Alternating fan controller with RC timer and MOSFETs (70 s + 3 s dead-time)  - Page 1
Forum section: electronics / projects

author: Muraste
author_rank: Newbie
timestamp: August 23, 2025, 05:40:35 pm
content: Hello everyone,I am looking for a simple and reliable circuit to alternate two fans (FAN A and FAN B) as follows:• FAN A runs for about 30-70 seconds• Then a pause of ~3-5 seconds (both fans OFF)• FAN B runs for about 30-70 seconds• Then again a pause of 3-5 seconds• The cycle repeats continuously until power is turned offRequirements:- Power supply is 10 V (both fans are 10 V type)- No relays, no counters, no microcontrollers – only analog solution with RC timing and MOSFET switches- Each fan should be driven by a MOSFET (low-side switching)- I would like to have that 3-5second "dead-time" between switching from one fan to the other- Fan about 1-3 W- Timing stability 10%- Or do a digital solution with day timer

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2025, 06:11:44 pm
content: Those are an odd set of requirements. It sounds like a recipe for something more expensive than necessary.How have you tried to solve this?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 23, 2025, 09:18:50 pm
content: QuoteWhat you describe cycles through 4 states, so it is an illusion to claim ‘no counters’Looks like badly phrased homework.If you want simple and reliable and 10% timing, pick a HEF4541and HEF4017Addit: Or a HEF4521 + HEF4052, needs only passives.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2025, 09:52:01 pm
content: Quote from: PCB.Wiz on August 23, 2025, 09:18:50 pmIt should be possible with an astable and a monostable.The astable has a period of 150s. The monostable has a period of 5s and is triggered from both the rising and falling edges of the astable. The outputs of the astable are connected to AND and NOT gates which disable the outputs when the monostable is high.

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2025, 03:21:52 pm
content: A variation of this without any logic gates is four 555 monostables in a ring oscillator. Of course, the devil is in the details and arranging for only one out of four to be initially triggered is essential, as its quite capable of oscillating at double the frequency with two pulses chasing round the ring and both fans on at the same time!

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2025, 04:19:27 pm
content: The classic dual BJT astable can have long rise times if relatively high value pull-ups are used. Add a comparator to each output to trigger at the correct voltage and will give roughly the right waveforms.The problem with long delays is monster capacitors are required. Try higher value resistors (multiply R1 to R4 by 10) and using low leakage tantalum capacitors: divide C1 & C2 by 10.The tolerance of the capacitors is so wide it's not feasible to achieve the sort of accuracy the OP needs. Perhaps trimmer resistors can be used for part of the timing resistors to adust it.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 25, 2025, 03:29:44 am
content: Quote from: Zero999 on August 24, 2025, 04:19:27 pmThat's cute as it relies on the collector rise times to give the much shorter pause times, however that also imposes limits on the resistor rations.Monostables have a both-on lockup mode which is rare, but possible.If I check this for 'both-on' recovery, by checking the both-on AC loop gain is > 1, interesting to see the VBE protection diodes have a real negative impact.With them present Rb's needs to raise very high, > 1Meg, but with the diodes removed, a Rb.Rc of 100k, 8k2 has a saturated AC loop gain of just under 2 (at 10Khz test).Appx 82k is where saturated loop gain drops to unity.There must be a better method to protect VEBO.Reduce operating voltage a little, clamps on the collectors. or pick high VEBO parts, typically used for muting?A simple resistive divider on the collector limits the flyback -ve voltage

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2025, 12:46:27 pm
content: The collector rise times are determined by the ratios of R1 to C1 and R2 to C2. Normally fast times are desirable so R1 & R2 should be as low as possible, but the hFElimit the lower values, as the bases are driven via R3 & R4. In other words, making R1 and R2 high, for longer rise times makes this circuit easier to build and reduces the hFErequirements.I don't see the issue with using diodes to protect the transistors. Reducing the power supply voltage would work, but hat's more components.There are other ways to do this.I wouldn't seriously consider using RC circuits. I'd go for an MCU but that wouldn't answer the homework question.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 25, 2025, 11:21:50 pm
content: Quote from: Zero999 on August 25, 2025, 12:46:27 pmExactly, imagine testing this?The other wrinkle in a simple BJT monostable is the timing C's do get reversed biased during the timing cycle, which is not going to be great for predictable leakage on electrolytic caps.A LED or Zener in the collector can avoid that, (which still needs loop-gain care for possible lock-up avoidance), so there are enough wrinkles hiding here to make a CMOS solution like HEF4521/4060+HEF4052 look simple - no coding needed. Very easy to test, and easily meet the 10% spec.For high volumes, a tiny MCU would be the better choice.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 26, 2025, 01:45:40 am
content: For fun, here is another branch.This uses a NMOS monostable, using high threshold FETS.NMOS are worse for polarised caps, but they allow much higher R's, so the cap becomes MLCC territory. Here a nominal 10uF 10M gives ~ 115s period.There is enough gate-effect slew to allow the simple comparator threshold of #5 for slight dead times.You can get 5% ceramic caps, in the 10uF region, tho somewhat rare. 10% are much more common.Higher voltage ratings help reduce the voltage effects, so this solution would be vendor-locked to specific part numbers for CAP and FET

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2025, 07:18:52 am
content: Yes, the polarity is briefly reversed but it's <1V which isn't enough to be a problem, especially so because it's only a short time and it charges up to near the supply voltage fairly quickly afterwards.The downside with a MOSFET astable is it's dependent on the threshold and supply voltages.

author: Seekonk
author_rank: Super Contributor
timestamp: August 27, 2025, 02:26:14 pm
content: While not in the spec sheet, I remember a wave generator (ocean) that used a TL494 to create almost this same action. They can run quite slow. Use the deadtime to create the pause.