voltage converter need help badly / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

voltage converter need help badly

Move Thread

LAN_403

teslacoolguy

Thu Jan 24 2008, 05:59AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

im not sure if this should be in this fourm or the coilgun fourm but anyway i need to take 12v dc at 1A and step it down to 3v at about 750ma using minimal components for a cap charger using a tiny camera inverter transformer and just wondering if anyone has any ideas on what i can do and remember simplcity is key

Shaun

Thu Jan 24 2008, 06:22AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

If the charger draws exactly 0.75A at 3 volts, lets assume it has constant impedance of 4 ohms. Maybe you can make it part of a resistive divider?

You need exactly one quarter of input voltage, so if we equate charger to 4 ohm resistor, putting a 12 ohm resistor in series with it will give 3 volts across the charger and 8 volts across the 12 ohm resistor. Keep in mind the resistor will be dissipating 6 Watts, so rate accordingly.

All this is of course assuming the charger is an ideal resistive load, which it is not. In my opinion, its still worth trying just because chargers are available on the cheap.

As for the power resistor, Ive found lots of em in old VCRs, loudspeakers, and TVs.

Dr. Slack

Thu Jan 24 2008, 08:00AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If the charger draws exactly 0.75A at 3 volts, lets assume it has constant impedance of 4 ohms

Bad assumption, a camera charger will draw widely varying current as it charges. As it tapers off towards the end of the charge, the current will drop, and the input voltage will rise and fry it.

A 3v zener shunt used with a suitably sized resistor above is as untidy and inefficient as hell, but is simple and bomb-proof, as long as it is adequately heat-sunk. A BJT to "amplify" the zener, or a small string of silicon diodes, would make obtaining the parts easier.

You could drop 9v with powerful zeners, but that puts you at the mercy of the battery voltage, I assume the 12v comes from a lead acid accumulator, which may be anything from 11v to 14v depending on the state of charge.

If you want efficiency, simply (and by that I assume you mean low parts count and low design effort and high probability of sucesss) then go to somewhere like Maxim, sample one of their low power switchmode controllers, and copy their application circuit.

If you want simplicity and can tolerate only 25% efficiancy, then use a 317 linear regulator. Two resisitors to set the output voltage, an output cap and possibly an input cap for stability, losing 7w off the IC should be easy enough with a reasonable heatsink. They're pretty much bomb-proof for over-current and over-temperature.

A crude and consequently rarely used linear regulator can be built from a low power zener shunt and an emitter follower to boost the current, but they're not short circuit proof, have poor regulation, and are only useful if the shops are shut and all you have to hand is your box of bits.

Of course a cap charger starting from a 12v supply should be a much better option than starting from 3v, but that breaks the simplicty assumption if you have to go and build one!

Tetrafluoroethane

Thu Jan 24 2008, 02:01PM

Registered Member #127 Joined: Fri Feb 10 2006, 03:36PM
Location: Cincinnati, OH - USA
Posts: 44

This might be what you need:

If you need exactly 3 volts, they have an adjustable regulator too. Very efficient since they are switching regulators. Otherwise, just use a normal regulator (1 chip and 2 caps).

teslacoolguy

Thu Jan 24 2008, 05:40PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

You could drop 9v with powerful zeners, but that puts you at the mercy of the battery voltage, I assume the 12v comes from a lead acid accumulator, which may be anything from 11v to 14v depending on the state of charge.

actually i am using a 12v 1A power supply to power it but the only reason i did this is because i can simply drop the voltge with a 220ohm resistor to trigger the scr and 12v is very easy to work with but i could possibly just have a 3v at 1A power supply and use like a couple ohm resistor to drop the voltage to the scr like to like 2.3v from 3v

Dr. Dark Current

Thu Jan 24 2008, 07:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I would say somthing like this

if you don't need it stabilised.

Or just buy a switching regulator as Tetrafluoroethane said if you want easy (but not as entertaining) solution

teslacoolguy

Thu Jan 24 2008, 08:52PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

i have found out that the transistor will push 4v but anything higher it blows because to many amps. the inverter transformer will handle 5v input but i was wondering if anybody knows what kind of transistor to use as a substitute that can handle at least 3 amps at 5v preferably avaliable at radioshack such as a tip120 or a tip122 because i dont want to order for just one part the transistor that was in there is a d2470 and it blew within seconds at 5v operation

Weston

Thu Feb 21 2008, 11:28PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

you can buy LDO voltage regulators. why does efficency matter?

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob