I'm designing a system integrating:

  1. an industrial PC with 230V AC input (~300W)
  2. various peripherals requiring 24V DC (150W max)

In case of external AC power loss, the DC components (2) need to be supplied with backup power for up to five minutes. This is not a problem and suitable PSU are available.

However, for a reason I cannot control, the industrial PC requires an advance signal (about 1 second in advance, possibly less) indicating that its supply power is about to be lost.

So I'm looking for suitable 230V AC buffer with power loss notification. Compliance with IEC 60601-1 (3rd) would be a big plus.

I don't know exactly what I'm looking for. I would like to avoid a full-blown UPS, even though a battery is needed for the 24V PSU. Could anyone direct me to the right kind of component?

  • 2
    \$\begingroup\$ A properly-sized UPS solves both issues: keeping the 24VDC power supplies up and providing adequate warning to the industrial PC. I doubt you'll find anything simpler. \$\endgroup\$ Oct 25, 2013 at 16:20

2 Answers 2


Placing UPSes here and there is a primary instinct for almost every automation engineer. But it is not a good solution, especially on big systems (which the discussed one is not). The two main disadvantages of powering from UPS is:

  1. The UPS has batteries inside. All types of batteries degrades with the time and need replacement. But any support work on the industrial machinery is not good by definition.

  2. Using UPS on some part of the machine (usually computers) means that on power loss, the system goes to inconsistent state, that is rarely processed properly by the software. As a result there are often restart problems after power restoration. Such a machines need very careful design and extended testing.

So, on the topic. I would suggest following variants:

  1. Think again - is it really, really so important to keep the PC working after the power fault? Isn't it possible by some software methods to make that shut-down to not lead to information loss?

  2. If the answer of 1. is "No", then use industrial PC powered by 24VDC and use capacitive backup device that will keep the voltage for at least several seconds or minute and will switch it for the whole machine. In addition, it will not need support and battery replacements.

Note: We have the cited Wöhrle devices in several of our plant machines.

  • \$\begingroup\$ Thank you! The answer is "no". It's not because of the PC, but because of a motor component. We need to be able to engage a fail-safe brake prior to removing power from that motor. Trouble is, due to certification issues, we cannot modify this part of the system. Do you know of a capacitive backup device with AC output? \$\endgroup\$ Oct 27, 2013 at 20:31
  • \$\begingroup\$ @DanielGehriger - If you want to turn an AC motor after the power loss, you need UPS. What is the power of this motor? The normal approach to this problem is to use motor with electromagnetic brake - when the brake is powered, it is free, when the power is switched off, the brake stops the motor. \$\endgroup\$
    – johnfound
    Oct 28, 2013 at 4:57
  • \$\begingroup\$ it's about 200W. Yes, we already have fail-safe brakes. Trouble is, they have some inertia and the motor axis will move by <1° after power loss before the brake fully engages. So I need to engage the brakes just before power to the motors is lost. \$\endgroup\$ Oct 28, 2013 at 10:20
  • \$\begingroup\$ @DanielGehriger - And how <1° can affect the safety? Anyway, I am afraid you have to share more details on your project in order to get more adequate design suggestions. You better edit the question. I hope it is not top-secret project. :) \$\endgroup\$
    – johnfound
    Oct 28, 2013 at 10:44
  • \$\begingroup\$ @DanielGehriger if you have a capacitive backup device with DC output that is rated for 12V, you could probably connect it to an AC inverter for an AC output. \$\endgroup\$
    – ajs410
    Nov 20, 2013 at 22:12

I saw a novel solution to this same issue once. An AC motor spins a flywheel to a few thousand rpm. The opposite shaft of the flywheel from the motor is a reduction gear drive that feeds a genset head. When AC power fails, the flywheel stores enough energy to run the genset for several minutes. An led on one side of the flywheel and a phototransistor on the other side allowed a simple pic chip to estimate the amount of time the backup would continue. The PIC also tied a port input line directly to the ac line via a current limit resistor. So a port pin detects presence of AC. and another port pin detects flywheel speed. Another port pin issued a NOT SAFE output for start up when the flywheel had not yet spun up to an acceptable speed. Note that the whole thing was enclosed inside a concrete case for safety, due to the energy stored in the flywheel.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.