voltage regulators v's voltage dividers?...

[deadastronaut]

just curious, why use regulators, (example 7805), when voltage dividers can be used to go from 9v to 5v....

is it just a preference/space saving thing?...1 component v's 2 resistors....

or are regulators more stable , suitable etc...i'm curious as i have a  3x  3v 5mw lasers ( red green, blue) i want to run all night for a halloween party..

cheers rob.

[petemoore]

  Lasers...what do they want ?
   Do they need to be regulated ?
  What are the min/max's on the power voltage ?
   

[deadastronaut]

hi pete: well i was planning on using a 9v psu for each of them. and dropping the voltage to 3v on each one. (seperate psu's)

they each take 2xAAA 1.5v batteries.and are each  5milliwatt ..but they eat batteries quickly...

these are the ones i have...scroll down for specs..

http://www.ebay.co.uk/itm/Powerful-Green-Laser-Pointer-Pen-Beam-Light-5mW-NEW-/220565489382?pt=LH_DefaultDomain_0&hash=item335aba46e6

[acromarty]

Regulators provide a low impedance output suitable for providing power to other devices.

Voltage dividers are usually higher impedance to minimise the current taken from the battery through the divider.
They tend to be used to reduce larger voltages to smaller ones, for example dropping 9V signals to 5V for input to logic devices, or to provide an intermediate reference voltage for opamps, such as the midpoint 4.5V from a 9V battery.

If you try to take a larger current from a divider, the output voltage just drops because of the resistance in the top half of the divider.
A regulator maintains an approximately constant voltage output up to its current limit rating (or the limit of its input supply).

[deadastronaut]

cheers acromarty: so i should go with 9v- 3v regulators then?...

[egasimus]

^ It's definitely possible, but it's a waste of power IMO... Try a 4.5V or 5V transformer instead.

[deadastronaut]

^ thing is i have 3x  9v transformers spare, i only want to use 3 lasers for 1 night....dont fancy buying 3 more..

[egasimus]

then why not wire the 3 lasers in series, from one transformer? provided they're gonna be located close, and the transformer can dish out enough power, of course

Also, have a look at this. I saw something based on it a while ago - it was a microcontroller-based light dimmer powered directly from 120V/220V mains. IDK if it can be of use to you, and it looks a tad more dangerous, too, but it's a nifty trick.

[deadastronaut]

yeah they'll be in different areas...but good idea, might try that, and bounce em with mrrors..

i was just looking at universal power adaptors,  3v  4.5  up to 12v.....

but if i can get away with 3v regulators that'd be better/cheaper..and less dangerous!!!!

[egasimus]

Sure you can, but make sure they have big enough heatsinks, or it's gonna get dangerous again

[deadastronaut]

   yeah...

i'll stick one of my lasers on my breadboard and leave it for a while, see if it gets hot...and dangerous!!..

[Mark Hammer]

Voltage dividers simply divide whatever one feeds them.  If what you feed them has changed over time (e.g., as a battery dies out), then the result of that division changes too.

In contrast, regulators provide a stable unchanging output, provided one meets the conditions for the regulator to do its job.

I've mentioned this before in other threads, but circuits like phasers and BBD-based circuits will use regulators or zeners as a way of providing a fixed reference voltage that does not change as the battery voltage changes.  That lets the circuit be biased once, with the bias voltage remaining valid as long as the battery has some life in it.

[deadastronaut]

^ yep gotcha, i never use batteries, only psu... so reg's it is then!....cheers guys. 

i'll try it out...

[garcho]

Would part of the reason be the cost difference? A voltage regulator can be ten times the price of a pair of resistors.
Along similar lines, would a voltage divider work well in regards to biasing the circuit if a wall wort is used? In theory, it's regulated voltage as well, right?

