What is a Limited Power Source?

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There are lots of technical particulars relating to Limited Power Sources (sometimes called an LPS) lined within the IEC60950-1 security customary, involving a number of functions, however I'll cowl simply the fundamental points relating to AC-DC power provides on this weblog article.

A number of security requirements discuss with IEC60950-1 for a wide selection of functions, and a kind of referrals entails the usage of Limited Power Sources.

What is, and why are Limited Power Sources vital? Merely put, if a piece or digital gear supplying DC power to external gadgets is to be put in by a third occasion, reminiscent of an electrician, the danger of wiring fires & electrical shock must be minimized. That electrician is not going to be anticipated to know all of the potential fault situations and use the suitable cable thicknesses and insulation to cowl these hazards. Through the use of a Limited Power Source, the system wiring may also be decreased, saving price.

If a Limited Power Source is used, then the electrician’s job is simplified, even when there is a (single) fault inside the power provide.

Some examples of testing that the power provide producer will do, to find out the utmost output present and power meet the “Limited Power Source” necessities are:

  1. Examine the power provide at most rated load
  2. Improve the load of the power provide till it is on the verge of overload
  3. Simulate an internal fault by shorting the current-limiting resistor
  4. Quick out the opto-coupler that gives suggestions to the management loop

The situations for a “Limited Power Source” AC-DC power provide are:

1. For a power provide rated at 30V or much less, the next have to be met even with a single fault situation:
    a. The output present should not exceed 8A
    b. The output power should not exceed 100W
2. For a power provide rated above 30V, however not exceeding 60V:
    a. The output present should not exceed 150 ÷ Vout
    b. The output power should not exceed 100W

This graph exhibits the boundaries for Limited Power Sources:

Taking some examples from TDK-Lambda’s DSP collection of low profile DIN rail power provides:


The DSP60-5 is rated at 5V 7A – That might meet (1b), however when put into overload it will actually not meet the (1a) restrict of 8A. This mannequin is not listed as being a Limited Power Source.

The DSP60-12 is rated at 12V 4.5A – Overload present can be round 7A which meets (1a) and the utmost power is ~ 84W (12V x 7A) which additionally meets (1b). This mannequin is listed as being a Limited Power Source.

The DSP100-24 is rated at 24V 4.2A – Proper out of gate this unit exceeds 100W, and so it is not listed as a Limited Power Source.

The DSP100-24/C2 is rated at 24V 3.8A – This mannequin really has a particular present restrict & over power circuit which strictly limits the output present and power beneath a fault situation, so it meets the <8A requirement of (b) and the restrict of <100W. Curiously although, a single fault on the management circuit made the output rise to 30.8V and so it now falls beneath the 30-60V restrict of 150 ÷ Vout = 4.8A most present (2b), which it handed. Due to this fact, this mannequin is listed as a Limited Power Source.


Check outcomes carried out by the protection our bodies, reminiscent of UL, CSA or TUV, are usually discovered within the CB report for the power provide.