Unsuitable Inrush Protection

Before we get to individually identifying the components that typically fail through bad power, we thought we'd describe one that can create a major challenge to power quality investigators called the NTCR.

The Negative Temperature Coefficient Resistor (NTCR) is used primarily as in-rush protection as it is cold when the equipment is first turned on, and then warms up as current flows through it causing a drop in resistance until a "maintenance" temperature is reached. This temperature is self regulating and is determined when the resistance falls till there is just sufficient power (equal to IČR) dissipated in the NTCR to keep it hot.

The problem is the warm-up cycle may be only a few seconds, but the cool-down time can be many tens of seconds. This cool down period, and therefore the time till the device again offers in-rush protection, far exceeds the time taken for any capacitors etc. to discharge. Should there be any dip in voltage, such as switching off and then back on again within a few seconds, then inrush currents can (and often do) far exceed the capabilities of the components the NTCR is employed to protect!

NTCRs are almost always found in switch-mode power supplies, these now being the standard power supply in PCs. However, the front of a PC also boasts a 'Reset' button, but this button is usually so small it is not even noticed by a user and a PC reset is often done by switching off and back on again (the On/Off button is big and easily used). This strains the input diodes drastically causing them, if nothing else, to weaken.

So now it takes nothing more than one little blip and...... pop! Some 'expert' then gets involved and blames "bad power" and the local electricity supply company is blamed and compensation sought - yet it was the users fault entirely!

One case involved a supplier of UPS systems and was having a problem at one particular site where PCs were being damaged after a fairly hefty UPS had been installed. The client had installed this on the advice of a company who supplied software that was installed on PCs that were prone to 'locking up'. The UPS had a lower feed impedance and the power supplies were now being strained everytime the user used the big 'reset' switch to get out of the lock-up. So much for a power problem!

NTCRs are not just used in PCs, they are used in all sorts of appliances. The common use is in-rush current limiting although some have even been used in timing circuits. The intention is not to discredit the NTCR, but rather to make you aware that they lose their effectiveness when the power is put through an 'Off/On' cycle.

Persistent Fuse Blowing  >>

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