0
\$\begingroup\$

I have a DC fan circuit that is intended to run off of a portable USB battery pack. The fan is a 5V brushless computer fan that draws approximately 250mA. The issue I am running into is that all the portable USB power supplies I have been able to find automatically shut off if the current draw is low enough, as they assume the attached device is "charged" and no longer requires power.

Is there anything I can implement in the circuit to increase the load on the power supply without decreasing the voltage on the fan? The power supply is able to supply at least 1.0A according to the specs on the packaging.

The exact portable USB power supply in use is this model: https://www.amazon.com/MyCharge-Style-Power-Portable-Charger-Black/dp/B01LZUL0BN/ref=sr_1_12?ie=UTF8&qid=1497299186&sr=8-12&keywords=mycharge+portable+charger

\$\endgroup\$
  • 2
    \$\begingroup\$ I have not once had a pack that shut off above the 10% threshold, In fact two of my cheap Chinese ones shut off between 30mA and 50mA. Do you have actual numbers, or just worries? Of course anecdotes are no guarantees. -- Misunderstood and realised seeing your comment below. Leaving the comment a while to avoid confusion. Apologies. \$\endgroup\$ – Asmyldof Jun 12 '17 at 21:21
0
\$\begingroup\$

If this is really the case then put a resistor in parallel with the fan, but I think you have the opposite problem. As far as I understand usb supplies 5V and the only time it shuts off is during an over current condition. For standard and high speed (1.0 through 2.0) USB this is 100mA so that could explain why your port is shutting off if you are plugging it into a PC or a hub that follows the USB spec.

If you are using out of spec chargers then there is no way to predict what will happen, they are out of spec and shouldn't have the USB label on them. The only way to predict what will happen is a schematic or experimentation. There are no guarantees with these chargers, its the wild west.

The other problem is most BLDC fans have a wicked load curve and can go from 0mA to full power in one fan cycle, this would wreck havoc on a charger that is expecting a constant current. You'll probably need a cap to smooth the power out.

\$\endgroup\$
  • \$\begingroup\$ It's definitely not overcurrent. The USB power supply is intended to fully charge a smartphone at least once on a full charge, at a rate of 1.2 A. Upon plugging the fan in, the charger turns on and the fan spins up fine. After a consistently measurable amount of time (~30 seconds I believe), it shuts off again. Pressing the power button turns the charger back on, and holding the button down keeps the charger from shutting off after 30 seconds, so it's definitely a situation where the charger thinks the fan is "fully charged" (as if it were a smartphone that would be drawing the full 1.2A) \$\endgroup\$ – Arcanox Jun 12 '17 at 20:24
  • 1
    \$\begingroup\$ I don't know what your plugging this into, I'm covering all the bases. Read the rest of the answer \$\endgroup\$ – Voltage Spike Jun 12 '17 at 20:25
  • \$\begingroup\$ This is the charger in question amazon.com/MyCharge-Style-Power-Portable-Charger-Black/dp/… \$\endgroup\$ – Arcanox Jun 12 '17 at 20:27
  • \$\begingroup\$ So the charger is really a battery-powered device intended to charge cell-phones or similar devices. Its battery is only 2200 mAh, so may not be able to power a fan for long, unless its battery is fully charged. \$\endgroup\$ – Peter Bennett Jun 12 '17 at 22:56
  • 1
    \$\begingroup\$ I've tested it...it can power the fan for 6+ hours which is more than enough for my purpose. The issue is that I have to wrap a velcro strap around the thing to hold the button down, and I'd rather not have to do that. \$\endgroup\$ – Arcanox Jun 14 '17 at 14:33
0
\$\begingroup\$

To increase current you have to put a parallel resistor. To calculate it you have to use Ohms law R= V/I and then you have to calculate the watt for the resistor W=V*I. A simple 1/4 (0.25) watt resistor will allow you to increase current only by 50 mA: 0.25 W = 5 V * 0.05 A, otherwise it will become hot and eventually die. To increase current draw above 50 mA you will need resistor with more watt or more parallel resistors.

So you can buy a few 110R 1/4W resistors (slightly less than 50 mA to be safe) and put them in parallel to increase the current to the point that you want.

*This is wasting of energy, the best solution is to hack feedback circuit inside power bank to 'think' that you draw more current but it is too hard and need experience.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.