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How to use a Variable Benchtop Power Supply
Written By: John R. Sewell

A Benchtop Power Supply is a term for any power source that you can set up on a “bench” and is self contained in that it has a power cord, case and a method of output connection. These can be heavy and large or they can be smaller and easy to move. We generally consider a Variable Benchtop Power Supply as those we use in testing circuits and are more or 0-30V, 0-3A Variable Benchtop Power Supply less portable. There are several classes of power supplies, all of which can be a Benchtop Power Supply from a simple transformer in a box to a processor controlled unit. The discussion here will be based around the more basic Test Bench setup.

In this case, we usually have at least one Variable Benchtop Supply. These Variable Benchtop Supplies have one or more common characteristics; the ability to vary what they output. This can be Voltage, Current, Resistance or Power in many various configurations. Resistance Supplies are sometimes called “Active” load or Forcing Supplies and are special and almost never seen as a Bench Supply but found in automated testers. Some benchtop supplies overlap in combinations of features while others are mutually exclusive. We will start with the most common type, which is the Variable Voltage Bench supply. These supplies have an adjustable output from essentially 0VDC to up into the kilovolts. The maximum voltage will be specified in the description of the Benchtop Powey Supply you are using. The most common of these are the units 0-50V, 0-3A Variable Benchtop Power Supply in the Range from 0V to 50VDC. These will provide the engineer, technician or hobbyist most everything needed to perform testing; a regulated (Constant) output voltage that you set. Variable Benchtop Supplies have a maximum output Current that they can supply as listed in their specifications.

All power supplies follow “Ohm’s Law” so if the voltage is constant (fixed) then only the current and/or resistance (Load) can change. An example is a simple Resistor, say 10 ohms; a 10 Volt supply output to the resistor yields a 1Amp of current, You change to a 20 ohm and the current drops to 1/2A. but the Voltage remains the same 10volts. This is what we call Current compliance. Where the voltage is constant and load changes then the current is forced to “Comply”.

Generally we just set the supply to max setting on the current controls (if available) and set our voltage and test away. BUT what happens if we have breadboarded incorrectly or connected something wrong? The Load draws more than the supplies current capability. Well, either the supply or your circuit can burn up or be damaged. Variable benchtop Supplies have a built in feature called Current Limiting. Even if it is not adjustable, it is factory set to shut down or supply current at the max. rating. Adjustable current limiting models have controls that allow you to adjust this max current between “0” and the specified maximum current to limit any damage to circuits.

Some Variable Benchtop Power Supplies have what is called “Constant Current” mode or True Constant Voltage/Constant Current operation (CV/CC) These will switch from constant Voltage to Current where the current stays at the setting and the voltage will “Comply”. Take Ohm’s Law & resistor again; This time we set our Variable Bench Supply to 10V and initial resistor to 10 Ohms yielding a load current of 1A. Now reduce resistor to 1 Ohm. This yields us 10A current which we’ll say exceeds our supply Max. of 5A. The supply will switch over to current mode and only supply the max of 5A. The output Voltage will “Comply” and drop to 5V (IxR=E or 5A x 1ohm=5V). Many variable Bench Supplies will not operate into a “0” ohm load (in electronics “A Dead Short”.) but limit the current between a control setting & max. They require a minimum output voltage to sense the load. These are “Adjustable Current Limiting” supplies.

Now we come to best feature of our Variable Bench Supplies. Remember that old math class “Can’t divide by Zero”? Take Ohm’s law (E/R=I) or for our example; 10V/0 ohm(Short)= ???). Using our 9616-PS supply; rated 0-30V at 0-3A. We can set this supply as with others to be a true Constant Current supply. Simply set the “Current” controls all the way counter clockwise to “0”, and Output Voltage control to a value safe for your use say 6V, then actually “short” the front panel terminals. After the spark you will adjust the current control to the value you want. Actually you should set a voltage, turn off the supply, apply the short and then turn on the supply to adjust the current control then replace the “Short” with your Load. Now you can also adjust the current up or down as needed. As long as your load is capable of drawing the current available the Voltage will comply to supply the set current. This Constant Current mode is a must when testing LEDs, testing batteries and other devices that test using a fixed current. As a note, we didn’t repeal Ohm’s Law. Our supplies have a built in “Pre load” sensing that allows the output terminals supplies enough energy to allow the supply to ‘think” it’s not a “0” ohm load.

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