sorry about this

Post by **Adverse Effects** » Wed, 20 Jul 2011, 05:55

sorry about this but i didnt want to sing up to yet another forum for a one time question

i am looking for the smallest circuit possible to do this

3 to 8 Vdc in
out what ever the input Vdc is with a % of a duty cycle on / off

and i want to pules it sort of like we are doing with the car controller

if it could fit in side a area of no more than circler 25mm across and no more than 15mm high but the lower the better

what i am trying to do is replace a controller that controls the voltage up and down from 3 to 8 input to modify it to 5 volt out constant
but i want to replace it with something that will give me verifiable volts but control the amount of time it is on instead as we do in DC cars

this has to be able to do up to 3amps as well

[EDIT] that makes more sense now

Post by **Richo** » Wed, 20 Jul 2011, 20:49

Sounds tricky.
Probably need some SEPIC controller chip.
Here is a module in the ball park:
ZY1207 7A

Post by **Adverse Effects** » Wed, 20 Jul 2011, 21:12

umm no that is
3V to 14V Input 0.5V to 5.5V Output

i am trying to have same v input as output
like 7vdc in 7 vdc out but only on X% (veritable) time on adjustable with a small trimpot

i was looking at something useing a 555 chip but its going to be a bit to big and it wont take the amperage i need

Post by **coulomb** » Thu, 21 Jul 2011, 16:09

Adverse Effects wrote: I was looking at something using a 555 chip but its going to be a bit too big and it wont take the amperage i need

555 chips are pretty small, especially SMD versions. I would consider a small PCB with a CMOS 555 (for the very variable voltage) driving a smallish low threshold MOSFET (so it would turn on properly at 3 V). The PCB could be mounted vertically if necessary. The PCB could just be Veroboard, if using a through-hole 555C.

Post by **Richo** » Thu, 21 Jul 2011, 20:55

Just be careful as some 555 won't do 3V-14V.
Also you may not get true full 0-100% duty cycle.

Post by **Adverse Effects** » Fri, 22 Jul 2011, 00:11

Richo wrote: Just be careful as some 555 won't do 3V-14V.
Also you may not get true full 0-100% duty cycle.

that is cool
i am looking at inputs from 3 to 9 or 10VDC (depending on battery configuration)

and anything from say 20% to 99% would be good

Post by **Adverse Effects** » Sat, 23 Jul 2011, 01:33

ok like i said b4 i know almost nothing about about electronics

but with a lot or research i think i have made a circuit that SHOULD work can someone tell me if i have wasted all my time (almost a full day)

the red area id the unit i am trying to power and control with the unit

Post by **Squiggles** » Sat, 23 Jul 2011, 04:20

Do you only want it to operate when the "touch switch" is touched?
Why not just use a miniature momentary contact switch?
Then you don't lose 0.7V through the transistor.

Post by **Adverse Effects** » Sat, 23 Jul 2011, 04:49

yes i can cut out the touch switch its in there just because its "cool" lol

but the rest of the circuit should work as a PWM ?

Post by **coulomb** » Sat, 23 Jul 2011, 06:17

Adverse Effects wrote: but the rest of the circuit should work as a PWM ?

Yes. However, the frequency will change as the potentiometer is changed; if that matters, you could rearrange the potentiometer and limit resistor(s) to connect the wiper to pins 2 and 6. The capacitor charges through the top 2 resistors, and discharges through R2.

You should use a low threshold MOSFET instead of a bipolar transistor to drive the load; at 3 V you will lose proportionally way too much voltage across a bipolar.

I'd lose the touch switch. But if you decide to keep it, get the touch transistor to drive another MOSFET for lower voltage drop. You will need very high gain with just one transistor, and you will lose too much voltage across a Darlington pair.

Post by **coulomb** » Sat, 23 Jul 2011, 06:18

I looked briefly at a 555 and 555C datasheet, and they both listed 4.5 V as the minimum supply voltage. You will have to hunt a bit to find a 555 equivalent that runs on as low as 3.0 V.

Post by **Adverse Effects** » Sat, 23 Jul 2011, 07:19

ummm ok i lost you about 1/2 way through your first line lol

any chance you could redraw the pic for me?

like i said i am a total noob at this and that is the first time i have drawn a circuit and i just copped each component off other things

do you know a smaller way to do it as it needs to be as small as possible

it will be driving a heating element of about 2.4 to 3 omes like these Atomizers

i dont think the voltage will get as low as 4.5 volt so as long as the lowers is not much higher than they it will be ok

Post by **coulomb** » Sat, 23 Jul 2011, 15:58

Adverse Effects wrote: any chance you could redraw the pic for me?

Not much to it:

You want a low threshold MOSFET that can handle at least 3 A. Digikey (and not Element14 or Mouser) have the gate threshold in their parametric search. This one is 2.0 V threshold, TO220, 12 A continuous:

http://search.digikey.com/scripts/DkSea ... 055VLFS-ND

You may need to search for one in a smaller package. TO220 transistors are about 15 x 10 x 5 mm plus leads. It probably won't need a heatsink.

Post by **Adverse Effects** » Sun, 24 Jul 2011, 05:01

thanks for that

so that will do what i want then?

so all i need to do now look for thew smallest of each component i can get to do the job

i need to make it fit in to 1/2 the top part of one of thes torches