AC-DC Transformer Advice

[ericmark]

minimum load problems. It is only the high frequency AC supplies used for halogen lights that have this problem.

Are you sure about that ?

Yes.

Ask yourself what is switch mode and how does it work? It switches on for a time and off for a time and the ratio between on and off is what controls the power and in the main the volts output. There is a minimum time it can be on for and once that point is reached it can't control the voltage. By having a small internal load that is switched on when the external load is not large enough then it's possible to get to zero or use other means to get around the problem, but standard switch mode without some extra circuitry can't start at zero there has to be a lower limit although that limit can be very low.

Using a high impedance volt meter many switch mode power supplies will show zero volts with no load.

 

[winston1]

I'm sure power supplies can be designed to switch off without minimum load.
But I have just measured a selection of switch mode DC supplies, phone chargers, wall warts for set top boxes etc and they all show normal output voltage on no load.

I understand what Eric says about minimum switching time so assume there must be some form of bleed resistor or losses elsewhere.

 

[plugwash]

Ask yourself what is switch mode and how does it work? It switches on for a time and off for a time and the ratio between on and off is what controls the power and in the main the volts output. There is a minimum time it can be on for and once that point is reached it can't control the voltage. By having a small internal load that is switched on when the external load is not large enough then it's possible to get to zero or use other means to get around the problem, but standard switch mode without some extra circuitry can't start at zero there has to be a lower limit although that limit can be very low.

It really depends on the control algorithm. You can for example have a control algorithm that below a certain load keeps the pulse width the same but increases the time between pulses essentially performing a small top-up each time the output voltage drops below a certain level.

Using a high impedance volt meter many switch mode power supplies will show zero volts with no load.

In my experiance it's quite common for power supplies to have worse specifications under no-load but for them to outright fail under those conditions is unusual.

 

[Monkeh]

Ask yourself what is switch mode and how does it work? It switches on for a time and off for a time and the ratio between on and off is what controls the power and in the main the volts output. There is a minimum time it can be on for and once that point is reached it can't control the voltage. By having a small internal load that is switched on when the external load is not large enough then it's possible to get to zero or use other means to get around the problem, but standard switch mode without some extra circuitry can't start at zero there has to be a lower limit although that limit can be very low.

It really depends on the control algorithm. You can for example have a control algorithm that below a certain load keeps the pulse width the same but increases the time between pulses essentially performing a small top-up each time the output voltage drops below a certain level.

This is called pulse skipping and virtually any laptop power supply made in the last decade will demonstrate it beautifully.

Also very common in various set top boxes during standby (and is why I can hear the bluray player across the room when it's switched off).

 

[ericmark]

I did say or other means. With DC a large capacitor can bridge the gaps between pulses but with an AC output more of a problem. Halogen lamp power supplies did seem to always start well above zero but today you can buy switch mode regulated AC supplies which claim the work from zero.

However the guy only want to run a computer fan which is a set load so not really a problem for him.