What do you consider "high temperature"? The Pi will run safely to over 80C, and will protect itself above that temperature by throttling its own speed.MarcosCD wrote: I want to use my Raspberry Pi to host a program and to prevent it to be at high temperature I want to put a fan on it
Why so? That will let the current caused by the back EMF of the motor flow all the way to the PSU and back, a potentially large loop that will radiate the pulse as EMI.Z80 Refugee wrote: It is more important to have a diode across the transistor than it is across the fan motor.
When the transistor turns off, there will be a large negative spike at the transistor collector as a result of the collapsing magnetic field in the motor windings (or any other inductive load). Currently, you have nothing protecting the transistor, and the spike can get back to the GPIO.
An inductor (and, IIRC, a motor) wants to keep the current flowing in the same direction. When the transistor switches off, in the absence of the diode this produces a positive spike as the charge builds up at the transistor collector(remember the EMF comes from the inductor/motor, it is not an external voltage applied to it). That's why it is called reverse EMF. The diode across the motor or inductor clamps this voltage by allowing the reverse current to flow. Thus, with the diode in place, there is no voltage spike.Z80 Refugee wrote: When the transistor turns off, there will be a large negative spike at the transistor collector as a result of the collapsing magnetic field in the motor windings (or any other inductive load).
So what causes a negative spike? Unless I have misunderstood the theory (always possible), it is not due to the action of switching off an inductive load. It may be good design practice to protect power switching elements from negativwe collector voltage, but surely the cause is different?
Agreed for profesional, modular design. Not necessary in a permanently connected homebrew circuit -- unless there are serious consequences to accidental fan operation (in which case, I wonder if a Pi is a suitable controller?)
Ringing? That requires a degree of resonance, which is unlikely here, or driving some form of transmission line (again not likely here).Z80 Refugee wrote: I think a negative spike is the result of ringing. If the diode across the transistor is a zenner, it will also protect the transistor from excessive positive voltages. The diode across the inductive load suppresses positive spikes at the collector to V+, but this spike is only a problem if it exceeds the breakdown voltage of the transistor. Whether the flywheel diode helps with EMC is a moot point - it's not so much voltages as currents which induce EM noise.
Agreed.Z80 Refugee wrote: "Fly back" is definitely not the right term for it, that comes from CRT scan circuits (when the spot flies back to the beginning of the next line or frame).
As I was viewing this in a small tablet it was off the screen unless i scrolled right down.
Sometimes you may not like your Pi to throttle while watching a youtube video via chromium.boyoh wrote: ↑Sat Jun 30, 2018 8:30 am
If you use the Pi as it should be , there is no need for a fan or extra heat sinks
You only have to look at the size of the Pi to tell you it is not meant to run power
hungary circuits directly,, Just use the Pi to switch low power signals to con troll
Buffer stages for high power circuits using a separate power supply.
Fitting a fan on the on the Pi is counter productive, as this will ad load to the
Pi power supply, If you still intend fitting a fan , use a separate power supply.
Regards BoyOh Retired Electrical / Electronic Technician