switch mode power supplies?

[Hal]

Are these just normal transformerless power supplies?  I read somewhere that they require a load to function.

If i have a 5V, 2.4A switch mode power supply, what kind of load would be required to sustain a tolerable voltage ?

[niftydog]

Are these just normal transformerless power supplies?

"Transformerless" is a furphy.  They still may have transformers in them, but some just have inductors.  The transformers they use are smaller compared to their linear supply cousins.

I read somewhere that they require a load to function.

some do.. not all.  Some have a "dummy" load in circuit so that when the real load is disconnected the circuit isn't damaged or stressed.

If i have a 5V, 2.4A switch mode power supply, what kind of load would be required to sustain a tolerable voltage ?

Use ohms law!

R = V/I
R = 5/2.4
R ­­= 2.1ohm MINIMUM!

However... using the power equation

P = I²R
P = 2.4² x 2.1
P = 12WATTS!

Got any 2.1ohm 12W resistors lying around!?!?  Didn't think so...

Perhaps try 5ohms?

I = 5/5
I = 1A

then;
P = VI
P = 5 x 1
P = 5W

More likely you might have a 5ohm 5W... or atleast be able to get one.

or 10ohms

I = 5/10
I = 0.5A

then;
P = VI
P = 5 x 0.5
P = 2.5W

Still use a 5W, but it won't get bitchingly hot!

[sir_modulus]

Um... Niftydog, don't mean to be rude, but Switching P.S.'s work different than you think. They take the voltage up high and switch it realllllll fastt than at high freq lowers the voltage, get high current.........and after a really long boring, and complex way it gives you low voltages (<24V usually) at high current. Switching ps's need no resistors. 2.4A!!!!!!!!!!! What kind of ps is that. get a comp. supply. up to 50A @ 5V and have 12 V, -5, -12, etc... (small voltages such as 1.5, 3.3, etc....) no load is needed as long as you don't hook up two or more.

[Hal]

so moral: I can play with this thing without being afraid of blowing it up?

[Paul Perry (Frostwave)]

Niftydog, don't mean to be rude, but Switching P.S.'s work different than you think.

Ah, I think they DO work like he thinks, but the explanation was very brief!
Some switchers have a mains tranny at the front, some just go straight to a rectifier driving an oscillator (which, controlled by the switcher controller chip, runs atr a high freq enabling a tiny output tranny feeding a rectifier).
Jim Williams at Linear Technology really wrote the book on switchers, at least the modern version.

http://www.hills2.u-net.com/electron/smps.htm
is a useful intro.
I don't recommend DIY for any connected straight to the mains.
Low voltage switchers (eg make a pair of D cells into a regulated 9v supply) is very useful, though.

[niftydog]

Um... Niftydog, don't mean to be rude, but Switching P.S.'s work different than you think.

Well... I guess four years working on switch modes that run this TV studio has been a complete fluke then...

What exactly are you getting at by saying that?

They take the voltage up high and switch it realllllll fastt

Not necessarily.  fast yes, high, doesn't have to be.  This bears no consequence to the output load that I calculated.

Switching ps's need no resistors.

Some do, some don't.  Trust me.  The small mountain of switch modes on my bench do nothing but whine if they don't have an appropriate load resistor.

after a really long boring, and complex way it gives you low voltages

hmmm.... simple buck convertor;

Sure, low voltages... high current.. that's ONE possible config.  The 5V 300AMP supply in the racks here attests to that.

But what do you make of this?

Abstract. The design and development of various high-voltage switch mode power supplies (SMPSs) for use with streak cameras are described. This work details -15 kV(),  (1 mA) and ±2kV (1 mA) SMPSs for biasing the streak and image intensifier tubes, driving the sweep circuit and positioning the photoelectron beam on the phosphor screen of the streak camera respectively. The design of the high-frequency pulse transformer - the main part of these SMPSs - is also presented.

Switch modes can pass the same voltage, convert it up or down or even turn it upside down.


Look, the upshot is that I know how switch modes work, and ALL switch modes produce a DC voltage at the output.  SOME switch modes require appropriate current to be flowing out in order to maintain proper duty cycle.  DC voltage and a required current can be worked through ohms law to come up with an appropriate load resistance, just like ANY other DC supply, linear, switching or otherwise.

Ignore what the switch mode is doing, forget the high frequency stuff... that's irrelevant.

What the OP is interested in is firing up his SMP and having it work properly.  This might happen without a load resistor if it's designed to cope.  But having a appropriately sized load resistor is NOT going to hurt!

[niftydog]

so moral: I can play with this thing without being afraid of blowing it up?

I'd be more worried about blowing myself up...

What I would do is, if I actually didn't know anything about SMPs, is this.

Hook up a DMM to monitor the output voltage.

Switch it on.  If the DC voltage isn't what you expect, or the thing is making strange noises, immediatly switch it off.

Try connecting a load resistor and try it again.

BTW, usual warnings... these devices can be real dangerous, especially if connected directly to mains power.  Be careful.

Also, they're often fairly useless for audio applications unless extremely well filtered and shielded.

[Hal]

niftydog - thanks for the great information!

I think this is slightly different than those you're used to working with.  I'm pretty sure its from a cable box - my uncle gave my a box of random electronic stuff that he's picked up over the years and doesn't need anymore.  It seemed to fit in the box.

its small - reminds me kinda of the OneSpot adapters.  I tested it wtih my meter (carfully) and read a beautiful 5v with no additional load.  So...looks good :-D

Im not totally sure what i would use it for, just wanted to know if i could do _anything_ with it.

[niftydog]

clean 5V, run some logic!  Buy a PIC and start playing?!

I'm sure it's different, but the concepts are the same.

[Paul Perry (Frostwave)]

Switch mode power supplies are one of the very few things in electronics that can be actually dangerous. Because, if things go wrong (with the ones driven from the mains directly0 stuff doesn't just smoke, it can simply EXPLODE :!:  so, safety goggles are essential. Plus, if it is the type that is directly attached to active mains wiring (no input tranny) you can just 'count me out'! (rather than CARRY me out!

[sir_modulus]

Man I made an idiot out of myself :oops: I was only used to computer switching ps's and custom ones, so they all didn't need resistors, or had them built in. Sorry 'bout that.

[Mark Hammer]

In their favour, however, I will note that small switching supplies have revolutionized wall-warts.  I have a 12vdc/2.5A switching wallwart  that is light, small, and cheap.  Perfect for a small practice amp supply or to power the radio/deck you yanked from your car and set up on your workbench.

[niftydog]

Sorry 'bout that.

is cool.  BTW, my post could have been read two ways.. it was intended to be more of a query than an inquisition!  Don't sweat it!

[niftydog]

Plus, if it is the type that is directly attached to active mains wiring (no input tranny) you can just 'count me out'!

welcome to my world.

Got 13 (eurrgghh, unlucky no.!) such supplys on my bench at the moment.  120W mains powered dual polarity.

What's worse is, they're about 15 years old and Australian made...