Low Cost and Simplest DC-DC Converter

A large resistor.

No, really! Do the math. You know Ohm's Law. You need to drop 110Vdc to 2.1Vdc. Your LEDs probably need, say, 15mA of current. If you've got your LEDs in parallel, multiply 15mA by 12. Now, you've got all the info you need to calculate the value of your resistor and the wattage.

To make your resistor a little smaller in wattage, string your LEDs in series.

I always tell my people, knowing how to use Ohm's Law is very important if you want to work in the electrical or electronics field.

Dear Vulcan

Thank you for positive response. Your suggestion is totally acceptable and in the past I allready worked very hard on it. When I failed in this experiment then I approached to the CR4. In this regard you can refer back issues of CR4 and all related comments. In this suggestions the heat generated is matter of concern and which required to be addressed. So based on the suggestion for dc-dc convertor of the honourable Members I wanted to try out the dc-dc conertor. So I have again approched to the Forum to find out if anybody is having idea for simple and cheap dc-dc covertor.

Still I appriciate your suggetion and hopes you understand the facct.

Warm REGARDS,

- Workflow

I can't see how you could fail with a series resistor. I've done this before and never had a problem.

There's something strange in your figures though. A typical high-brightness LED is 2.1V, 20mA when forward biased. A few can go up to 30 or 50mA. If you have 12 of them parallel connected, that's 2.1V, 240mA which equals 0.504W (versus 5W). Could you have made a typo or are you using a different, high current type of LED (something like 200mA)?

In any case, with your figures of 110V (input) and 2.1V/5W (output), your converter will be dropping 256.9W. Whether you use an IC or a resistor you're going to have to deal with the heat.

A power FET banging the primary of a transformer?

Cheap and cheerful!!

Resistor x 1. Cheap, simple and effective!

Google "Voltage divider". You can also Google "Make it as complicated as possible"

Wangito.

If you put the LEDs in parallel, you'll need a 250 Watt resistor ($30 ish) if you put the LEDs in series you'll need a 20 Watt resistor (about $4). If the 5 Watts was accurate then you're looking at 200 mA so you want about 400 Ohms (470 will probably do).

Where is the 110 V DC coming from? Can you afford to waste 80% of the energy in heat. By the way: don't let the resistor heat the LEDs and try to avoid grouping the LEDs so they don't "self heat" too much.

Buck converter and LEDs in series. Krass

google "dc-dc converter". There are hundreds of them out there.

look at this site

http://www.kpsec.freeuk.com/components/led.htm

Use RC series circuit.

Use diode bridge in series with the RC , provide rectified DC voltage to series connected LED.

This circuit will reduce power loss and heating.

As its dc bharat I don't see the point of a diode bridge...

However if we're all talking of using a huge power resistor, why not just use a 50 cent ic that you stick 110 volts in and it can be configured as a constant current source to your string of leds?

You will still need to remove the waste heat from the chip and you will need a small resistor or two but that's it!

John.

Yeah, a simple op-amp could do it couldn't it? You don't necessarily have to amplify, you could attenuate the source. Then like John said, it can be used like a current source.

Nick

Aren't you still going to have truckloads of waste heat?

His LEDs are pulling a couple of amps @ 2.1 volts. Anything that doesn't involve some form of voltage conversion is going to pull a couple of amps @ 110 volts and leave heaps of unwanted and unloved watts to dispose of.

Actually, although it's obviously no answer to his problem, but I wonder how a 5 watt, 110 v light globe would do for dropping the voltage. Suppose the initial inrush would kill the LEDs but it'd be a cheap resistor!

I dunno, I thought that an op-amp would be pretty efficient attenuating a source. I'll ask my circuits prof, he's a genius with stuff like this.

Nick

Dear Nick,

Thank you for extra initiative. Hopes he brings some solution.

Regards,

Workflow

He said that it would create just as much heat as a resistor would, so it wouldn't be worth it. What kind of leds are these (because 5 watts is a whole lot!), and where are you getting 110 volts DC? I have some Luxeon 1 watt white LEDS but they operated at 4 volts and 250 mA, and they were about as bright as a 25-35 watt light bulb or so.

Dear Nick,

The LEDs are normal of 2.1 Volts, 20mA. For you 2nd querry for 110 Volts DC it is available on main busbar of Electrical Locomotive. So how to bring down 110V DC to low voltage with space constrient and heating are main factors to bother.

On your suggestion I will try on Luxeon LEDs which may reduce the problem. Thanks. Regards

Ooops... I remembered wrong, I used the "Cree XLamps". I forgot, I only looked at Luxeon Leds, but never used them.

Here's where I bought the Cree Xlamp Leds:

http://www.allelectronics.com/cgi-bin/item/LED-110/340500/1_WATT_WHITE_ULTRA-ULTRA_BRIGHT_LED_.html

They are $11.50 there, but it's a lot cheaper than Digikey, which was selling them for I think over $40 a piece. These LEDS you are supposed to fasten them to a small heat sink if they are going to be in a warm environment (probably a small aluminum plate would be fine, you should be able to figure something out). They generate very little heat, but when they are that expensive and easy to destroy, it's better to be safe than sorry.

Are you sure that some cold cathode fluorescent wouldn't work better for the price? The LEDS are just sooo expensive.

Hope that helped.

Nick

If the LEDs are only 20mA and twelve in number, surely the original suggestion of a resistor will do the trick. Put them all in series with a 4.2k, 2 watt resistor and away you go. I thought we were looking at over 2 amps?

v

The best bet is a transistor, a small hf transforer with two windings at list,(its easy to do) and tree small resistor(1/8w) and a capacitor of about 103, enough!!!(oh dont forget one resistor for limited current for your led, when you suupply) the transistor's parameter would be 300v and 150mA. and >200k hz. easy find! to construct a converter to supply your load, either led or lamp. if you do better, you can add a diode to output of transformer seconder winding.of cuase it neednt for your led supply. all is less than 2usd with less heat disspate if you would add a bridge, you could lik it to main supply of 110v ac or 220zc directly. its not fun. if you need it large amount, I can build them for you at once.

Dear cnpower,

Good idea of hf transformer. Yes it is best in case of AC to DC conversion! But what about DC - DC Converter?

Common do somthing and let's come out with good idea for the DC-DC

Regards,

Surely his suggestion is for DC-DC? The transistor is converting the DC to AC, then transforming it down where it is then rectified back to DC. Or am I misinterpreting?

A workfolw, almost all ac to dc device can be use directly to dc to dc. no problem! vicevisa, just add a diode rectifier!

I think the obvious possible solution is to use an old mobile phone charger or other unused plug top power supply which is switch mode as they will (or should) work happily with a dc input converting it down with very little heat to a more useable voltage...

Cost = nothing, if you keep these things lying around as I do!!

John.

That's a neat idea. I didn't realise they'd work with DC. I'll be viewing them with more interest now!

I didn't know about that either... I'll have to check that out, that's cool

Nick

Oh yes!! they are great little things...

They can work from 85 to 230 volts ac and because they convert the ac to dc using a bridge rectifier, you don't even have to worry about which side is positive or negative!!!

I don't know why I didn't mention it earlier, I was at my bench removing the innards from a mobile phone charger to use inside another instrument and thinking what a cheap skate I was, when I suddenly remembered this thread

John.