Understanding Amp Readings on a Lab Bench Power Supply
When you increase the amps knob on your lab bench power supply (PSU) and the meter amps remain the same, with the current increasing only when you increase the voltage, it actually reflects normal behavior under certain modes of operation. This article will help you understand the fundamental concepts and the reasons behind this behavior.
Constant Voltage (CV) Mode
Most lab bench power supplies operate in either constant voltage (CV) or constant current (CC) mode. In CV mode, the PSU maintains a fixed voltage output, and the current (amps) flowing through the circuit is determined by the load resistance.
Current Limit Setting
The amps knob on the PSU is typically used to set the maximum current limit. If the load draws less current than this limit, the actual current will not exceed the load's requirements. Increase the voltage, and if the load demands more current, the meter amps will increase accordingly. This is because the relationship between voltage, current, and resistance is governed by Ohm's Law.
Ohm's Law: I V / R, where:
I is the current (in Amperes), V is the voltage (in Volts), R is the resistance (in Ohms).Load Characteristics
The behavior also depends on the nature of the load you are powering. For resistive loads such as resistors or incandescent bulbs, increasing the voltage will lead to an increase in current as per Ohm's Law. However, if the load is already consuming its rated current at the set voltage, increasing the voltage further will not increase the current beyond the load's limit.
You Are Missing Something Fundamental
While what you are experiencing is normal, there is a misunderstanding of the underlying principles. The amps knob on your power supply is not meant to control current at all. Instead, it sets the maximum current limit the PSU will provide, in case the load shorts out. The actual current drawn by the load is determined by Ohm's Law and the resistance of the load.
Increasing the voltage while keeping the load fixed will cause the load to demand more current, as per the formula I V / R. However, the PSU will not force more current into a fixed load at a fixed voltage. It is crucial to understand Ohm's Law to grasp the behavior of electrical circuits.
Why Ohm's Law Matters
Let's review a historical perspective. In the second day of your first electronics class in grade 10, you were introduced to Ohm's Law. This principle is fundamental to understanding electricity and electronics. Without a solid grasp of Ohm's Law, other concepts in these fields will not make sense. Understanding this will enable you to troubleshoot and operate your lab bench power supply effectively.
On the first day, your class learned the resistance color code. To fully comprehend electronics, you must memorize Ohm's Law and understand its implications. In 1966, you got the concept on the first day it was introduced, and it took you two more days to memorize it. If you or your classmates could not grasp it after a week, they likely dropped the class because nothing else made sense without this foundational knowledge.
Switching to Constant Current (CC) Mode
If you want to limit the current to a specified value, you can switch the PSU to CC mode. In this mode, the PSU will maintain the set current, even if the voltage needs to be reduced to achieve that. This mode is useful for testing devices that require a specific current but can vary their voltage.
Conclusion
The amp reading on your lab bench power supply will remain constant until the load demands more current based on the voltage you set. Understanding the characteristics of the load and the role of Ohm's Law is crucial for any electronics work. If you have any specific scenarios or setups that need further clarification, feel free to ask.
Remember, Ohm's Law is your baseline for understanding the behavior of electrical circuits. Make sure you master it, and you'll find the rest of electronics much more intuitive.