# Switch mode PSU minimum load

Alright, I know I am going to get a lot of flak for this, but let me reassure you that I have already spent the last few days reading on Google while going back and forth from the bench to test different things. I have taken numerous seated/structured classes on basic circuit design and basic electrical engineering courses, yet it seems every time I take a break and come back to the material in application, I can never seem to grasp the most basic concepts. The math of Ohms law is simple and makes perfect sense, but am I the only one that seems to still confuse things in application? Ok, enough setup (and please dont edit my question. I included that information for a reason...).

I am testing a new off the shelf switch mode power supply. I don't have all that much experience with them, in fact I just recently learned about their minimum current requirements. That's the problem I am trying to overcome.

This power supply requires a minimum load to turn on. Can you please help me determine the proper minimum dummy load to get the power supply to turn on and stabilize?

If the supply (RT-125D http://www.mouser.com/ds/2/260/RT-125-SPEC-806385.pdf) has a minimum current of .1A on V3 (12V rail), does that mean I can throw a 120 ohm resistor (R=V/I R=12v/.1A) across the terminals to get that minimum load? Also, that resistor should be rated for at least 1.2W but realistically, should probably be rated for quite a bit higher right? Maybe 2W - 4W? (P=V^2/R P= 12v^2/120ohms) Is my thought process right so far? That should be enough minimum load to at least get the supply to turn on right?

Side note: This supply may not always have a load in its final application which is why I need this dummy load. Or... I guess I need to find a supply with no minimum load requirement (which I understand does essentially the same thing I am trying to do here anyways, just inside the supply itself)

Any help here is greatly appreciated.

Thanks

• Can you write just a single phrase what's the question? May 19, 2016 at 18:23
• When I dont write enough detail people get mad and tell me im stupid and didnt do my research. When I put too much detail people get mad and tell me to write just a single question... There is no way to please everyone on this site. Can you please just read my entire post? The question is pretty clear in there. May 19, 2016 at 18:27
• You have a question in your text which makes no sense 'How can I test the power supply (achieve that minimum load) so I can use this supply for my application?' I think it would be an improvement to delete the bold section. May 19, 2016 at 18:35
• I edited the bold section so it makes more sense. May 19, 2016 at 18:37

Your calculations are correct. A 120 Ohm, 2W resistor is fine for the 12V rail. A 4W resistor might not get as hot if it is physically larger.

The spec sheet implies a minimum current for each of the 3 outputs, but you may get reliable operation with less load than this implies. For the 5A rail, you would need a 10 ohm, 10 watt load to keep it in spec, but the spec also relates to the regulation (+/- 1%) and you probably don't need such a load to keep the PSU turned on.

You need to apply a dummy load to account for the worst-case operating conditions, but can safely use a higher current load temporarily. If you want to experiment and save some power, I'd try ~220 Ohm on each rail, and see if that made it happy (but I'm guessing here).

• I have several of these chassis and each one may use a different combination of voltages. For example, one may only use the 12v rail, while another chassis may use 5v and 12v or possibly all three. If I only use the 12v rail, do I still need to put a dummy load on the other unused rails too? May 19, 2016 at 18:52
• Maybe. It depends on the design. You may be able to get a more detailed specification which has more detail. May 19, 2016 at 18:57

These types of power supplies use 2 output transformers in series. The main supply is 5 volts with a minimum load of 2 amps. The 24 volt and 12 volt outputs are supplied by a second transformer in series with the one that supplies 5 volts.

I have worked with and installed many of these many years ago. By placing a minimum load (2A) on the 5 volt line the 24 volt and 12 volt outputs would work as expected. We placed a 2 ohm 20 watt brass screw-mount resistor on the 5 volt line as a minimum load.

The 24 volt and 12 volt lines have min/max specs due to the linear IC regulators they use. The 5 volt line is the one used for the main voltage ref and may not be stable without a minimum load on it.

You do have the option of using the 12 volt line (use a 2 watt 120 ohm wire-wound resistor), or consider buying one with built in minimum loads, and the extra part issues go away.

• You wouldn't happen to have a Mouser part number would you? May 19, 2016 at 19:35
• I can check, though I tend to buy some things from Digi-Key, which has a larger inventory. What voltages did you need? What amp rating?
– user105652
May 19, 2016 at 19:40
• If you only need one voltage, a sealed SEPIC type power supply might work for you. The (-) output is usually earth grounded. They cost about a dollar per watt.
– user105652
May 19, 2016 at 19:46
• Already picked up a few per your recommendations. Went with 25W though as they were available. Will give these a shot and see how they work. Thanks for the tip. Learning more every day :) May 19, 2016 at 19:46
• Sparky, I initially picked these supplies with 5v, 12v and 24v because I bought them in bulk to go in several very similar chassis. Each chassis may have slightly different voltage requirements in the future, but I wanted to bulk purchase a supply that would be standardized across all my systems so its easy to replace. May 19, 2016 at 19:50

The outputs CH2, CH3 are not regulated, simple diode rectifier, caps,..when there is no load at the output, the voltage is probably not in the tolerance window. So, if you don't need those voltage outputs, you don't have to put anything, this has no influence on 1st channel.

• Hmm are you positive? With no load the power supply does not work. I turns on and off constantly as if it is ringing up and shutting itself off. Once I load it up slightly (x2 12v fans) the power supply finally turns on and stays on and the voltage stabilizes to about 12v. May 19, 2016 at 18:35
• @Atomiklan 1st: I didn't know that you know that it needs minumum load - this because you wrote too much for description, now it's more clear when bolded. 2nd thank you for info, I had never such issues with industrial brands SMPS, so good to know, I won't buy it. May 19, 2016 at 19:05