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Full disclosure: I'm fairly new to all this hardware-stuff

Prologue:
I have old bluetooth speakers that I wanted to be run from a normal 5V USB-Powerbank. Which worked perfectly but the music was pretty quite.

So I installed a Boost Converter Link to step up the voltage to 9 Volt (Standart power supply voltage)

Here a picture:
enter image description here
I simply replaced the battery input with a USB input

Problem:
As soon as I connect the Powerbank it shuts itself off.
When connecting the Boost Converter to the Powerbank via a USB to TTL Converter it works just fine. So what is the difference? I only have ground and 5V hooked up.

I think I maybe need a capacitor, but I have no clue what kind of capacitor and what other specs to pay attention to.
I went for the cheapest Boost Converter I could find straight from China. Should I maybe invest a bit more?

Thanks in advance

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    \$\begingroup\$ Show a schematic/overview if what you are trying to connect and achive. What is the input requirements of your speaker? \$\endgroup\$
    – winny
    Aug 3, 2017 at 12:15
  • \$\begingroup\$ I added a picture and clarified what i'm doing. The input is 9V and 1 Amp at max volume. My Powerbank can provide up to 2 Amp at 5 V \$\endgroup\$ Aug 3, 2017 at 12:23
  • \$\begingroup\$ How does the boost converter request more than 100 mA from the USB? You need to "open the gate". Google USB power negotiation. \$\endgroup\$
    – winny
    Aug 3, 2017 at 18:37
  • \$\begingroup\$ The USB power is provided by a power bank and not by a computer. I have no clue about the details but it seems to work. Adding a resistor resolved my problem. Have a look at the accepted answer \$\endgroup\$ Aug 3, 2017 at 18:46
  • \$\begingroup\$ Yes, the resistor was probably needed anyway. But if the power bank follows specification, it will only deliver 100 mA unless you "comunicate" with it. Play some Enya or dubstep at full blast and report back how much current you where able to take out. \$\endgroup\$
    – winny
    Aug 3, 2017 at 19:41

2 Answers 2

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When you first run the boost converter, it may draw a huge current (Can go up to 10 times the nominal input current) for a short time (say, a few micro- or even milli-seconds) due to the "empty" input capacitor(s) (That is because an empty capacitor acts as short circuit. As it charges up, its resistance increases). That high current which flows until the input capacitor charges up is called "inrush current".

So, perhaps, the built-in (if any) overcurrent protection shuts down the power bank. Slowing down the start-up action of the boost converter may solve the issue:

schematic

simulate this circuit – Schematic created using CircuitLab

NTC has a high resistance at room temperatures. When you apply the supply voltage, powerbank's output sees high resistance so a small current flows. As the current flows, NTC's resistance decreases (because its temperature rises) so the current increases. That current keeps increasing until the NTC reaches its minimum value. This characteristic of NTC helps reducing the inrush current.

So, how to select the NTC?

Since your equipment draws 1A @ 9VDC input at max volume, your 5V-to-9V boost converter will draw nearly 2 Amps. So you should select an NTC with a current rating of min. 3A.

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  • \$\begingroup\$ Thank you for the answer. I don't have a NTC but I added a normal resistor (actually just a very long wire). Now it works as intended. \$\endgroup\$ Aug 3, 2017 at 16:21
  • \$\begingroup\$ @PeterLudolf Glad to hear that! \$\endgroup\$ Aug 3, 2017 at 17:19
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If you know how much current your bluetooth speaker is using at max volume and how efficient your converter is you can add a limiting resistor in series with it.

Let's say your bluetooth speaker needs 500mA@9V at max volume and your boost converter works at 90% power efficiency(probably more if it's a switching one) but lets just assume that cause it's a cheap module.

0,5A*9V = 4,5W. Those 4,5W are 90% of the energy your power bank has to supply. So we calculate the 100%: (4,5/9)*10=5W, 5W = 1A * 5V. Your powerbank is probably just a 1A rated one anyways, but if the boost converter doesnt have a limitation build in and for example charges a cap from your 5V it can easily exceed 1A. Adding a 5 Ohm Resistor should limit your 5V output to exactly 1A, but as 5 Ohm resistors aren't common you can use a bigger one (10 Ohms) and instead add a big capacitor(+100µ) on the converters output.

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  • \$\begingroup\$ The max current of the speakers is 1 Amp at 9 Volt. My Powerbank can provide up to 2 Amp. But the Powerbank shuts itself off as soon as I connect it and not a max volume. When I'm using a USB to TTL Converter to connect the power bank to the converter it works fine. \$\endgroup\$ Aug 3, 2017 at 12:30
  • \$\begingroup\$ So your bluetooth speaker used 6 AA/AAA Batteries? Or how did you supply 9V before? What kind of battery was inside? \$\endgroup\$ Aug 3, 2017 at 12:35
  • \$\begingroup\$ It uses 4 AA batteries. But the power supply runs on 9 V. \$\endgroup\$ Aug 3, 2017 at 12:38
  • \$\begingroup\$ And you are sure that applying 9V to the battery terminal works as intended despite the fact its 50% more voltage than what the batteries supplied? It's possible that even tho the power supply provided 9V it got divided down to 6V or so. \$\endgroup\$ Aug 3, 2017 at 12:40
  • \$\begingroup\$ Yes I am sure that it works, but I am not sure if it might be harmful to the speakers. When using a USB to TTL Converter it works with 9V. \$\endgroup\$ Aug 3, 2017 at 12:48

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