ACS NTU-7550 (RTU) Power Supply Replacement

Power Supply Problems

We have a SCADA system from Advanced Control Solutions.   While it is an older system, it works well for us.  Due to the antiquated relays we are using in many of our substations, we don’t have much more than a bunch of binary states to monitor.  Analog is connected to PT/CT direct to the NTU.  This configuration is going to change in the future when we upgrade the substation’s equipment.  For now, we must work with what we got.

Steve came to me about some AC power supplies that were failing.  He mentioned that the ones that are failing have been in service for over 15 years.  Since being here, I have been adamant about moving all AC power supplies to 12-48VDC – whatever voltage our batteries are on site.  This has eliminated the need for placing a UPS in every cabinet.  In our communications tower sites, I have installed DC-to-AC inverters.  Since going this route, we have had a significant decrease in power related failures and reduced the number of batteries we must maintain at each site.  DC-to-DC converters are being used for equipment we can’t get DC power supplies in higher/lower voltages.  Up to this point, one component would fail due to a power issue rendering the entire system useless during outages.  The AC UPS’s that powered the AC equipment were usually the culprit in all of the failures.  Since moving to an all DC power plant, there is only one power source to worry about.  Failures decreased.  Maintenance costs also decreased.

The ACS NTU’s were powered by AC power supplies.  Knowing that the NTU series of RTUs are becoming obsolete, there are only two power supplies ACS manufacturers as replacements.  They cost $1,500-$1,700 depending on input voltage.

Steve told me that he used up all of the replacement stock and needed to order some new NTU power supplies.  After seeing the cost of each, I asked what was so special about them.  He gave me the manual and we hacked open an old busted power supply to see its guts.

What we found was nothing special.  It was a switched power supply that looked like many others.  The manual was nondescript.  It didn’t describe all the output pin voltages.

Direct from the ACS NTU manual

Testing the output voltages without load was interesting.  They varied from the NTU-7550 documentation.  Once we used a resistor to add load, I determined that the NTU required two +24, +12, -12, and +5.  “Well, these voltages a simple ATX power supply can provide.  Want to try it?”

The Idea

I went to and purchased two DC-DC PicoPSU ATX power supplies.  These power supply’s intended use is to put computers in a vehicle and power them off of the 12VDC supply.  An automobile environment is a rugged place.  The automotive voltage going into the supply are not constant nor are they entirely clean.  These ATX power supplies are ROHS compliant, are filtered, made for harsh conditions, and have a 2 year warranty.  This is a perfect fit for a SCADA field environment.  The PicoPSU can accept voltages from 6-34VDC.  Being that all of our NTU cabinets have 12VDC batteries, the PicoPSU could be used to power the NTU and eliminate the need for an AC UPS.  I figured that I could cut a ATX extension cable to make pigtails and peel off the necessary voltages. Also, we could short the PC power button so that the power supply is always on. So, I ordered some extension cables.  The total order was $110 including shipping for 2 ATX DC-DC power supplies.


Bench testing was a snap.  After hooking up 12VDC to the supply, a green SMT LED began to flash.  We shorted the PC power switch with a jumper (pins 13 & 14) on the ATX header.  The green LED went solid.  We were then able to measure voltages from the ATX header with a volt meter.  Steve came up with the following schematic to hook the PicoPSU to an ACS NTU-7550:

Steve's ATX power supply schematic. Note: +24V to J3 is provided by wiring the ATX PicoPSU -12VDC (pin 12) and +12VDC (pin 10) in series.

We ran the NTU on the bench for two weeks while running random relay operations once every minute during the burn-in test.  We experienced no problem during the test weeks.

Burn-in testing

Steve used a hot-glue gun to mount the PicoPSU to a DIN rail clip and cleaned up the extra cables.  The end result was impressive looking.



This power supply has been in service since August 2011.  We have experienced no issues with it.  So, you can use an ATX power supply to power an ACS NTU.  This will save you between $1,400-$1,600 and gives you a wider range of voltages to power your ACS SCADA system.

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