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Calibretto 09-06-2013 08:16 AM

7V "trick" safe?
Hi guys

I'm having trouble figuring out wether to use the 7 Volt trick where I plug my fans to the 12 and 5 volt connectors on a molex, or to use resistors to reduce voltage.

I have some fans that I intend to run at 7 Volt and I don't really know enough about DC current, let alone power supplies, to decide where to go.

Is the 12V to 5V trick safe enough? It seems contrary to common sense not having anything grounded. Sorry if asked before, search didn't yield any answers and other websites have been very ambiguous in their answers.

Thanks in advance. I hope I'm in the right place.

mariush 09-06-2013 10:22 AM

No, it's not safe to use it. It may work fine with some power supplies, it may not work with others.

Just use a resistor to drop the voltage, or a voltage regulator, or an npn transistor.

For the resistor option... see

Calibretto 09-06-2013 01:00 PM

Thanks a lot. That certainly isn't so ambiguous.

Can anyone explain what exactly the problem with it is?

CM Phaedrus 09-06-2013 01:32 PM

You can use an LM431 shunt regulator driving a power transistor to drive several fans

View figure 18 on page 8

The problem with using the 7V trick is that it causes current to flow from the +12V rail to the +5V rail. The circuit generating the +5V rail is only designed to move current in one direction; it is not designed to sink current. If the load from your 7V fans exceeds the total load on the +5V rail, the circuit will be trying to sink current, and will be unable to. Depending on the circuit topology the unit may shut down; or become unstable and malfunction; or fail catastrophically. One or two fans is usually safe enough, but if you seriously want to run a lot of fans at 7V, use a voltage regulator.

Calibretto 09-06-2013 02:40 PM

Greatly appreciated.
I'll go read up on my resistors and voltage regulators.

Calibretto 09-16-2013 12:53 PM

I found some 42.2Ohm 0.6W resistors.
Using the tool Mariush linked I got this:

Target resistance: 42.2 Ohms
R = 4.955 / 0.117 = 42.2
Power dissipated by resistor: 4.955V * 0.117A = 0.582W
That's assuming I want to reduce a 12V, 0.2Amp fan to 7.045V
Does it look OK to use those resistors as long as each fans gets its own?
Is 0.58W dissipated from a 0.6W resistor fine? Or is it cutting it too close?

walterm 09-17-2013 08:12 PM

Gelid fan controller
Zalman fan controller

Either will do the job.
Actually a dedicated fan controller is usually a better, and variable alternative. Though buying quiet slow fans also works ( my preferred method)

Calibretto 09-19-2013 12:35 PM

Thanks for the suggestion Waltherm, but this is to avoid fan controllers like that altogether.
I'm crimping the cables for this build by hand anyway, so I may as well stick the resistors on there, since everything is already custom.
I just wanted to see if someone know about these resistors. If not, I'll find some electronics guys.
Appreciate the reply though.

cypherpunks 09-19-2013 11:58 PM

People usually get away with it, but it's tricky.
Well, it depends on how safe you want it to be. Certainly many people get away with using the 7V trick on a couple of low-power fans. But there are some nasty gotchas.

The bit about no connection from a piece of electrical equipment to ground is a complete non-issue; don't even worry about that. Everything battery-powered works that way!

The two things you have to be careful of the with the 7V trick are:
  • The fan RPM sensor (yellow wire) cannot be used, and in fact it's likely to damage your motherboard if you connect it. The fan signals RPM by periodically connecting that wire to its ground, the black wire. If the fan is "grounded" to +5V, that's not good for super I/O chip, which generally operates on 3.3V or lower. (I know the protection diode makes this worse, but I think the explanation is good enough.)
  • (The really important part.) You need enough load on the +5V rail to consume the power the fans are feeding it. Power supplies can only source current to the +5V rail. The least they can do is supply zero power; they can't consume the power the fans are dumping onto the +5V rail. You need some "sink" for that power, or the 5V rail will rise to a higher voltage. The problem is that modern motherboards don't use +5V all that much. The main load on +5V is USB devices. If you have a lot of fans transferring power from +12V to +5V and nothing taking power from +5V, bad things can happen.
Good power supplies have an overvoltage crowbar, and will shut down if the 5V output rises to 5.5V or so (out of spec, but the magic smoke won't escape), but that's not well tested or even included in cheap ones.

My problem is that I don't have a good feel for what a typical +5V system load is. I know hard drives often specify a maximum of 0.5 to 1A, but we need to know the minimum. And computer makers keep working to save power and heat by turning unused equipment off when not needed, which reduces that minimum. So numbers from a few years ago aren't necessarily today's numbers.

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