# How to choose NTC thermistor?

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 ?

I have made some voltage measurement on the DCDC output.

DCDC Output, on USB plug

DCDC Output, on battery

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)

• Can we see your schematic? – Ron Beyer Mar 5 '20 at 13:52
• 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? – Scott Seidman Mar 5 '20 at 14:01
• 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. – Elliot Alderson Mar 5 '20 at 14:11
• @Ron Beyer, I have added a schematic and some informations, – RomainD2 Mar 5 '20 at 14:22
• That's more of a "block diagram" than a schematic. Can you show how your grounds are shared? – 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.

## Suggestion

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.