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.9042311.0.0.24db4c4dn31JsH

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.

DCDC Output, on USB plug DCDC Output, on USB plug

DCDC Output, on battery DCDC Output, on battery

My project only use chinese modules, so I have no real datasheets... My schematic :

Schematic with NTC

Siren consumption under 12 V : between 0,25 and 0,35A

Edit 2 : Voltage on the 5V UPS output (yellow curve)

5V UPS output (yellow curve)

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    \$\begingroup\$ Can we see your schematic? \$\endgroup\$ – Ron Beyer Mar 5 '20 at 13:52
  • 1
    \$\begingroup\$ Why do you think the siren has a major onrush current? Can't it just be asking for too much current, period? I can't get at your siren link. Can you post the relevant data sheet sections? \$\endgroup\$ – Scott Seidman Mar 5 '20 at 14:01
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    \$\begingroup\$ Yes, please, in order to offer any useful advice we need to see the schematics for your board, siren, and battery backup device. If you don't have that documentation and the items are not working properly then you should return the items to the seller. Consider it a lesson learned: always buy from reputable vendors who offer the needed documentation. \$\endgroup\$ – Elliot Alderson Mar 5 '20 at 14:11
  • \$\begingroup\$ @Ron Beyer, I have added a schematic and some informations, \$\endgroup\$ – RomainD2 Mar 5 '20 at 14:22
  • \$\begingroup\$ That's more of a "block diagram" than a schematic. Can you show how your grounds are shared? \$\endgroup\$ – Ron Beyer Mar 5 '20 at 14:25

Obviously to me is your DCDC convert is unable to meet the inrush current and the NTC is too high impedance.

To solve the problem you need to define some design specs for step load regulation ESR and current limit based on voltage and impedance. There may be a better bandaid NTC solution but likely inadequate.

  • perhaps a reed relay bypass of NTC is best. with a closure time of 20 ms where the DCDC has recovered from the step load.

The only siren specs I could find with my english corrections are:

Rated voltage: 12V DC
Sound index: 120dB
Sound frequency: 3.8KH2+10%
Transmit speed: 3Hz increase or decrease 10% (me) this is frequency modulated +/-10%

Siren DC resistance

  • Measure it then model the impedance as a RC//R to match test results with 20ms voltage recovery time of 22ms with USB connection.


simulate this circuit – Schematic created using CircuitLab

  • I presume this is a magnetic speaker style siren not piezo.
  • If it were like a tweeter speaker the DC resistance would be 3/8 of the rated impedance
    • or 3 Ohms DC for an 8 Ohm speaker.
  • If it were like a linear motor the DC resistance would be 8 ~ 10% of resonant frequency impedance meaning like a DC motor inrush is 10 ~ 12x the operating current.


If it were me , I would use a tiny 12V battery with ESR < 100 mOhm charged by "UPS" with a float charge. This gives the >>> 1 Farad needed with low ESR needed to start the siren. A Buck then Boost DCDC will never hack it. Not enough stored energy in a small package and too high ESR.

  • fix up all power and signal interfaces to suppress 5V Oscillations from Boost regulator.
    • Start with a transistor active load test to simulate Alarm step load 400mA@12V= 5W .
    • For example disconnect Boost regulator and test with NPN or switch and 0.5 to 2 Ohm resistor with a loop smaller than a C with your fingers.
    • Use 20MHz filter because your probes will have step resonance >20MHz with a long ground lead.
    • Use better EMI techniques for RF interference, Like twisted pairs only. You have an EMI problem easily fixed with experience.


simulate this circuit

  • \$\begingroup\$ Thanks for your answer. I still doesn't understand why it can be the dcdc converter as it is working when the ups is on the power line? \$\endgroup\$ – RomainD2 Mar 5 '20 at 22:30
  • \$\begingroup\$ Why did you now show the 5V output? It is shutting off it seems \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 5 '20 at 22:43
  • \$\begingroup\$ The UPS has two output : 3.3V/1A - 5V/4A. I use the 3.3V to power the ESP01, and the 5V to power the DCDC step-up converter which give me a 12V to power the siren. Graphs above are measured at the 12V DCDC output when I am switching the siren ON. The first graph is with the USB power connected. We can notice a voltage dip for 20ms, but after that, we come back to the 5V, ans the siren is still working. The second graph is with the USB power disconnected, so the UPS is on battery. Here, we can see the same voltage dip, but after 10ms, the UPS enter in security mode and switch off. \$\endgroup\$ – RomainD2 Mar 6 '20 at 8:00
  • \$\begingroup\$ Again. Where is the scope trace of the 5V input to DCDC boost conv.? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 6 '20 at 8:02
  • \$\begingroup\$ Again, when the siren is turned on with the USB power, and I unplug it in order to be on battery supply, the siren is still working. That why I think about an inrush current that the UPS only on battery can't accept. \$\endgroup\$ – RomainD2 Mar 6 '20 at 8:03

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