Switching circuit

[quioxz]

Ok, here's my original concept:
- I need a circuit with LED that the LED only lids when load is present
With some work, I came up with this, please correct if my concept is wrong anywhere:

The R1 is the actual load, it may or may not present, I put switch S1 to clarify so don't bother about moving the switch
In my theory,
I have a +5VDC constant supply
The lower line is the LED, with R2 to reduce current which would be around 15-20mA
The upper line has very little resistant so the current may go up to max of 500mA
Now I want the LED to lid only when the load is present or, in this case, the switch is pressed
So I thought of the transistor and hook it in
I believe the base can not take in high current so I put a high resistor in R3 = 50k and parallel it with R4 which I don't know it's value now and R4 should by very low so very low current goes into R3 just to make the base high and the rest goes to GND via R4

Question:
- Are my concepts correct?
- Can I optimize it?
- With appropriate resistors, will it work? I need as much as 5V 500mA at R1
- If things are fine, what is the appropriate value of R3 and R4?

Since R4 will have a lot of current flowing through, as much as 500mA with 5V, so the power that crosses R4 will be as much as 2.5W and normal resistors are 0.25W, is there such thing as 2.5W resistors? Cost?
I came up with a very bad solution: hook up 10 resistors in parallel to share the current

Question:
- Do I really have watt problem with R4?
- If so, is there any solution to it beside my method?
- Is there a better way to make a circuit that works the way I want??

I know it's a lot but thanks for any and every info

 

[Resqueline]

The power developed in R4 won't be 5V*0.5A, it will be 0.6V*0.5A because that's the target voltage for its resistance calculation: R=U/I where U is a voltage sufficient to turn on Q1, say 0.6V.
The base can actually withstand quite a lot of current (look in datasheets) so R3 can be reduced considerably.
The problem is that the voltage drop over R4 will vary with the load. Task: can you think of some other component that has a more or less constant 0.6V drop?

 

[quioxz]

Thanks, that lid me up a lot
I need another method then?
Because I need the voltage drop over the R1 to be at least 4.8V and 0.6V drop to the transistor would be too much...
So, I assume my diagram will work then, but not up to my expectations

Can you suggest any other method that makes the LED lid when the switch is pressed while having voltage at R1 at least 4.8V and current range from 100mA to 500mA?
And I don't want to make the circuit complex
I am a bit new to electronics so I don't have much knowledge..

You can just give me a guideline on eg. what to use, how it works, and I'll do the research myself

 

[Laplace]

Since you want the voltage drop across R4 to be no more than 200 mV at a current of 0.5 A, you will need to sense when the R4 voltage is just 40 mV at a current of 0.1 A. That seems to be a job for a low voltage comparator, or op-amp run as a comparator, powered from the 5 volt supply. The reference voltage would need to be set around 20 mV.

 

[quioxz]

Will the transistor work then, even though I got R4 right?

 

[Laplace]

Depending on the comparator you select and what the output stage looks like, you might not need the transistor at all to drive the LED. But the transistor would have no function in sensing the voltage as is shown now.

 

[quioxz]

I'm sorry, but I have absolutely no idea what a comparator is since it's not in my high school text
And wikipedia gives too much technical term, so would you mind explaining what it is and how it works?
I assume I can replace the transistor with the comparator?

 

[Laplace]

A comparator is an integrated circuit, similar to an operational amplifier, with two voltage sensing inputs (plus & minus) and a logic level output. Over a wide range of input voltage, if one input is a few mV higher than the other, then it sets the output to that logic level. For your application you need to find a comparator capable of operating from a single supply of 5 V with a sensing range near ground, i.e., 20 mV.

 

[quioxz]

So it acts similar/like a transistor?
Only allow current to pass through a line if current passes through another line?

 

[Laplace]

A comparator is not like a transistor in any way at all.

 

[quioxz]

Then how is it applicable for me then?
I *think* I need something that operates at very low volt and can only let current flow through a line only if current flows through another line, I'm not sure
Maybe there is a better solution

 

[Laplace]

Here is an example of how a comparator might be applicable for you. The LM393 has a open collector output and can directly drive the LED.

 

[quioxz]

Thanks I'll give it a try, one question:
The resistors with no units are in ohms right?
So R4 is 0.4ohms?

 

[(*steve*)]

If this is in answer to an assignment question, I can pretty much assure you that you have the wrong answer.

If this is an assignment question and not something you thought was interesting then it may be better for you to just tell us what the question is and we'll try to point you in the right direction (without actually giving you the answer of course).

If it's just "interesting" then your constraints are probably able to be changed and there are alternative ways of doing what you want to do.

 

[quioxz]

Actually, no. It's not an assignment.
I'm trying to make a device charger that's all.
If there's an alternative way, would you mind sharing?

 

[(*steve*)]

What are you wanting to charge, what are your options for powering it, what current does it require(max), and for what range of currents do you want the LED illuminated?

I'm assuming it required 5V at around half an amp max.

 

[55pilot]

Adding to what Steve has said, what SPECIFICALLY are you trying to charge? Charging a battery is not as simple as applying a voltage and letting the current decrease.

---55p

 

[quioxz]

USB devices.
Standard USB chargers are 5V 500mA
But PC USB gives 5V 100mA

Edit: I don't mind the brightness of the LED, it's a standard 5mm one

 

[(*steve*)]

So you want a device with a usb socket, and a LED that illuminates when power is being drawn?

btw, A PC USB port should give you 500mA unless it's an unpowered hub.

Some USB devices require specific conditions on the data lines or they will remain unpowered.

 

[quioxz]

Thanks for telling, but I've got everything else worked out but only this part, and if it's too complicated then I'll just ignore it.
So, can I actually do it? I only need a small device