To measure the output characteristics of a DC voltage supply without a current-limit adjustment, the load resistance should be varied from
The correct answer and explanation is:
Okay, let’s break down how to characterize a DC voltage supply, especially one without a current-limit adjustment.
Correct Answer:
To measure the output characteristics of a DC voltage supply without a current-limit adjustment, the load resistance should be varied from a very high resistance (approaching open circuit) to the lowest resistance that corresponds to the maximum safe operating current of the supply.
Explanation (approx. 300 words):
Measuring the output characteristics of a DC voltage supply involves understanding how its output voltage (V_out) behaves as the current drawn from it (I_out) changes. This relationship is fundamentally governed by the load resistance (R_load) connected to the supply terminals, following Ohm’s Law: V_out = I_out * R_load.
To map this relationship, you need to vary the load and measure the corresponding voltage and current.
- Starting Point: Very High Resistance (Approaching Open Circuit): When the load resistance is extremely high (ideally infinite, like an open circuit with nothing connected), the current drawn from the supply is very low (ideally zero). This condition represents the “no-load” state. Measuring the voltage here gives you the supply’s open-circuit voltage (V_oc), which is typically its highest output voltage. Starting from a very high resistance ensures you begin safely with minimal current draw.
- Varying the Resistance Downwards: By gradually decreasing the load resistance, you cause the supply to deliver increasing amounts of current (I_out = V_out / R_load). While doing this, you simultaneously measure both the output voltage and the current for each resistance value. Plotting these V-I pairs reveals the supply’s characteristic curve. As current increases, the output voltage of a real supply will typically drop slightly due to its internal resistance.
- Stopping Point: The Lowest Resistance for Safe Current: This is where the lack of a current limit is critical. A standard voltage supply is designed to deliver a maximum rated continuous current. If you attempt to draw current beyond this limit by using a load resistance that is too low, you risk overheating components, potentially damaging the supply permanently, or triggering internal (non-adjustable) protection mechanisms like fuses or thermal shutdowns if they exist.
Therefore, you must stop decreasing the load resistance when the measured current reaches the supply’s specified maximum safe operating current. The corresponding resistance value (calculated as R_load = V_out / I_max_safe) is the lower limit for your load resistance variation. You cannot safely test down to zero resistance (a short circuit) on a supply without active, adjustable current limiting, as this would demand excessive, damaging current.
In summary, varying the resistance from a very high value (no load, minimal current, max voltage) down to the specific resistance that draws the maximum safe or rated current allows you to characterize the supply’s performance across its intended operating range without risking damage.