Off Grid Solar delay on relay switch

Question

Delay-ON relay in noisy 24 V DC (golf-cart converter) environment keeps failing—looking for a field-hardened, minimal-parts topology. Rail is ~24 V from a floating, capacitive converter with ripple/ghost voltages and brownouts. Load is a 24 V relay coil ≈60 mA. Goal: delay-ON (single, decisive energize) with true brownout reset (no chatter, no instant re-arm). We tried classic RC→gate schemes: Zener/SBS/PUT triggers, AC/DC coupling, and robust SCRs (e.g., TYN608/TYN806; gate needs ~15–20 mA and has ~100 Ω G-K). Core problem: high-impedance RC can’t reliably source the required gate pulse; leakage/strays cause false/missed triggers. Off-the-shelf H3Y-type delay modules also chattered on dips. We’re now leaning to “rail fires, RC only inhibits” (default-ON logic): clamp the SCR gate hard to 0 V until C⁺ crosses a threshold, then release so the rail (through a fixed resistor) delivers the gate current—must include brownout-safe reset and be tolerant of converter spikes/ghosts. Looking for proven circuits (schematic + values/parts) that meet this: 24 V supply, ~60 mA coil, decisive delay-ON with hysteresis, low component count, and documented immunity to brownouts/ripple (automotive-grade comparator + open-collector clamp? opto-MOS SSR in series with coil? other hardened patterns). Practical guidance on resistor/cap sizing, clamp strength, and reset strategy appreciated.

Answer 1

Classic case of an RC→gate circuit getting bullied by a dirty 24 V line: ripple, spikes, and ghost voltages throw off the SCR trigger. The gate needs a solid 10–20 mA pulse, but a high-impedance RC can’t deliver it reliably, so you get false or missed firings.

The most field-proof fix is to drop the SCR and use a small logic-level MOSFET to drive the relay coil, controlled by an automotive-grade voltage supervisor or comparator with hysteresis. The supervisor holds the output low until the rail is stable, then releases a clean signal to start your RC delay (about 1–3 s). You get a crisp, single turn-on, real brownout reset, and no chatter. Just a few parts: supervisor + RC + MOSFET + flyback diode, it's very robust in noisy 24 V environments.

If you really want to keep the SCR, use a small "reservoir" capacitor that dumps a defined current pulse into the gate when released, sized for at least 25–30 mA, and make sure it resets cleanly on brownout. It works, but it’s fussier.