24vdc power supplies

Mickey

Guest
M
Not knowing exactly what a switching 24 VDC power supply is versa a linar 24 VDC power supply, what is the difference and which is better?

Mickey
 
Well, Mickey, the answer to which is better is "it depends".

Older power supplies were all of the linear type. Big as barns, more expensive, and not very efficient. They were based on traditional electronics devices like transfmormers, diodes, capacitors, etc. The advantage was the output power had very little ripple.

I don't know precisely the technology switching power supplies use, but the name implies some kind of modulated pulse. The output power of switchers isn't as clean, with more noise and ripple. They are more compact and less expensive.

Some sensors, PLC manufacturers etc. reccommend linear power supplies, but I've been using noting but switching power supplies for years without problem.
 
switching 24 VDC power supply: Is it chopper based ? Is it lowering output voltage in short-circuit ? => fusing problems ?
 
Mickey-
Like Tom said, 'better' is a very subjective term. The use of a switcher versus a linear is more application driven that anything else. However I would say that I would not use a linear unless I absolutely have to due to size and capacity issues.

Linear and switching power supplies operate in fundamentally different ways. I don't know what kind of electronics background you have, but as a baseline technology comparison linear power supplies are built around Class A transistor aplifiers while switchers are built around Class C transistor amplifiers.
A linear power supply uses a transformer to get the AC voltage down to something close to the desired DC voltage. This AC is then rectified and is used as a source for the output transisitor. The output transisitor (usually the big can transistor you see sticking out of the bottom of the power supply case) sits between the rectified DC and the output terminals of the power supply. It acts kind of like an automatic variable resistor letting more or less current through to keep the output voltage at the desired level. Because it is in series with the output terminals it will see the full load current of the power supply output. Also, because it is 'on' to varying degrees you end up burning up quite a bit of power across the transistor as resistive heating. So a linear power supply of a given power rating needs to be quite big to get rid of all this lost heat.
A swithing power supply, as seppoalanen pointed out, is based on a chopper circuit on both the input and output sides. On the input side the AC is rectified and then is fed through a transisitor followed by an inductor. The transistor switches the power to the inductor. When the power to the inductor is cut, you get a pretty good sized voltage (works like a coil in a car). this voltage is used to make an internal high voltage bus, in the 300 VDC area if I remember right. This bus is then switched through another transistor driven by a PWM signal to the output of the power supply. The power supply output is filtered with a capacitor(s) so you don't see too much voltage ripple. The two tricks that switchers use to keep the efficiency up are:
1) The transisitors are always either fully on (very low resistance) or fully off (very high resistance) so you don't burn up too much power as heat across the transisitor.
2) Power supplies are power converters. This means that the power going in is the same as the power going out plus losses. The transistors are all on the high voltage side, which keeps the current through the transistors low. This further reduces the heat losses across the transisitors.

Switchers are not without their shortcomings. The biggest is that they sometimes need a minimum output current to minimize voltage ripple. This voltage ripple will show up as a sawtooth wave on top of the DC. Some switchers have a problem with this and some don't. So back to the 'which is better' point, my general feeling is that if the devices you are powering can handle a little voltage ripple the switcher is the way to go.

I suspect this is more than you ever wanted to know about power supplies. I hope it helps some.
Keith
 
kamenges, very illustrative!
You told output voltage have done from 'high-voltage' ~310V with 220VAC supply. In my mind devices must earth for that reason. Linear Power Unit could be made safaty for leakage hihg-voltage. Sorry my English, I dont know all your words !
 
seppoalanen -
Most switchers don't directly rectify the incoming AC and out it on the high voltage bus. They use the inductive kickback of a lower voltage pulsed through an inductor to get the high voltage they need. This is why you get such a large input voltage range going into a switcher (90 - 260 VAC is not uncommon).
I don't know enough about switcher details to know what, if anything, they do about fault connection from the high voltage output directly to the bus. I could see this as a very bad thing, though.

Keith
 
24VDC Power supplies

Thanks to all.Keith, it is a little more then I wanted
to know but a little knowledge always helps.
I was looking at a flyer advertising AutomationDirect PS Series
Power Supplies, it offers switching power supplies at linear supply prices. At that point the question came to me that I didn't know the
difference. Thanks again.

Mickey
 

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