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Need 5V and 1A

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Tan9890

New Member
Hi..
I'm making an 8051 programmer. Our sir has asked us to put in a transformer to convert main AC to 5V dc 1A..
But the circuit is getting too heavy, so i thought of getting the power supply from a regular USB port..

Now, the USB gives 5V n 500ma.. All i need now is to somehow convert this 500mA to 1A..

Can any1 suggest a method to achieve this?

Or simply, is 500mA enough for the programmer circuit?
Thanks.
 

MikeMl

Well-Known Member
Most Helpful Member
Hi..
...
Now, the USB gives 5V n 500ma.. All i need now is to somehow convert this 500mA to 1A..

Can any1 suggest a method to achieve this?
If I did, it would be worth about $100,000,000,000 :D

What's wrong with a 5V 1A regulated Wall-Wart like this.
 

Mikebits

Well-Known Member
Sounds like Alchemy, make gold from sand. No such thing as a free lunch as it were.
You should be able to make a 1 amp supply without it being to large.
 

Mikebits

Well-Known Member

Tan9890

New Member
Actually, I too didn understand why 1A is required.. The only thing is that, the transformer gives 1A current. And as i'm trying to avoid the bulky transformer, i thought i can get the curent directly by adding some current booster..

Anyways, i think it is possible to make the circuit work on 500mA, right?

I mean its just the microcontroller, leds, n low value capacitors on the circuit..
So will it work on 500mA current then?
 

leonidus

New Member
Originally Posted by Tan9890 View Post
Anyways, i think it is possible to make the circuit work on 500mA, right?

I mean its just the microcontroller, leds, n low value capacitors on the circuit..
So will it work on 500mA current then?
Hi Tan, LEDs require current in Amps. to drive them. So, I don't think this much current would be sufficient for the ckt..:rolleyes:
 

Sceadwian

Banned
The total current draw of most micro controllers doesn't exceed more than a few hundred ma's in most cases. If you want a small power supply grab an old modern cell phone charger. They're all very small, and usually provide 500+ma of current. Your programmer may be safe to run off the USB port, but if you hurt something it's your own fault.
 

Tan9890

New Member
i'm using just the power from the usb port like this, in the image..
So, i'l be connecting a resistor in series like the diagram so that it gives lesser current.

I need to clear 1 doubt tho, how much current should i actualy use so that the circuit will work fine?

That way, i'l just attach the appropriate resistor..

Also, so at least, the usb port idea for power supply is fine right? I'l not use the transformer then..
 

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dougy83

Well-Known Member
If you need the full 1A, use 2 USB ports in parallel - a lot of cables do this for power hungry devices e.g. some modems and external HDD.

By the sounds of it, you won't need 1A anyway.

You mentioned connecting a resistor in line with the supply; what, to the micro? You may find that it doesn't work very well. I would think that the LEDs should have limiting resistors, but not the micro.
 

Sceadwian

Banned
47 ohm resistor to put 25ma into an LED? What LED has a 4.4 volt forward drop at 25ma? Most white LED's are around 3.1 volts at 20ma. For 25ma into a normal white LED would be closer to 75ohms.
 

audioguru

Well-Known Member
Most Helpful Member
i'm using just the power from the usb port like this, in the image..
So, i'l be connecting a resistor in series like the diagram so that it gives lesser current.

I need to clear 1 doubt tho, how much current should i actualy use so that the circuit will work fine?

That way, i'l just attach the appropriate resistor..
The datasheet for your LED lists its max allowed current (maybe 30mA) and its range of voltages.
You must calculate the value of the resistor to match the spec's of your LED.

Most white LEDs are around 3.5V. But LEDs have a range of voltages so your white LED might actually be only 3.2V. Then for 25mA the resistor value is (5V - 3.2V)/25mA= 72 ohms. Nobody makes a 72 ohms resistor so use 75 ohms.
If your LED is actually 3.6V then the resistor should be 56 ohms.
 

Sceadwian

Banned
Damn audioguru, are you following me? =) I think that's the 2nd time in the last few days I posted at the same time as someone else with the same answer.
 
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Sceadwian

Banned
LED math is one of the few things I'm good at =)
I would like to know where the original poster got that image from though, it says that resistor is for 25ma, even though the LED will probably draw 40ma with it. That's not a small error.
 

Tan9890

New Member
Thank you guys..
But, i'v just added the diag to show what kind of supply i'm going to use..
Just think that,ther's my whole circuit in place of the led.

The port should give 5V and a suitable current value to power the microcont Vcc as well as all those places where 5V is reqd.

I'm not worried about the leds, coz i'm adding current limiting resistors to all. Its about the microcontroller.. It should not remain off or something if the current's low (dont know what'l be the consequences) or shouldn't even get damaged, if the current's high..

Again, Thank you all for all this help.. Really appreciating it all..:)
 

Sceadwian

Banned
Can't just think that Tan... The diag you showed is wrong.. so we have to assume your circuit is wrong as well. For an IC you wouldn't use a resistor at all....
 

Hero999

Banned
I'd go for 68Ω because it's an E6 value and much easier to come by than 75Ω. If the LED is 3.2V then the current will be 26.5mA which is probably all right at the maximum rating will be 30mA.
 
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