defibrillator protection circuit using neon lamp

Suhale Mohammad

Other Parts Discussed in Thread: ADS1198

TI document (in link: http://www.ti.com/lit/an/sprab36b/sprab36b.pdf) gives circuit diagram of neon bulb used for defibrillator protection circuit. Can you explain how series resistor (=22k) was selected for the neon lamp (MC08010000) as highlighted shown in the attached image. .

over 7 years ago

0 Brian Pisani over 7 years ago

TI__Mastermind 38745 points

Hey Suhale,

The neon lamp will clamp the voltage to ~70-80 V in the case of a defibrillator pulse. The purpose of the resistor is to then limit the current to i = V_clamp/R = 0.36 mA. We want to limit the current since the lamp has some maximum allowable current.

Brian Pisani

0 Suhale Mohammad over 7 years ago in reply to Brian Pisani

Prodigy 35 points

Hi Brian,

Thanks for the reply. Current through series resistor Ir= 0.227 A. I have given calculation in the attached image. Correct me if i am wrong. MC08010000 datasheet gives nominal current as 0.5mA which is less than 0.227A.

0 Brian Pisani over 7 years ago in reply to Suhale Mohammad

TI__Mastermind 38745 points

Hey Suhale,

Ah yes my math is wrong. Looking at the datasheet for the neon lamp, it actually does not seem to specify a maximum current. From what I can tell, I do not believe it would hurt the defib protection circuit to increase the size of the resistors. It would, however, have signal chain implications.

Brian

0 Suhale Mohammad over 7 years ago in reply to Brian Pisani

Prodigy 35 points

Hi Brian,

What are the implications of increasing the series resistance on the ECG waveform. Will it increase the noise in ECG waveform. We are using ADS1198 as front end, what is the max series resistor allowed.

0 Brian Pisani over 7 years ago in reply to Suhale Mohammad

TI__Mastermind 38745 points

Hey Suhale,

Increasing series resistance will increase noise in any signal chain via the Johnson-Nyquist noise. You can find the relevant formula in equation 68 in this document.

Regards,

Brian Pisani

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defibrillator protection circuit using neon lamp