[EATyourGuitar]

like Mark Hammer said. dividers move with the input voltage. two wall warts might be a bit different. its perfect for opamps where you always want half the supply. but for a midi to CV or a vacuum display that wants EXACTLY 5v you would use a regulator. another thing to consider is that regulators can offer some degree of ripple protection. most regulators have a loss in them so putting a 9v reg on a 9v supply wont work as planned. you need at least 10v or more to get 9v from a 9v regulator. not true with a zener however. a 9v zener will turn on at exactly 9v.

as some said about the divider limiting current, could you just use a 1k+1k divider if you wanted more current? I have used dividers to starve fuzz pedals and I think its great how you can control the resistance of the supply by changing your divider pair to different values.

[Gurner]

What you need to remember is that with a voltage divider, you've a couple of (normally high-ish) resistances .....whatever you attach to the junction (ie the load) is essentially in parallel with one of the resistors (normally the lower one) .....and as we all know, when you put one resistor in parallel with another the ooverall resistance goes down - so now you have a different voltage divider vs the one you started out with ...so unless you are connecting a load that is much higher that the resistor values you've used, then as soon as you connect up, the voltage divider junction voltage is gonna change.

[EATyourGuitar]

What you need to remember is that with a voltage divider, you've a couple of (normally high-ish) resistances .....whatever you attach to the junction (ie the load) is essentially in parallel with one of the resistors (normally the lower one) .....and as we all know, when you put one resistor in parallel with another the ooverall resistance goes down - so now you have a different voltage divider vs the one you started out with ...so unless you are connecting a load that is much higher that the resistor values you've used, then as soon as you connect up, the voltage divider junction voltage is gonna change.

and everything affects everything. good to know. I did it by ear when I tried it. I didn't even check voltages! I always check voltages now though. I guess in the case of a fuzz face, I starved it and then I hooked up something in parallel and that made it bias up hotter? doesnt that pretty much just cancel out the starvation? its kinda funny looking back.

[garcho]

Wow, thank you, that makes a lot of sense to me now. As you can infer, I haven't made any circuits with voltage regulators, but I was eying the Rebote Delay v2.5 delay pedal at Tonepad, which has a 5v regulator and looks to be simple enough for my abilities. So far the most complicated thing I've built is a CMOS oscillator with some LDR controls, that goes through a light controlled Dr. Quack. The sound trigger is played with your foot so your hands are free to manipluate the LDRs, which sound really cool when LFO-ed by LED bicycle lights.
Anyway, the Vr is always half the Vcc, and that's how the circuit biases itself regardless of the actual Vcc? That's the usefulness of a differential amplifier? Am i on the right track?

[PRR]

> why use regulators, ..when voltage dividers can be used

Resistor dividers sag. Regulators don't.

A small-sag divider eats ten times more waste heat than the load it supplies.

Divider sag may be acceptable if the load is _very_ light. Such as biasing amplifier signal inputs, the Vref in many one-supply audio plans. The amp input demand is microwatts, the amp's +9V power demand is many milliwatts, the divider can waste a few milliwatts without much harm.

> i have a  3x  3v 5mw lasers ( red green, blue) i want to run all night for a halloween party..

How big is that load?

I assume the "5mW" is OUTput light power. Laser efficiency is probably not 10%, so it must need 50mW or more of input electric power.

So a voltage-divider approach will waste 500mW or more. Which I guess is not a huge amount, but more than is convenient with 1/2W resistors.

Further, I find there are two types. Naked laser diodes, and lasers with onboard driver circuits.

The basic laser is not a constant-voltage device. The critical thing is that Current is at least 20mA (to get into lasing mode) but not more than 40mA (so it does not melt). The voltage at these currents is "around" 2.7V, but not specified exact, and very variable with production tolerances and in-use temperature.

Which is why the with-driver version exists. Laser needs a constant current, constant-voltage supplies are common, adding the driver makes a more useful product.

Find out exactly what you got. And what its voltage and current ranges are.

> run all night for a halloween party..

One party? Bah, don't think too hard. It will probably run on a couple flashlight cells, and a couple D-cells will probably live past dawn. Yes, a 6-night/week show could run-up battery costs, but One Special Night is no big expense.