help regulating voltage

[pato_llaguno]

i found this MC34063A switchable regulator on a shop, but i just dont know how to wire it.

[jack wp]

You don't give us a URL?

The example I presented to you cost $3.80. Do you find one less expensive. What's the URL ?

[pato_llaguno]

https://www.sparkfun.com/products/317


i am not buying it from here, a local store sells for $2.90, but i belive i need extra components

[jack wp]

I don't know anything about that IC. I would suggest spend a dollar more, and get all the components on the board.

[pato_llaguno]

can you help me on how do i need to design it?

[polymorph]

If you have a dollar store near you, see if they have any cigarette lighter plug to USB charger adapters. You are nearly 100% guaranteed that it'll have a MC34063A buck regulator in there. Complete with schottky diode and coil.

Many of the cigarette lighter plug phone chargers also use the MC34063A chip.

The Dollar Tree here has them now and again, and the ones they sell use a rewindable inductor and the chip itself, the diode, and inductor are through-hole components. You'll have to replace the input caps to withstand 24V (use 35V capacitors) and change some other components.

There are Javascript calculators on several websites, I like this one because it automatically selects buck or boost, and you can use a percent variation of input voltage to account for changing supply voltage:
http://dics.voicecontrol.ro/tutorials/mc34063/

This chip has a built-in current limiter, it can output a maximum of 750mA. However! You should not be daisy chaining regulators. The LM317 in your circuit, besides having insufficient input smoothing capacitance, has to carry all the circuit current. You never said, what is the input voltage there? Is it AC, or is the bridge rectifier there just so it doesn't matter which way a battery is connected? Either way, there should be more capacitance there.

The MC34063 can withstand up to 40V of input voltage.

[pato_llaguno]

if i use that calculator i get this

Vin =   24.00 ±10% V
Vout =   5.00 V
Iout =   500.00 A
Vripple =   0.10 V

Vin min =   21.60 V
Ton / Toff =   0.34
Ton + Toff =   10.00 µs (micro seconds)
Toff =   7.49 µs (micro seconds)
Ton =   2.51 µs (micro seconds)
Ct =   100.47 pF
Ipk =   1,000.00 A
Rsc =   0.00 ?
Lmin =   0.04 µH
Co =   12,500.00 µF
R1 =   10.00 k?
R2 = ((Vout - 1.25) / 1.25) * R1 =   30.00 k?





isnt 12,500uf a bit unrealistic?

[CrossRoads]

http://www.pololu.com/catalog/category/131

[pato_llaguno]

i cant afford the time that would take me to get those here

[polymorph]

Isn't 500 Amps just a little unrealistic?

Vin =   24.00 ±10% V
Vout =   5.00 V
Iout =   0.50 A
Vripple =   0.10 V

Vin min =   21.60 V
Ton / Toff =   0.34
Ton + Toff =   10.00 µs (micro seconds)
Toff =   7.49 µs (micro seconds)
Ton =   2.51 µs (micro seconds)
Ct =   100.47 pF
Ipk =   1.00 A
Rsc =   0.30 ?
Lmin =   38.93 µH
Co =   12.50 µF
R1 =   10.00 k?
R2 = ((Vout - 1.25) / 1.25) * R1 =   30.00 k?

[pato_llaguno]

why is it Lmin and Vin min, there is no problem if it is greater??

[polymorph]

Vin min is simply Vin - 10% of Vin, as you specify.

As for the inductor, if the inductance is too low, the chip cannot switch off fast enough. The lower the inductance, the faster current rises. I'd not go -too- much higher than the minimum, but I can't tell you what that amount might be. 25%?

[dc42]

If im good at my estimates, i am not drawing more than .5A so i think i could design a 1a regulator, so i am trying to figure out how

Where did you get the figure of .5A from? Does all of that .5A need to come from the 5V supply? For example, if you are running relays from the 5V, then you would be better off using 12V or 24V relays, which draw less current.

If you can reduce the demand on the 5V supply to about 100mA or less, then you can go back to using a linear regulator. Otherwise, you need a switching regulator, and I strongly advise you to buy a ready-made board. There are lots on offer at eBay.

[bobthebanana]

Do you really only have .1 uf on tbe input of the 317? You're going to need a lot more if it's coming directly from a bridge rectifier.

[polymorph]

Is there some reason why you won't tell us what the source of power is?