PC Fans in Parallel - Voltage Drops

[mspring501]

Hello. I have wired 2 PC fans (identical, 12v, 0.24 A each) to a) a USB charger (12v DC, 0.93A) and then b) to a PC USB hub (12V DC, 2.86 A). The results are as follows according to my multimeter:

USB Charger
With no fans plugged in: 11.97v, 0.93A
With one fan plugged in: 10.60v, 0.93 A
With two fans plugged in: 7.58v, 0.93A

PC USB Hub
With no fans plugged in: 11.97v, 2.86A
With one fan plugged in: 11.9v, 2.86 A
With two fans plugged in: 9.9v, 2.86A

I am at a loss to understand why the voltage varies as I am using a parallel circuit design with both fan +ve (yellow) wires gonig to the +ve USB connector and both fan -ve (black) wires going to the -ve USB connector. Both can be turned off and on independently. I have also swapped the fans around and go the same results - both singly and together.

I can only assume that the fan is having some type of impact I don't understand - and I would very much like to understand and see the maths. I'm making a small air drying unit for cheese and want to understand the effects on the voltage etc. of introducing more than one fan.

Can anyone enlighten me please.
Thanks.
(PS - I'd attach a picture but it isn't obvious how to do that - sorry)

 

[basroil]

It's pretty obvious that your 12V input is NOT 2.86A and likely under 500mA

Not sure why you are connecting PC fans to a USB hub though, pretty stupid thing to do when you can just use your PSU's 12V

 

[mspring501]

It's pretty obvious that your 12V input is NOT 2.86A and likely under 500mA

Not sure why you are connecting PC fans to a USB hub though, pretty stupid thing to do when you can just use your PSU's 12V

 

[mspring501]

I'm not powering the fans on a PC and wanted to see if I could Power via USB. I'm using a voltage booster to make the 5v from many of the USB options available to me up to 12v to match the fans maximum voltage so I can later control the speed and step it down in stages accordingly. I have measured the current from kindle, Apple etc USB hubs and get different figures for the ampage ranging from 0.5 to nearly 3 amps. I have remeasured and the multimeter's figues are identical to those posted.

It is disappointing that you chose to be insulting than be helpful. A sign of the times sadly. I'm hoping someone else will be more helpful. Please don't bother replying... This isn't Facebook and your response and language is unacceptable. Go and sit on the naughty step young man.

 

[basroil]

I'm not powering the fans on a PC and wanted to see if I could Power via USB. I'm using a voltage booster to make the 5v from many of the USB options available to me up to 12v to match the fans maximum voltage so I can later control the speed and step it down in stages accordingly. I have measured the current from kindle, Apple etc USB hubs and get different figures for the ampage ranging from 0.5 to nearly 3 amps. I have remeasured and the multimeter's figues are identical to those posted.

It is disappointing that you chose to be insulting than be helpful. A sign of the times sadly. I'm hoping someone else will be more helpful. Please don't bother replying... This isn't Facebook and your response and language is unacceptable. Go and sit on the naughty step young man.

1) Who the hell insulted YOU? I merely said that powering 12V fans though a 5V 500mA maximum port was a stupid idea.
2) I'm sure you're confused as to the actual amperage at 12V vs 5V. Most 5V USB hubs and chargers alike will put out between 500mA and 1.5A AT 5V, which means even with a 100% efficient 12V DC-DC converter you will get between 210mA and 625mA at 12V. In reality, it's closer to ~500mA max at 12V with a 1.5A 5V device as input.

If you want to power fans independently you can easily purchase a 12V AC to DC switched mode adapter, most don't cost more than $10. I've powered plenty of PWM and nonPWM fans that way without issues.

 

[mspring501]

I'm not powering the fans on a PC and wanted to see if I could Power via USB. I'm using a voltage booster to make the 5v from many of the USB options available to me up to 12v to match the fans maximum voltage so I can later control the speed and step it down in stages accordingly. I have measured the current from kindle, Apple etc USB hubs and get different figures for the ampage ranging from 0.5 to nearly 3 amps. I have remeasured and the multimeter's figues are identical to those posted.

