FAQ
TL;DR: From 6.3 VAC, expect ~8.1 VDC after rectification; “use a CMOS part that has a max voltage higher than the VDD supply.” [Elektroda, Steve Lawson, post #21670594]
Why it matters: This FAQ shows how to build a reliable 3‑minute, high‑side‑switch timer for a 12 VDC load under 1 A, replacing an Amperite tube, with parts you can still source.
Quick Facts
- Supply math: 6.3 VAC → ≈8.1 VDC after rectifier and drop (6.3×1.4−0.7). [Elektroda, Steve Lawson, post #21670594]
- Load target: 12 VDC, under 1 A (relays and lamps). [Elektroda, Harbach Electronics LLC, post #21670589]
- Device limits: 74HC4060 max 7 V; MC14060B max 18 V—choose headroom over VDD. [Elektroda, Steve Lawson, post #21670594]
- Switch losses: MOSFET ≈negligible at 1 A; SCR ≈1 V drop when on. [Elektroda, Steve Lawson, post #21670594]
- Sourcing: CD4060BE and IRF9Z34PBF noted as available via major distributors. [Elektroda, Steve Lawson, post #21670596]
What does this 3‑minute timer actually do?
It delays power‑up. After turn‑on, it counts 3 minutes, then triggers a switch that applies 12 VDC to the load until power is removed. [Elektroda, Harbach Electronics LLC, post #21670589]
Can I latch the high‑side switch on until power‑down?
Yes. Once triggered at the 3‑minute mark, the SCR or MOSFET remains on and passes 12 VDC until the unit is turned off. [Elektroda, Harbach Electronics LLC, post #21670589]
Why use a P‑channel MOSFET instead of an SCR here?
A P‑channel MOSFET used as a high‑side switch has near‑zero drop at 1 A, while an SCR drops about 1 V when conducting. [Elektroda, Steve Lawson, post #21670594]
Which timer/counter IC fits this supply and what are safe limits?
Use a CMOS 4060 variant with headroom above VDD. 74HC4060 maxes at 7 V; MC14060B tolerates up to 18 V. [Elektroda, Steve Lawson, post #21670594]
How do I set a 3‑minute delay with the 4060 oscillator?
Tweak the RC. 1) Start with R2 and C1 that land near 180 seconds. 2) Adjust R2 to fine‑tune frequency. 3) Change C1 if coarse shift is needed. [Elektroda, Steve Lawson, post #21670594]
What does the R7–C3 network do?
It forms a power‑on reset. On startup, C3 and R7 reset the CD4060 so the count begins from a known state. [Elektroda, Steve Lawson, post #21670596]
What is diode D2 doing in this design?
D2 blocks current from 12 V through R5 when Q2 is off. Without D2, that path would skew the oscillator’s timing. [Elektroda, Steve Lawson, post #21670596]
Can I power the timer from the 12 VDC rail instead of 6.3 VAC?
Yes. A variation uses the 12 VDC supply directly, reducing parts versus rectifying and filtering the 6.3 VAC source. [Elektroda, Steve Lawson, post #21670596]
Are the suggested parts available right now?
At the time of the thread, IRF9Z34PBF and CD4060BE were listed with distributor part numbers for purchase. [Elektroda, Steve Lawson, post #21670596]
Any lifecycle or package concerns for the 4060 family?
Yes. PDIP 4060 parts were near end‑of‑life; consider SOIC packages and adapter boards for production builds. [Elektroda, Steve Lawson, post #21670597]
What MOSFET specs should I target for a 12 V, <1 A load?
Choose P‑channel, RDS(on) ≈100–200 mΩ, VGS(max) >12 V, VDS(max) >24 V, and >0.5 W dissipation. “Hundreds… of other possibilities.” [Elektroda, Steve Lawson, post #21670597]
What happens if I omit the discharge resistor on the timing capacitors?
Without R8, the capacitors may not discharge after power‑off, causing incorrect timing on the next start. [Elektroda, Steve Lawson, post #21670596]
