12V, 15V, 18V, 24V - What are the pros and cons?

Thread Starter

mengjialyu

Joined Sep 4, 2020
4
Dear community,

I am interested in power management in medical devices and am doing research on ventilators.
I have found through internet that most ventilators have an operating DC voltage of 12V or 24V. I also note that most medical AC/DC power supplies provide one of these four voltages (12/15/18/24). I would like to know:

  1. 1. Why those numbers specifically?
  2. 2. Are there pros and cons associated with picking one voltage level versus another? (assuming that buck/boost converter are readily available to transform the AC/DC voltage to desired levels.
Thank you !!
-Mengjia
 

Papabravo

Joined Feb 24, 2006
14,393
I don't think you'll find an overarching set of principles that drive those choices. They can all be conveniently realized with one or more battery technologies but I wouldn't want to hang my hat on that proposition.
 

MisterBill2

Joined Jan 23, 2018
6,997
Three of those voltages are related to the nominal voltage available from storage batteries, while the 15 volt standard was used for analog IC devices and semiconductor analog circuits because it provided adequate "headroom" for analog processing of signals with a zero to ten volt range, a long-time standard.
12, 18, and 24 volts arose from multiples of the 3-cell(nominal) six volt battery of olden days. 12 volts was and is an automotive standard and thus there are many products designed for that voltage. 24 volts was the standard voltage for a whole lot of military equipment and aircraft equipment, as well as a current standard voltage for a large amount of industrial control equipment. So there is a huge selection of 24 volt devices. In addition, 24 volts allows twice the power of the same current as a 12 volt device. One other thing is that 6 volts was the filament (heater) voltage for the majority of vacuum tubes for many years, and it was very convenient to have them in series and use 12 volts for power.
I am not familiar with 18 volt systems , other than they may derive from some number of lithium cells.I am not aware of 18 volt devices being available and so I would not regard it as a "standard" voltage.

In addition, there are shock hazard considerations and all of these voltages are considered to be non-hazardous.
 

Reloadron

Joined Jan 15, 2015
5,646
I have found through internet that most ventilators have an operating DC voltage of 12V or 24V.
I believe you will find those are likely the two most popular battery voltages. They are common in many devices like inverters and other medical devices. We had a ventilator for my mother-in-law before she passed away. While it ran on AC mains power it also had a 12 VDC backup battery internal and a jack for 12 VDC external power. The idea was if there was a power outage the internal battery offered 20 min of support. You will also fine devices have a power use rating normally expressed in watts. So if I have a 24 volt 48 Watt unit it will draw 48 watts / 24 volts = 2.0 amps. Getting the same work done using a 12 volt battery would require 48 / 12 = 4.0 amps. With that in mind you have consideration for things like battery and wire gauges and internal components, like the motor(s) in a ventilator or other devices. About all I have seen is 12 and 24 volt devices here in the US. As to pros and cons? Depends on how much power a device uses making one battery or another the better choice.

The newer ventilators we see in use today are pretty nice machines able to easily precisely control the pressure, volume and rate of breathing oscillations on a patient. The unit we had here in the house while being smart was nothing like today's systems. It was able to display how much of the work it was doing verse what the patient was able to do on their own. Eventually as more and more equipment was moved in I worried about a power outage so eventually got a generator for backup. Anyway as I am sure you know backup power is essential in devices like a vent machine.

Ron
 
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