Stabilised Power Supply with Current Limiting

[Roff]

I've seen many transformers feeling so hot to the touch by just idling without any load connected. This is a sign of inferior core material or some core saturation occurring. Could be all cost related.

Several years ago I bought a Triad isolation transformer. It gets too hot to touch, even with no load. I emailed Triad, saying that I thought it had a shorted turn or something. They wrote back saying that that's the way it's designed. Didn't give me a lot of confidence in Triad.

 

[audioguru]

A cheap transformer gets hot without a load because its primary doesn't have enough turns to make enough inductance. The cheap primary is just a long piece of wire. Sure it gets hot.

 

[Nigel Goodwin]

Didn't give me a lot of confidence in Triad.

Don't you mean gave you none at all?, and never used them again!.

 

[Roff]

Don't you mean gave you none at all?, and never used them again!.

I guess I'm master of the understatement.

 

[Roff]

A cheap transformer gets hot without a load because its primary doesn't have enough turns to make enough inductance. The cheap primary is just a long piece of wire. Sure it gets hot.

Yeah, I thought about that. More inductance means more turns of heavier gauge wire, and that leads to a larger core, both of which raise the cost. Well, I seem to recall selecting it because it was cheap (in the true sense of the word, as it turns out). You get what you pay for.
When I get home, I'll check to see if it's rated for 220V also. If it is, that sucker should be glowing red on 220.

 

[Hero999]

I don't think a transformer manufacturer would make a transformer so poor that its core saturates when it has a rectifier and filter capacitor feeding a load and it is operating within its ratings.

I don't either, but if you connect a 1A load to a capacitor and rectifier connected to a 1A transfomer it will draw huge 3A pulses with an RMS value of 1.414, which will overload it.

I've teasted my theory using a scope, a LM317 constant current source, a transformer, capacitor and a small 0.5ohm current sensing resistor (two 1ohms in paralell.

 

[Nigel Goodwin]

I don't either, but if you connect a 1A load to a capacitor and rectifier connected to a 1A transfomer it will draw huge 3A pulses with an RMS value of 1.414, which will overload it.

The relevent word there is 'PULSES', it's NOT 3A continuous, and the transformer is (or any decent one will be?) designed for it.

 

[audioguru]

I think a rectifier charges the main filter cap with current pulses that are about 10 times the load's current. All power supply parts are designed for it: the transformer, rectifier bridge and main filter cap.

 

[Hero999]

The magnitude of the pulses will depend on the filter capacitor. Somehow I find if difficult to believe that a 10A pulse won't cause the core to saturate, there again the pulses are quite short so I don't know.

 

[audioguru]

I have never measured a cheap transformer that has a cheap soft iron core instead of carbon steel, maybe has a core that isn't even laminated with insulated laminations, and without enough turns on its windings to be efficient. Some guys have and insist that the peak voltage is the same as the RMS voltage due to the transformer compressing the voltage peaks.
I wouldn't say the cheap transformer saturates its core, I would say that its peak current operates it on the curve of its magnetic hysteresis graph, so it is approaching core saturation.

 

[4RESTER]

Use ADJUSTABLE VOLTAGE AND CURRENT REGULATOR:

L200

**broken link removed**

 

[cocostarz]

I need help building a power supply for a project at school... 110 ac to 24 v dc at least 2.5 amp.. please can any one help me out with schematics and diagram and a list of all parts i need thanks..

 

[mneary]

Is this a project for-credit? What level of contribution is expected of you?

 

[bananasiong]

I need help building a power supply for a project at school... 110 ac to 24 v dc at least 2.5 amp.. please can any one help me out with schematics and diagram and a list of all parts i need thanks..

Read from the beginning of the post you will find out something..

 

[skydemon2008]

Do you have any better power supply that is suitable for a radio system?

 

[Hero999]

It depends on what sort of radio you're talking about.

 

[zlatkoA]

original answer

15 million parts instead of a single LM350 3A adjustable voltage regulator IC?
Even an inexpensive LM317 is good for 1A.

audioguru is 100% in right

 

[ranggaperdana_06114]

how about the isolated power supply, do you know it and how to make it?
tanx

 

[audioguru]

I do not know medical isolation requirements but I bet the insulation must be double-insulated.

 

[ranggaperdana_06114]

do you have the circuit of it?