Regulated power supply

[eblc1388]

Question:

Two DC regulated power supplies, with negative terminals connected together, are set to output different voltages. A resistor is then connected across their positive terminals to form a complete circuit.

Let say one is at 8V and the other at 12V. What is the current in the resistor if it has a value of 4 ohm?

 

[dknguyen]

|12V-8V| = 4V.

So it is the equivelant of a 4V supply being connected across the resistor.

V=IR says that passing through the resistor is 1A.

It's just like when batteries are placed in series + to -, the voltage across them adds up. When placed + to + and - to - they subtract. Same dealy.

 

[audioguru]

The 8V supply can source current, but probably not sink current. So I think the current in the 4 ohm resistor will be very small.

 

[Nigel Goodwin]

The 8V supply can source current, but probably not sink current. So I think the current in the 4 ohm resistor will be very small.

This is a school question, not real life!.

 

[AllVol]

My sim shows 1 amp...

 

[AllVol]

... unless you reverse PSU's, then you get negative 1 amp.

 

[OutToLunch]

I'm wondering why eblc1388 is asking this in the first place... I'm familiar with some of his past postings and he appears to be well beyond this simple type of question...

 

[audioguru]

It is a trick question.
The spec's didn't say that the 12V regulator is a series-regulated one and that the 8V regulator is a shunt-regulated one.
If the 8V regulator is a series-regulated one then hardly any current will go into its output.

 

[Hero999]

... unless you reverse PSU's, then you get negative 1 amp.

I don't mean to be rude but try actually caculating the answer rather than relying on a simulation program, I know this isn't an issue here but they can't always be trusted.

 

[AllVol]

My Hero,

I'll try not to be as rude to you are you were to me with your presemptous reference to my lack of ability to work a simple equation. I'm sorry that I and other newbies occasionally darken the threshold to your ivory tower, but we simply want to share a piece of your vast storehouse of knowledge.

My use of two sim calculations was just as you said, an attempt to show that even your brightest products can be skewed.

I will, of course, continue to listen to your words of wisdom as they are issued to we underlings.

 

[eblc1388]

I'm wondering why eblc1388 is asking this in the first place...

Just thought it is an interesting question to ask.

It is a trick question.

By regulated power supply, I meant just ordinary supplies most hobbyists, schools or people are using. Nothing special and there is no trick in the question.

 

[audioguru]

By regulated power supply, I meant just ordinary supplies most hobbyists, schools or people are using. Nothing special and there is no trick in the question.

Sure it is a trick question. It has two answers:
1) An "ordinary supply" could be a zener diode which is a shunt regulator. If the 8V supply is a zener diode then the 4 ohm resistor will supply it with an extra 1A from the 12V supply and probably burn out the zener unless it is huge (more than 8W).

2) An "ordinary supply" could also be a series regulator like an LM7808 or an LM317. Its output is the emitter of an NPN transistor which would become reversed biased by the 12V through the 4 ohm resistor and would barely conduct any current.

 

[Nigel Goodwin]

there is no trick in the question.

There is, as AudioGuru points out!.

Which is why I said it's a 'school' question, and not a real one - typical of the badly thought out questions you get given in exams (usually written by an Emglish or Maths teacher - and NEVER by anyone with any electronics knowledge).

 

[Hero999]

My Hero,

I'll try not to be as rude to you are you were to me with your presemptous reference to my lack of ability to work a simple equation. I'm sorry that I and other newbies occasionally darken the threshold to your ivory tower, but we simply want to share a piece of your vast storehouse of knowledge.

My use of two sim calculations was just as you said, an attempt to show that even your brightest products can be skewed.

I will, of course, continue to listen to your words of wisdom as they are issued to we underlings.

LOL

 

[HiTech]

next time he should sit on a step ladder and post his comment so he feels a bit up in the world.

 

[eblc1388]

Hi gentlemen,

Don't get too carried away with this thread. It's nothing but an interesting question.

It looks easy but the answer requires some thinking and assumptions based on the information given.

Thus I have no idea why it was referred as a trick question, school question or "unreal".

Audioguru had posted two possible answers and I am to add a third. Though his first answer assumes a shunt regulator setup which in my opinion does not fall into regulated power supplies that "ordinary supplies most hobbyists, schools or people are using"

The third possibilies of current in resistor is 3Amp.

Some regulated power supply has output overvoltage crowbar protection.

Bridging a 12V output via a resistor would very likely triggered the crowbar protection(SCR across output) and blown the fuse or protection of the 8V power supply cutting off the AC source. The SCR, however, is kept in conduction via the 4ohm resistor and the 12V supply thus having a continuous current of 3Amp.

 

[audioguru]

The third possibilies of current in resistor is 3Amp.

Sneaky, sneaky and sneaky!

 

[Nigel Goodwin]

It looks easy but the answer requires some thinking and assumptions based on the information given.

Thus I have no idea why it was referred as a trick question, school question or "unreal".

Because it's such a badly written question, it doesn't give enough information to provide a definitative answer - so in that respect it's a 'trick' question, because it doesn't have a single correct answer. It's a 'school' question, because it's the sort of badly written question you get at school or in exams (NOT written by anyone with Electronics knowledge or experience - and they are looking for ONE specific answer, right or wrong!). Lastly it's 'unreal' because it's not a question anyone with a reasonable electronics knowledge would ask.

 

[eblc1388]

Cool it man. Enjoy life.

Because it's such a badly written question, it doesn't give enough information to provide a definitative answer - so in that respect it's a 'trick' question, because it doesn't have a single correct answer. It's a 'school' question, because it's the sort of badly written question you get at school or in exams (NOT written by anyone with Electronics knowledge or experience - and they are looking for ONE specific answer, right or wrong!). Lastly it's 'unreal' because it's not a question anyone with a reasonable electronics knowledge would ask.

Assuming you are the one with Electronics knowledge or experience(trick question as the statement itself is already badly written because it doesn't has enough information to provide a definitative answer), show us all how the question should be properly worded? I'm more than happy to learn from your expertise.

Or perhaps you choose not to.

Lastly it's 'unreal' because it's not a question anyone with a reasonable electronics knowledge would ask.

Sorry I'm still lost at your logic in the comment but that won't affect whether your are real or not.

Edited: Oh I see. That's your compliment that I'm a person with reasonable electronic knowledge, of which I'm not.

 

[eblc1388]

Sneaky, sneaky and sneaky!

I cheated.

Most variable regulated power supplies do not have overvoltage protection in this case of several volts increase.

They do have protection against pass element failure instead.