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wouldnt a zener diode placed backwards do the same trick only with more effect?
Considering fans are typically brushless, is it safe to use a diode? I'll be putting it on the fan in my power supply to slow it down. Thoughts?
It would work fine for OPs application. Once a zener is in breakdown it still has the zener voltage dropped across it, accomplishing the goal of reducing the voltage applied to the fan. It would, however, be silly to use one since it offers no advantage over a resistor.as for zener diode they resist up to a certain voltage beyond that the diode gate is open and allows current flow.... only practical if your using a thermistor in a circuit with the zener to activate the fan at a certain temp....... >_>
Diodes are not meant to be used in any particular way. They have certain properties, and you can exploit those properties in different ways depending on your needs. Since the voltage they drop is relatively constant with the current through them, they can be useful for dropping a fairly constant voltage when the load is unknown or variable.the other thing.... diodes are check valves for current only allowing one way flow...... they are simply not meant to be used in place of a resistor......
Some fans (usually super-high-RPM monsters, blower fans, etc) have a big inrush current when they start up, and using too large a resistor causes the fan voltage to sag when the fan starts up. In these applications, a zener works better than a resistor.It would work fine for OPs application. Once a zener is in breakdown it still has the zener voltage dropped across it, accomplishing the goal of reducing the voltage applied to the fan. It would, however, be silly to use one since it offers no advantage over a resistor.
just tryin to save the boy some money.....Your conclusion I agree with though. Since the fan load is constant, there's no reason not to just use a resistor.
gee said:Some fans (usually super-high-RPM monsters, blower fans, etc) have a big inrush current when they start up, and using too large a resistor causes the fan voltage to sag when the fan starts up. In these applications, a zener works better than a resistor.
The choice between a resistor and a diode won't affect the current draw from the PSU, so that worry is misplaced. Here's a quick difference between the two:The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.
Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.
The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.
Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.
if this is the case consider then use a capacitor to compensate the initial high current draw of the fan
*facepalm* how did i not regurgitate that right......? sorry guys uni is getting to me......An ideal capacitor appears as a dead short (ie. infinite current) when power is applied to it, decreasing to zero current after a long time, assuming DC is applied. Like the fan, it will have a high initial current draw, eventually reaching ~0 once the capacitor has charged. Slowly charging a capacitor and then using it to start the fan is possible, but requires more than just the cap.
The main problem I can see with using a resistor might in fact be this increased initial draw. The fan needs more current to start than to run, but as it tries to pull more current, more voltage drops across the resistor. This might mean that the fan will work properly with a lower voltage if you use diodes, since the initially high current draw won't affect the voltage on the fan, allowing it to start successfully at a lower 'run' voltage.
Don't even worry about the PSU. They're rated for many amperes, while the fan probably draws under 0.25A even at startup. It's negligible, diodes or no.
The resistor would introduce more current draw into the PSU's fan circuitry which is something I don't want to do in the off chance that the circuit can't take it. That's why I would rather lower the voltage using a diode or zener diode, which would not add more current draw.
Anyway, the operation is a go as I have the parts, it's just a matter of balls and time, haha.
i knew i wasn't completely losing it......yet...... any ways getting back to the meat of the thread.... just use a resistor ......Its the opposite
Resistors resist current flow.
The current flowing through the resistor causes a voltage drop across it, therefore feeding less voltage to the fan.
This reduces the power load, it doesnt increase it.
Careful that the resistor you use can "easily" carry the current otherwise it may get very hot and even burn up.
If the resistor heats up much, put it somewhere near airflow or get a higher wattage resistor.
Why do you say that?
A resistor divider is great...as long as anything attached to it is high-impedance. As soon as you attach something with a low impedance, the voltage divider no longer works as designed. The reason is this: whatever load you attach to the voltage divider will draw additional current through the top resistor. That additional current changes the voltage (V=IR and all that). If the load is high-impedance (like an op-amp), the additional current is small enough that the change in voltage is insignificant. If the load is low-impedance, like a fan, the additional current will very strongly affect the voltage.Why do you say that?
3.3V is too low to spin up big part of fans. Without knowing behavior of particular fan already 5V can be very risky if it's low speed fan variant.Since it's going into your power supply, you already have the option of tapping 12v, 5v, and 3.3v (fast, medium, and slow respectively)
5V is often too low a voltage and the fan wont start, so 3.3V is very unlikely to work.
Tap onto +5V as gnd/earth and +12V as V+ to get 7V.
This has worked great for me, quietening down some pretty loud fans in the past.
not a wise choice.... for several reasons.... it is much wiser to take +12V to a resistor to fan to ground.....
Several years ago, there were isolated cases of unloaded power supplies reacting badly to having a fan across the 12V and 5V lines. In essence, there was current entering the 5V rail when the power supply was only expecting current to go out, and the typical result was that the power supply shut down.pls explain why its not wise
Several years ago, there were isolated cases of unloaded power supplies reacting badly to having a fan across the 12V and 5V lines. In essence, there was current entering the 5V rail when the power supply was only expecting current to go out, and the typical result was that the power supply shut down.
If there are other things drawing off the 5V power supply (like, say, the rest of a computer), there's no issue.
pls explain why its not wise
i really should not need too....
i said it was not wise, i however did not say that doing so would not work.... so i will share why i would not do so....
1. noise bridging: any induced noise from things like fans now has a direct bridge from the 12V rail to the 5V rail......
2. uneven loading: as mentioned by moho.... some of the regulator circuits on the market are quite sensitive to load jumping from one rail to another......