Re: SMPS needing a load to startup myth?

Question

back when these first came out [remember Boschert?], most OEM units did   require a load. Thus, the start of the myth. some did NOT work during brown out and would burn up. Their specs actually gave a 'range' of input voltage AND a 'range' of   load. Outside those ranges and catastrophe. But that seems stupid because to get from 0 AC to ACmains values, you MUST   be outside that spec range AND to attach a load really makes sense to be   able to go through a zero to max load range, too. Thus, as components came down in price, functionality went up, but the   memories of having a fire burn up your SMPS at 50 Vac stuck.

RobertMacy · Accepted Answer

oops, forgot. at zero load where some of the SMPS's 'skittle' around; they   will fail EMC. Good design [by definition] no problem.

mroberds · Answer

I have one that delivers +5 V, +12 V, and a couple of other things at a couple hundred watts total - along the lines of a PC power supply but not the same shape. It needs a load on the +5 V to start up; I use a

12 V automotive lamp. It probably dates from the early 1990s.

I recall talking to someone online who had a PC power supply that had

1/4 watt resistors across the outputs, each sized to draw a few mA from the output. This was too much power for one of them and it had burned, but I think there were also some other failures too.

I also have a couple of 5 V, 0.5 A wall-wart cell phone chargers, and they at least don't draw much current from the AC line/mains with no load, according to my Kill-a-watt meter. If I unplug/replug the charger several times, I can sometimes catch a 1 W reading on the meter for a second, but it then goes back to 0 W.

For the cell phone chargers, I suspect they are deliberately designed to "gulp" a little power when first plugged in, charge a cap, and then look for a load every so many milliseconds. If the cap gets low, it takes another "gulp", but it doesn't have to do that very often. The goal is that you can leave them plugged in to the wall without wasting a lot of energy.

Matt Roberds

JW · Answer

On Mon, 30 Mar 2015 22:37:29 +0100 "Gareth Magennis" : I work on Tek TDS700 series of scopes quite often, they will not work if there is no load. In fact, if you disconnect the two loads on the +25VDC rail (The HV deflection board and the fan) it will shut down, or oscillate on and off.

Cydrome Leader · Answer

This was true for old switchers, but I've not seen minimum load reqirements on modern ones.

Old switchers with no load would sometimes not work or even make horrible sounds in the magnetics as they failed to regulate properly.

Re: SMPS needing a load to startup myth?

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