Posts Tagged ‘harmful interaction’

Systems Engineering In Medical Device Design – Instructions, Part 4

Sunday, January 27th, 2013
     Last time we wrapped up our discussion on the development of quality control instructions for use during the Development stage of the systems engineering approach to medical device design.  These instructions are used to guide quality control inspection and testing during the Production stage.  Now let’s continue our discussion on the development of instructions for the Utilization stage, the stage when the medical device is actually put into operation by the end user.

     In the systems engineering approach to medical device design, design engineers must work closely with technical writers, those responsible for writing operating and product service instructions.  The objective here is to share the engineering staff’s intimate knowledge of the medical device’s design with the writers in order to ensure that instructions are clearly written, comprehensive, and follow a logical progression.  Instructions must be written so as to be easily understood by lay people outside of the engineering profession and medical device industry, because most of the individuals using the device will be healthcare professionals and service technicians, individuals lacking a background in engineering or medical device development.

    Instructions are not only meant for the eyes of end users.  They are also subject to review by governmental agencies.  This fact acts as a safeguard to ensure device compliance both with regulatory requirements and industry standards as regards cautions and warnings.  For example, instructions may be required to caution the user to allow the device to warm up for a certain period of time before use to avoid patient discomfort when coming into contact with cold metal.

     Instructions might also warn against a harmful interaction if the device is used in conjunction with other devices. For example, an electronic muscle stimulator may send electrical pulses into a patient’s body that can interfere with the operation of their heart pacemaker.  No doubt this is something that the operator of the device and the patient would want to be informed of.

     At this point our medical device design has been completed, and instructions and procedures written, but the Development stage is not yet complete.  Next time we’ll continue our discussion on this stage to see how a systems engineering step helps us to be safe rather than sorry after full production of the device has begun.


medical device design

Transistors – Voltage Regulation Part V

Sunday, August 19th, 2012
     I’m sure you’ve seen the television commercials warning about harmful interactions between prescription medications.  By the same token electronic circuitry can also be adversely affected by certain combinations of electrical components, as we’ll discuss in today’s blog.

     Last time we looked at a circuit schematic containing an unregulated power supply.  This power supply was connected to an external supply circuit containing a number of components such as electric relays, buzzers, and lights.  Each of these components has a resistance factor, and combined they have a total resistance of RTotal.  We saw that when RTotal increases, the electrical current, I, decreases, and when RTotal decreases, I increases. 

     In contrast to this increasing/decreasing activity of the total resistance RTotal,  the fixed internal resistance of the unregulated power supply, RInternal, doesn’t fluctuate.  Let’s explore Ohm’s Law further to see how the static effect of RInternal  combines with the changing resistance present in RTotal to adversely affect the unregulated power supply output voltage, VOutput, causing it to fluctuate.

unregulated power supply circuit

Figure 1


     In Figure 1 RTotal and RInternal are operating in series, meaning they are connected together like sausage links.  In this configuration their two resistances add together as if they were one larger resistor.  

     Generally speaking, Ohm’s Law sets out that the current, I, flowing through a resistor in an electrical circuit equals the voltage, V, applied to the resistor divided by the resistance R, or:

I = V ÷ R

     In the case of the circuit represented in Figure 1, the resistors RInternal and RTotal are connected in series within the circuit, so their resistances must be added together to arrive at a total power demand.  Voltage is applied to these two resistors by the same voltage source, VDC.  So, for the circuit as a whole Ohm’s Law would be written as:

I = VDC ÷ (RInternal + RTotal)

     But, Ohm’s Law can also be applied to individual parts within the circuit, just as it can be applied to a single kitchen appliance being operated on a circuit shared with other appliances.  Let’s see how this applies to our example circuit’s RTotal next week.