Connecting Power Supplies in Series

The outputs of two or more power supplies can be easily connected in series to obtain a combined output with a higher voltage than provided by a readily available standard supply. The outputs of two or more power supplies can also be connected in series if the object is to obtain a higher current at the required voltage, since lower voltage supplies usually have higher current ratings. The power supplies could have different voltages allowing greater flexibility of the final output voltage.

However, three precautions should be observed.

First, the combined voltage should not exceed the output breakdown voltage rating of the supplies. So, for example, don’t connect the outputs of three 200 volt supplies in series if their output breakdown voltage rating is 600 volts or less.

Second, the output connections for each power supply should have a reverse biased diode wired across them (see drawing). This will assure that, if one supply comes up to voltage before the other(s), it won’t apply a reverse polarity voltage to the other(s) through the load and possibly damage it (them) or cause it (them) to lock up. Each diode’s PIV and current rating should exceed that of the entire series string. Note that when remote sensing is being used, the voltage drop in the output wire connecting the supplies is compensated by extending one supply’s sense lead to the output terminal of the other supply.

Third, since the supplies are in series, the current rating of each power supply must equal or exceed the maximum current rating required by the load.

Also, intermediate voltages can be tapped off the connections between power supplies.  For example, if four 12V supplies are connected in series to obtain 48V, the intermediate connections can be tapped for 12V, 24V and/or 36V outputs of the same polarity.  Each power supply must, of course, have a current rating at least equal to the sum of the currents of all outputs that will be flowing through it.  Therefore, if (using the same example) 10A will be drawn at each voltage, the supply between Common and 12V needs to be rated at least 40A, but the others need be rated at only 30A, 20A and 10A respectively.

Some possible combinations:

• 24Vdc@30A power supply in series with a
  24Vdc@18A power supply can be used as a
  48Vdc@18A power supply.
• 12Vdc@60A power supply in series with a
  12Vdc@37A power supply can be used as a
  24Vdc@37A power supply.
• 18Vdc@40A power supply in series with a
  6Vdc@68A power supply can be used as a
  24Vdc@40A power supply
• 18Vdc@40A power supply in series with a
  6Vdc@40A power supply can be used as a
  24Vdc@40A power supply
• 5Vdc@70A power supply in series with a
  6Vdc@68A power supply and in series with a
  7Vdc@66A power supply can be used as an
  18Vdc@66A power supply

Add Comments

Sequencing Power Supplies

It is certainly not very difficult to have a power supply turn on after another is on - simply connect a relay (‘Relay 1’ in the diagram) across the output of the first, and use it to turn on the second. But that doesn’t address the usual additional requirement for the second power supply’s output to also turn off before the output of the first.

Using another relay (‘Relay 2’ in the diagram) across the output of the second supply and a separate input switch pole (‘Switch’ in the diagram) for each supply meets that requirement. The relay shorts the input switch contacts of the first supply and keeps it on until the output of the second supply has collapsed.

A low voltage relay (‘Relay 1’ in the diagram) in combination with a zener diode to make up the difference between the relay coil rating and the power supply output voltage can be used to delay the turn-on of the second supply until the first is almost up to full voltage. And since a relay typically doesn’t drop out until the voltage is down to 25-35% of its coil rating, the output of the second supply will be almost collapsed before the contacts shorting the input switch pole of the first supply open and it can start to shut down. Alternately, relay modules with trip voltage adjustments may be used.

Add Comments

Quickly Alerting Personnel to a Blown Fuse is of Great Value

Time is often wasted before a blown fuse is detected and replaced. This is particularly true in prototype debugging, since the engineer may feel that his unproved design, rather than an accidentally blown fuse, is causing the problem. In unattended equipment such as component life-test racks and process-control systems, a positive means of quickly alerting personnel to a blown fuse is of great value. Fuse holders with neon indicators, though useful, do not adequately satisfy this need, since their signal may not be observed quickly enough.

An audible alarm wired across all system fuses, as shown in the figure, will be activated whenever a fuse blows. The alarm can be one of the small panel-mounting piezoelectric units available through electronic distributors. Diodes are required for isolation when more than one fuse is being monitored, but they also permit the use of alarms that require DC for proper operation.

Add Comments

On the Shoulders of Giants

Acopian manufactures a wide range of linear, unregulated, programmable, high voltage, redundant and custom power supplies, but that wasn’t always the case. In 1957, Acopian Technical Company would have sold you a solar powered radio.

As we launch our new blog to discuss “all things power supply,” we feel compelled to give a nod to the early innovations that helped define the company we’ve become. The solar powered radio applied applied modern science and engineering accomplishments to develop a unique product that was ahead of its time. Here’s a snippet from the original product instruction sheet: 

"The day will soon arrive when a great variety of your household appliances will be efficiently and economically powered by that eternal source of energy… the sun. Meanwhile, you can be proud to be among the first to enjoy the benefits of solar energy while in its very infancy."

Solar power has yet to live up to its potential in the decades since Sarkis Acopian and Thomas A. Workman developed, manufactured, and sold the first ever solar powered radio. The Acopian Technical Company, however, continues to engineer innovative products on the shoulders of giants.

Welcome to our blog. Subscribe. Read. Comment.

P.S. If you still have a working solar powered radio from Acopian Technical Company, let us know the comments!

Add Comments