Wednesday, March 28, 2012

What Is Going On When My Power Supply Displays “UNR”?

Most everyone is familiar with the very traditional Constant Voltage (CV) and Constant Current (CC) operating modes incorporated in most any lab bench or system power supply. All but the most very basic power supplies provide display indicators or annunciators to indicate whether it is in CV or CC mode. However, moderately more sophisticated power supplies provide additional indicators or annunciators to provide increased insight and more information about their operating status. One annunciator you may encounter is seeing “UNR” flash on, either momentarily or continuously. It’s fairly obvious that this means that the power supply is unregulated; it is failing to maintain a Constant Voltage or Constant Current. But what is really going on when the power supply displays UNR and what things might cause this?
To gain better insight about CV, CC and UNR operating modes it is helpful to visualize what is going on with an IV graph of the power supply output in combination with the load line of the external device being powered. I wrote a two part post about voltage and current levels and limits which you may find useful to review. If you like you can access it from these links levels and limits part 1 and levels and limits part 2. This posting builds nicely on these earlier postings. A conventional single quadrant power supply IV graph with resistive load line is depicted in Figure 1. As the load resistance varies from infinity to zero the power supply’s output goes through the full range of CV mode through CC mode operation. With a passive load like a resistor you are unlikely to encounter UNREG mode, unless perhaps something goes wrong in the power supply itself.
Figure 1: Single quadrant power supply IV characteristic with a resistive load

However, with active load devices you have a pretty high chance of encountering UNR mode operation, depending where the actual voltage and current values end up at in comparison to the power supply’s voltage and current settings. One common application where UNR can be easily encountered is charging a battery (our external active load device) with a power supply. Two different scenarios are depicted in Figure 2. For scenario 1, when the battery voltage is less than the power supply’s output, the point where the power supply’s IV characteristic curve and the battery’s load line (a CV characteristic) intersect, the power supply is in CC mode, happily supplying a regulated charge current into the battery. However, for scenario 2 the battery’s voltage is greater than the power supply’s CV setting (for example, you have your automobile battery charger set to 6 volts when you connect it to a 12 volt battery). Providing the power supply is not able to sink current the battery forces the power supply’s output voltage up along the graph’s voltage axis to the battery’s voltage level. Operating along this whole range of voltage greater than the power supply’s output voltage setting puts the power supply into its UNR mode of operation.
Figure 2: Single quadrant power supply IV characteristic with a battery load

A danger here is more sophisticated power supplies usually incorporate Over Voltage Protection (OVP). One kind of OVP is a crowbar which is an SCR designed to short the output to quickly bring down the output voltage to protect the (possibly expensive) device being powered. When connected to a battery if an OVP crowbar is tripped, damage to the power supply or battery could occur due to batteries being able to deliver a fairly unlimited level of current. It is worth knowing what kind of OVP there is in a power supply before attempting to charge a battery with it. Better yet is to use a power supply or charger specifically designed to properly monitor and charge a given type of battery. The designers take these things into consideration so you don’t have to!
I have digressed here a little on yet another mode, OVP, but it’s all worth knowing when working with power supplies! Can you think of other scenarios that might drive a power supply into UNR? (Hint: How about the other end of the power supply IV characteristic, where it meets the horizontal current axis?)

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