What's the cheapest DC to DC convertor that can handle 3A at 5v?

 

DC to DC? What are you trying to power?

 

Need to step 12v down to 5v to power a digital video recorder. Right now I'm using a 300 watt inverter, but I'd like to use DC-DC to get a cleaner power source and avoid interferance on the screen.

 

p/n 296-2261-5-ND $3.83

p/n LM2650M-ADJ-ND $7.27

you could also try searching:
www.mouser.com
www.alliedelec.com
www.newark.com

 

voltage divider -> should be less than $2 of stuff at radio shack or another electronic store

this link will help you set it up
http://ourworld.compuserve.com/homep..._Bowden/r2.htm

 

A voltage divider is not going to be nearly as efficient as a DC-DC converter. Plus, if you use that site, it will be hard for you to get an accurate result. They assume that all of the current is flowing through the two resistors (which would not be true in your case since you will also have a load connected to that circuit). If you simply plug in 12V (14.4V, whatever), 5V, and 3A, you will not get accurate results for R1 and R2 for your needs. That site can be used to find out what you want, but you'd have to do a lot more math to be able to use it. Also, with the power requirements of your circuit, I doubt the resistors you'd need would be <$2.

 

The calculator linked assumes a load. What do you mean the results won't be accurate? is the equation wrong? Please explain you're being very vague. The price of the parts is just where you pick it up at and how much the distributor jacks up the price in the end it will still be significantly cheaper than what you suggested. By the time that is setup and working correctly you're better of just finding a used cell phone charger with similar specs and splicing the wires. Even a brand new one at walmart would be cheaper.

 

The link assumes that you only have the voltage source and the two resistors (or resistor/LED). For this situation, the equations, and answers that the site gives you are correct. However, this would not be the circuit that Scruit would be using. He would have the voltage source, the two resistors, plus his load basically connected in parallel with the bottom resistor. In this situation, that site will not give you correct values because they do not take the load current into consideration.

I just priced out a circuit with one of the two suggested dc-dc converters, and it would cost ~$21.49 in parts. (I'm sure there are other dc-dc converters that would work with less components. I just didn't bother searching for them.) For the two resistors that he'd need for the voltage divider it would probably cost around $10. Also, the size of the resistors needed becomes prohibitively large with his power requirements.

 

Than what current are they referring to? From what I've read on its example was the current used by the led which would be the load.

 

That is correct. They assume that all of the current goes through the two shown resistors (the resistor and LED in the example). In which case, their numbers are absolutely correct. However, when you use a voltage divider (which their resitor-LED combo is not), you will have two paths for current to travel.

Let's call the point where the two shown resistors connect node 2. In Scruit's case, he would actually have his digital video recorder connected to Node 2 also. With this circuit, he would have current flowing through R1, into node 2. Current would flow out of Node 2 through R2. It would also flow out of Node 2 through his load.

Now, I suppose you could set up a divider with the load being the bottom resistor, but that is if there is only one path for the current to take. One shortfall of this method is that you will be limited to a specific current (and it might not be your desired current). Your load will have a set resistance, and you would adjust the top resistor to get your desired voltage across your load. By doing this, you are limiting yourself to whatever current is dictated by the top resistor. The top resistor is basically a current limiting resistor (this is what it is in the resistor-LED example).

If you need to regulate the voltage of a circuit with multiple current paths, then you should put your circuit in parallel with the bottom resistor of your voltage divider. I just assumed that his case would follow the latter scenario since that is what I've always encountered. With that said, that site may work depending upon his setup.