the 555 for switching and a separate 5‑minute delay network. “It is very easy to do. Just a few more parts.” [Elektroda, Peter White, post #21672690]
What’s the hysteresis window I should target?
Use an 11 V window: 180 V cut-off and 191 V cut-in. This gap avoids relay chatter and premature reconnects during marginal line conditions. Quote: “The difference between 180 and 191 is 11v.” [Elektroda, Peter White, post #21672693]
Why scale the mains by about 100× before the comparator?
Scaling keeps the sense voltage inside op-amp or 555 input limits. For example, 180 V becomes 1.8 V and 191 V becomes 1.91 V, which suits low-voltage circuitry and simplifies resistor and reference choices. This improves accuracy and safety. [Elektroda, Rohit Dubla, post #21672688]
How do I wire the 555’s trigger and threshold for voltage sensing?
Feed the scaled sense voltage to the junction of pins 2 and 6. With proper hysteresis and division, the 555 changes state as the mains crosses your engineered thresholds. The 555’s output then drives your relay or logic stage. [Elektroda, Peter White, post #21672693]
How do I add the 5‑minute power‑on delay?
Use a 555 delay stage configured for ~300 s. After the line recovers to 191 V, the delay times out before re-energizing the load. This reduces compressor short-cycling after sags or outages. A second 555 or RC timing around the first can implement it. [Elektroda, Peter White, post #21672693]
Is this suitable for air conditioners and refrigeration loads?
Yes. The stated use case is protecting air conditioning and refrigeration. The undervoltage cut-out, restore threshold, and 5‑minute delay align with common compressor protection practices to avoid rapid restarts. [Elektroda, DJS Lambs Marketing, post #21672687]
What if my build turns ON at 185 V instead of 191 V?
That indicates threshold-setting error or tolerance drift. In the thread, the builder saw 185 V activation while targeting 191 V. Adjust the divider ratios or reference elements, not only the zener, to shift the ON threshold accurately. [Elektroda, DJS Lambs Marketing, post #21672697]
Is swapping zener diodes enough to hit 191 V exactly?
Not always. The report showed zener swaps alone failed to reach 191 V activation. That suggests the divider and hysteresis network also need tuning. Component tolerances and temperature can move the threshold. This is a common failure mode. [Elektroda, DJS Lambs Marketing, post #21672697]
Is this safe for a novice to build?
Caution. You’ll work near lethal voltages. The forum advised direct help from someone experienced and a safety review before commissioning. Add isolation, fusing, and proper creepage to reduce risk. “Enlist direct help… ensure it is safe.” [Elektroda, Richard Gabric, post #21672695]
Can I rely on a bare 555 without a proper front end?
A bare 555 approach struggled to meet the 191 V turn‑on precisely. The poster using only a 555 couldn’t get it right. Add a proper hysteresis/front‑end to set clean thresholds, then let the 555 handle timing and switching. [Elektroda, DJS Lambs Marketing, post #21672689]
Three-step: how do I build the 180 V/191 V, 5‑minute controller?
- Create a zener string/reference so mains between 180–191 V maps to a 0–11 V window, then divide to a safe level.
- Feed the reduced signal to 555 pins 2 and 6; set hysteresis so OFF<180 V and ON>191 V.
- Add a 5‑minute delay stage on the 555 output before driving a relay. [Elektroda, Peter White, post #21672693]
What’s an example of a practical statistic from this design?
Scaling by ~100× turns 230 V-class mains into a ~2.3 V sense level. In this project, 180 V → 1.8 V and 191 V → 1.91 V, enabling low‑voltage comparators and timers to work accurately. [Elektroda, Rohit Dubla, post #21672688]
Can I tune it later for a different restore voltage?
Yes. Recalculate the divider/hysteresis so the scaled ON threshold equals your new target (for example, 200 V → 2.00 V). Keep the 11 V window concept if you want similar noise immunity, then re-time the delay if needed. [Elektroda, Peter White, post #21672693]
