Jump to content

Recommended Posts

Maybe a little off-topic, but I need to have redundant power supplies (24V for the PLC) in a system. What is the best approach to this? Two power supplies running all the time in parallel or a relay that switches when a power supply fails? I'm not sure the relay would be fast enough to prevent the PLC and other related 24V hardware from going down.

Any education in this area would be greatly appreciated.

@Joe Tauser, do you have relevant experience with this?

Link to post
Share on other sites

The question is do you need reliable 24V power or uninterruptible 24V power?  If you lose AC power to the system, can the PLC go down?

Reliable power is parallel power supplies as kratmel suggests.  Many power supply vendors have a model that allows connecting two together in parallel with a feedback wire between them so they don't have an amps wrestling match if the output voltages differ.

I had an application a while back which required battery backup, which is uninterruptible power.  If I lost AC power the PLC needed to have a few minutes of DC power available to park some air cylinders that positioned a powered roller.

Look at something like this-


The power supply keeps the battery charged and switches over in "zero time" when power is lost.

Joe T.

Link to post
Share on other sites

+ to all of what Joe says.  But in my cases I have used what used to be available as this:


I haven't needed it for a while, but in hunting it appears to be not available anymore from this supplier. 

But a whole host of them appear on this US site, some of which appear to be the same thing, so I guess they are still being made.

http://www.seco-larm.com/index.php?route=product/search&filter_name=power supply

They have been exceptionally reliable in all the instances of use, and they deliver smooth power at the rated volts no matter what they are running from.  The one headache I have encountered is that people forget to routinely check the battery is ok.  I can go in and find the same battery I put in ages ago, and it has long expired.  Even a good SLA on proper float will eventually die.

The other quirk to remember if you go for the paralleled method, with so much precision in manufacture these days the odds are that the MTBF on any 2 linked supplies might be identical!  Buy two that aren't from the same batch might help this?  But if it is that critical I'd definitely have some spares on hand ready!  And an inspection process to regularly monitor that each supply is still running ok, unless it can output a signal the plc can recognise.

cheers, Aus


Link to post
Share on other sites

Supercaps are great if they are the real thing, they have been around for ages but only now seem to be getting big enough for larger uses.  Many of my older plcs used small supercaps for RTC and values retention and it was amazing how long they'd last from such a small thing that recharged in no time at all.  But most of the makers I use have gone to button types now. 

There is talk around about how many of the supposed supercaps in the bigger supplies coming onto the market are just Lithium batteries in disguise.  Tread carefully.

cheers, Aus

Link to post
Share on other sites

The way we have it at work is just with diodes. Basically what they have done is two power supplies, fed from different UPS', with their positive connected to a bridge rectifier (graetz bridge) AC input. Both power supplies are connected to a different AC input. (If 3 PSUs are required, a 3 phase bridge can be used). The load is connected to the positive pin of the rectifier. During commissioning a load is connected (normally just the complete IO cabinet) and one PSU is switched off. Then the output voltage is regulated on the running PSU such that the voltage at the IO cabinet modules itself, is 24.0V. Then the other PSU is switched on, and the first PSU switched off, repeating the process. Once both are done, both are switched on and left on. The "power good" contact is wired in series with the primary MCB aux. contact, so failures can be detected easily.

The negative connections of the PSUs are wired in parallel to a common rail....

If a PSU dies, there is 0 voltage interruption. If the output voltages are 100% the same (which can easily be achieved by putting a voltmeter between the positive outputs of both PSUs and then voltage regulated to 0.00V) the PSUs will do proper power sharing too. If the voltages are not 100% the same, the one with the highest voltage will be the "primary" PSU. That is also the way to "force" a primary power supply.


All in all a total cost of a few dollars for a 25A (or greater) bridge rectifier, which can be mounted on the panel for cooling...

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...