It is disappointing that you chose to be insulting than be helpful. A sign of the times sadly. I'm hoping someone else will be more helpful. Please don't bother replying... This isn't Facebook and your response and language is unacceptable. Go and sit on the naughty step young man.

1) Who the hell insulted YOU? I merely said that powering 12V fans though a 5V 500mA maximum port was a stupid idea.
2) I'm sure you're confused as to the actual amperage at 12V vs 5V. Most 5V USB hubs and chargers alike will put out between 500mA and 1.5A AT 5V, which means even with a 100% efficient 12V DC-DC converter you will get between 210mA and 625mA at 12V. In reality, it's closer to ~500mA max at 12V with a 1.5A 5V device as input.

If you want to power fans independently you can easily purchase a 12V AC to DC switched mode adapter, most don't cost more than $10. I've powered plenty of PWM and nonPWM fans that way without issues.

 

[mspring501]

Thanks. I wanted to be able use whatever poster (USB) was to hand without resorting to an external adaptor. The voltage going into the booster is 5.07 at 0.5, 0.98 or 2.89 amps depending on if I using Kindle, Apple or front USB hub. Coming out of the booster, the voltage is stepped
Up but the amps remain the same. I know this clearly makes no sense but that's what the multimeter says and it'l matches the printed output on the bottom of the two plugs for Kindle and Apple so I'm thinking it's
Accurate enough. I'm more interested (maybe just academically) as to why adding another device on parallel reduces voltage as it's always been my understanding that voltage remains the same in parallel circuits (likewise current in series). Sometimes even curious it's needs an answer. Cheers

 

[TJ Hooker]

The voltage going into the booster is 5.07 at 0.5, 0.98 or 2.89 amps depending on if I using Kindle, Apple or front USB hub. Coming out of the booster, the voltage is stepped Up but the amps remain the same. I know this clearly makes no sense but that's what the multimeter says and it'l matches the printed output on the bottom of the two plugs for Kindle and Apple so I'm thinking it's Accurate enough.

You're inputting 5 V, X amps into a step up DC-DC converter, and getting 12 V, X amps out? That is impossible. Either your multimeter is faulty, you're using it wrong, or there's something wrong with how you're describing your setup.

I'm more interested (maybe just academically) as to why adding another device on parallel reduces voltage as it's always been my understanding that voltage remains the same in parallel circuits (likewise current in series). Sometimes even curious it's needs an answer. Cheers

You're correct, with an ideal power source the voltage would remain constant as you add more elements in parallel. However, the voltage regulators in the USB hub obviously aren't that great, and are struggling to supply the extra current when you add a fan. This is causing the voltage to drop.

Edit: Are the various current values you listed in the OP something you measured? The fact that they're exactly the same with 2, 1, or no fans is suspect.

 

[mspring501]

The voltage going into the booster is 5.07 at 0.5, 0.98 or 2.89 amps depending on if I using Kindle, Apple or front USB hub. Coming out of the booster, the voltage is stepped Up but the amps remain the same. I know this clearly makes no sense but that's what the multimeter says and it'l matches the printed output on the bottom of the two plugs for Kindle and Apple so I'm thinking it's Accurate enough.

You're inputting 5 V, X amps into a step up DC-DC converter, and getting 12 V, X amps out? That is impossible. Either your multimeter is faulty, you're using it wrong, or there's something wrong with how you're describing your setup.

I'm more interested (maybe just academically) as to why adding another device on parallel reduces voltage as it's always been my understanding that voltage remains the same in parallel circuits (likewise current in series). Sometimes even curious it's needs an answer. Cheers

You're correct, with an ideal power source the voltage would remain constant as you add more elements in parallel. However, the voltage regulators in the USB hub obviously aren't that great, and are struggling to supply the extra current when you add a fan. This is causing the voltage to drop.

Edit: Are the various current values you listed in the OP something you measured? The fact that they're exactly the same with 2, 1, or no fans is suspect.

 

[basroil]

Accurate enough. I'm more interested (maybe just academically) as to why adding another device on parallel reduces voltage as it's always been my understanding that voltage remains the same in parallel circuits (likewise current in series). Sometimes even curious it's needs an answer. Cheers

Very easy to explain: you're overloading the system and it has to drop voltage to keep under the maximum current.