Power Supply Connections
Let’s start by looking at simple ways to get a negative voltage out of a “positive” power supply (Fig. 1). Almost all power supplies are configured in one of the three ways (see the table). Isolated/floating supplies provide the greatest flexibility, as their outputs aren’t tied to a reference. You can connect them as positive supplies (Fig. 2a). Reversing their leads makes them negative supplies (Fig. 2b). And because they’re floating, you can reference them to ground in whatever way the application requires.
Changing Polarity
In some applications, it’s necessary for the voltage to be positive for some parts of the test and negative for others. For example, you might want to test a battery-powered device’s tolerance to installing the battery the wrong way. The supply voltage would first be applied correctly, then reversed. This is easily done with a relay that switches the polarity of an isolated supply.
Polarity-Reversal Relays
There are several important considerations when using relays to change polarity.
- You will be always be programming positive voltages. The relay determines whether the output voltage is positive or negative.
- Programmable supplies can take a significant amount of time to change voltage. Power supply voltage should be lowered before switching, then brought back up after switching. The time required can be a few milliseconds for a high-performance supply or hundreds of milliseconds for a basic supply. During automated testing, the time consumed when reversing polarity might significantly slow the testing.
- Most programmable power supplies cannot be set for 0 V. Many have a minimum output of 10 mV or a similar small value. Therefore, you might not be able to set up a voltage sweep that includes very low voltages (Fig. 3).
- Mechanical relays take relatively long to switch. The time can be tens of milliseconds. This might affect how quickly devices can be tested.
- The relay breaks the current flow when it changes polarity. This DUT therefore loses power during the changeover. This might not be acceptable if the DUT has to be continuously powered.
- It’s generally a bad idea to open a relay while current is flowing through it. This can cause arcing and lead to premature relay failure. When changing polarity, you should program the power supply to a low voltage, switch the relay, then return the supply to the desired test voltage.
- Multiple relays are sometimes needed to configure a polarity-reversal setup. You therefore need to sequence relay actuation to avoid shorting the power supply’s outputs or the DUT.
- When using remote sensing, you need to flip the polarity of the sense lines. This requires additional relays. As virtually no current flows through the sense lines, the remote-sense relays needn’t have high-current contacts. Be sure the remote sense lines are in the proper configuration before returning the power supply to the required voltage.
3. If a power supply cannot be set for “zero” voltage, there will be a discontinuity near 0 V when the polarity is reversed.
Some power-supply manufacturers offer optional polarity-reversal relays. Though designing your own solution might offer a lower hardware cost, the manufacturer’s solution saves time and minimizes configuration problems. More importantly, it offers an integrated approach. Relay capacity, control, and sequencing are handled for you.
Summary
If all you need is a negative power supply that never has to go positive, an isolated power supply connected “backwards” should work well. Programmable supplies are highly flexible, but if you need voltages “close” to zero, make sure the supply can provide them. Not all can. If you need to switch voltages between positive and negative, polarity-reversal relays provide an inexpensive solution, but with significant limitations:
- Power interruption during polarity reversal
- Increased test-execution time
- The need for careful design to avoid shorting out something
In a future article, I’ll cover two other ways to generate positive and negative voltages that overcome the limitations of polarity-reversal relays.
