Tuesday, April 23, 2024

Choosing The Right Bench Power Supply

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Programmable power supplies are equipped with remote sense capability. Remote sensing is required in applications where load is located at some distance, typically >3m (10-feet) from the power supply output terminals. It solves the problem of voltage drop in the leads by extending the power-supply feedback loop to the input of the load.

The difference in voltage is based on the amount of current and the load lead size and length. It uses a 4-wire connection (Fig. 2) to make sure the voltage you set on the supply is the voltage you get at the device under test (DUT) despite voltage drop in cables that carry current between the power supply and DUT.

Acceptable maximum level of noise in the supply’s output
For powering a very low-voltage circuit, or a circuit that uses or measures very low currents, such as a transducer detector that must pick up millivolt or microampere signals, noise from external sources may cause problems.

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The power supply itself is a noise source. This noise breaks down into two components: normal mode and common mode. Normal-mode noise, which is generated across the power supply’s output terminals, exudes from the power supply’s internal circuitry.

Common-mode noise is Earth-referenced noise originating from the power line and stray capacitance across the main transformer.

Two types of bench DC power supplies are commonly used today, namely, linear and switch-mode.

Linear supplies are simple and heavier because the 50Hz or 60Hz transformer and associated filters are physically larger. The linear topology generates minimum noise on the power supply’s output.

Switch-mode power supplies (SMPS) are significantly smaller, lighter and more efficient than linear power supplies, so these have replaced linear supplies for higher power requirements.

On the negative side, high switching frequency of SMPS generates five to ten times more noise than a linear supply. Whenever it commands minimising noise, choose a linear supply (if one is available) based on power requirements.

Number of outputs required
In many cases, a single output will be sufficient; however, multi-output supplies can sometimes deliver several important advantages.

Triple-output power supplies typically contain two higher-voltage outputs for analogue circuits (to power multi-voltage circuits or to create bipolar power supplies for testing bipolar analogue circuits) and a third output intended to power a digital circuit. For greatest flexibility, make sure that all three outputs are programmable.

If the DUT requires individual isolated power supply sections, a decision must be made to either configure multiple isolated supplies or buy a multi-output supply. The catch is that multi-output supplies can either have isolated outputs or output channels tied to a common point on their low side. When outputs are connected to the same common point, these are not suitable to power circuits that are isolated from each other.

For applications that require powering circuits up and down in a specific sequence, a multi-output supply with independently controllable outputs is usually better than a set of individual supplies.

For applications that require sourcing of more voltage or current beyond a single output’s capability, some multi-output power supplies allow outputs to be combined in series or in parallel. Non-isolated multi-output power supplies cannot parallel the two outputs. To ensure flexibility, look for a multi-output power supply with isolated outputs.

box 1

Interfaces needed
There have been numerous types of computer interfaces over the years with instrumentation. Two of the most popular have been IEEE-488, also known as general-purpose interface bus (GPIB), and RS232 serial communications. Network interfaces (for example, Ethernet) and USB interfaces have also been used.

One question to consider is: Would a digital I/O interface make it easier to generate fault status outputs or control an external relay or status lamp?

Placement of output connectors
Front-panel connections simplify frequent access and are more common with bench power supplies. Rear-panel connectors are generally considered superior for rack based automated test systems—these rarely require changes after setup. However, more manufacturers now offer power supplies equipped with both front- and rear-panel connections. This simplifies transition from bench experimentation to high-speed automated testing, because the same power supply suits both environments.

DIY bench power supply
A good power supply is very useful when working with electronics, but it can be costly. Any ATX (Advanced Technology eXtended) power supply can be plugged into the adaptor and can output 3.3V, 5V, 12V and -12V. (ATX is a motherboard developed by Intel in 1995 to improve on previous de facto standards.) It can be converted into a variable bench power supply. You can add female headers and banana jack (for appliances that draw more current) as output, or even a USB port.


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