I would like to make a siren on an UPS backup. I used this board with an 18650 embedded battery: https://www.aliexpress.com/item/32911096857.html?spm=a2g0s.90423184.108.40.206db4c4dn31JsH
And this siren: https://www.aliexpress.com/item/4000049267294.html
The UPS has an 5V output, and I use a DCDC step-up converter to drive the siren with 12V.
When the UPS is connected to the USB power supply, and the siren triggered, the system is working fine! When the siren is ON, and I remove the USB power supply, everything is still working fine.
However, when I try to trigger the alarm when the USB power plug is disconnected, the UPS goes in security and the siren never start.
I think that it is because of the inrush current of the siren.
I bought a 33 Ohm EPCOS NTC (Ref. B57153S330M51) to add it in series with the siren.
It is working now on the UPS only; however, the siren is not loud as without the NTC. It seems that the resistor value goes too slowly to 0 Ohms.
The consumed current of the siren is about 300-400 mA. How can I choose the right NTC in order to have the loudest sound of my siren after only 1s ?
Thanks for your help!
Aditionnal informations :
I have made some voltage measurement on the DCDC output.
My project only use chinese modules, so I have no real datasheets... My schematic :
Siren consumption under 12 V : between 0,25 and 0,35A
Edit 2 : Voltage on the 5V UPS output (yellow curve)