A have an Arduino Nano 33 BLE that I am using to control a automotive radiator fan. I would like to add a circuit to allow the fan to run an additional time of 10 sec to 90 sec. I have come up with 2 circuits that perform exactly what I need to do. But I'm not sure which is the best way to go. The first circuit uses a 555 timer the second circuit uses a MOSFET and transistor. What are the pros and cons of each of these circuits.

Circuit 1 using 555. active high of the ignition is ignition switch on.





Circuit 2 using MOSFET and transistor. Switch 1 represents the ignition switch of the car.

 

Simplicity suggests the second circuit is more appropriate for an automotive environment where it is notoriously noisy and the power is not very clean otherwise.

The simplicity is not in the parts count but in the operating principle. If both circuits are suitable, I would choose the second.

 

Simplicity suggests the second circuit is more appropriate for an automotive environment where it is notoriously noisy and the power is not very clean otherwise.

The simplicity is not in the parts count but in the operating principle. If both circuits are suitable, I would choose the second.

Thanks for the reply

Thanks for the reply. I kind of like the second. But the large capacitor is driving the box size. 3mF would be preferable but those are over an inch high. The second one actually has less parts involved. In the second one the battery and Sw1 and 470Ω resistor don't count. Sw1 is the ignition switch of the car, 470Ω resister gets replaced by the Arduino and the battery is the car battery, so 6 components. Circuit #1 has 7 components once the 1KΩ resister and LED are replaced by the Arduino.

Thanks John

 

If you use a small N-MOSFET for Q1, such as a 2N7000 or 2N7002, then the timing resistor can be a lot larger since the gate draws essentially zero current, allowing the use of a smaller capacitor.

 

Here's an LTspice simulation of my suggested circuit modification:

It shows the output off delay after the switch turns off (top blue trace) for various settings of the U1 pot.

I added diode D1 so that the other loads on the ignition switch won't discharge the capacitor, and R2 to limit the surge current charging C1 and preventing a short if the pot is set completely to its minimum resistance..

 

Here's an LTspice simulation of my suggested circuit modification:

It shows the output off delay after the switch turns off (top blue trace) for various settings of the U1 pot.

I added diode D1 so that the other loads on the ignition switch won't discharge the capacitor, and R2 to limit the surge current charging C1 and preventing a short if the pot is set completely to its minimum resistance..

View attachment 288467

Thank You very much that works a lot better and cuts my cap height down and gets me up to 2 min of post shutdown operation.

Interesting choice of a M2. The Si9803DY is obsolete, and because its obsolete when it did a search for it, I came up with Si9801DY. Thinking I found the right one I was looking for, I was naturally confused, it appears to be a chip that could replace M1 and M2 with a single chip. Both the Si9803DY and the Si9801DY are both available but not from the main stream suppliers. They are also only available in surface mount so that puts them outside my skill level.

As far as the P channel MOSFET M2, I can only use what is in stock today and available in a through hole configuration. So I am currently considering these 3. Could you recommend a preferance.

 

it appears to be a chip that could replace M1 and M2 with a single chip.

No, because the two MOSFET drain connections are tied together internally.

As far as the P channel MOSFET M2, I can only use what is in stock today and available in a through hole configuration. So I am currently considering these 3. Could you recommend a preferance.

My choice for M2 was one I arbitrarily selected from the LTspice models I have.

Just about any P-MOSFET should work, so pick one that's cheap, readily available, and fits you mechanical needs.

 

No, because the two MOSFET drain connections are tied together internally.
My choice for M2 was one I arbitrarily selected from the LTspice models I have.

Just about any P-MOSFET should work, so pick one that's cheap, readily available, and fits you mechanical needs.

Thank You

 

If you are already using a processor to control the fan ON and OFF, why not add a small bit of additional code to add whatever delay you choose??
Or is the delay intended to be after the processor switches the fan off??

 

In the second circuit, the timing resistor is actually R1, not the pot. The pot will make a slight adjustment, but not much.
Am I missing something here, but a 470Ω resistor doesn't seem like a suitable representation of an automotive fan?

 

One more time: Just what is the actual purpose of this timer?? If it is to keep the fan running after ignition switch off, what is the arduino doing? When does it control the fan operation??? There is a whole lot of explanation missing here and without that explanation the guesses are a waste of time.

 

I'd just use a Thermostat, ( Electronic or Mechanical ),
to run the Fan(s) until the monitored-Temps come down to an acceptable level.

If You have 2-Fans, run them in series during the cool-down period,
this will draw only 1/4 of the Power,
and will be more efficient,
and much more quiet,
and will only operate when needed.

This can be done with 3 common Automotive-5-Pin-type Relays, ( and the Thermostat ).
.
.
.

 

Just some words of caution. Be wary of simple designs that a) depend on MOSFET thresholds and b) don't have good, fast gate drive to a high-power device. MOSFET thresholds change with time, temperature, part selection, etc. making the operating point - in this case delay time - subject to some variability. For the application it may be OK, but it may not. If you have a slow gate drive situation during turnoff the power MOSFET will go through a linear region with a huge spike in power dissipation, which may destroy the MOSFET. If you use a MOSFET as a switch drive it so it switches, and minimize the time between on and off states.

 

If you have a slow gate drive situation during turnoff the power MOSFET will go through a linear region with a huge spike in power dissipation

That can be a concern with high current loads, but here he is likely controlling an automotive fan relay, where that should not be a problem.

 

Fans and relays are inductive loads. Don't forget a snubber such as a diode to protect your MOSFET.

 

Also makes me wonder why, if he has an Arduino, why that can't implement a 90 second delay.

 

Also makes me wonder why, if he has an Arduino, why that can't implement a 90 second delay.

I suspect the Arduino becomes unpowered when the ignition is off. Otherwise it would be on continuous, albeit a light load on the battery, that could drain it given a long enough time span.

 

I suspect the Arduino becomes unpowered when the ignition is off. Otherwise it would be on continuous, albeit a light load on the battery, that could drain it given a long enough time span.

The Arduino could switch on a power supply from the battery to power itself up, and then disconnect its own power supply when it has timed out.

 

The Arduino could switch on a power supply from the battery to power itself up,

Huh? If the Arduino isn’t powered up, how can it switch on a power supply… or do anything.

 

Also makes me wonder why, if he has an Arduino, why that can't implement a 90 second delay.

I already suggested that. But the Nuts and Volts mindset would not do that sort of thing. YES, I am being critical of that publication!!