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/diy/ - Do It Yourself

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File: 22 KB, 450x390, variable-power-supply-717366[1].jpg [View same] [iqdb] [saucenao] [google]
432888 No.432888 [Reply] [Original]

If i buy a variable power supply which can only supply whole voltages, could i use resistors in series to lower that voltage down from 12V to 11.7 Volts ?

>> No.432896

>variable power supply which can only supply whole voltages
why
>resistors in series to lower that voltage down from 12V to 11.7 Volts
why

>> No.432898

not in a practical sense, unless your load is non-varying.

and what kind of variable power supply isn't variable?

>> No.432903

Just use voltage divider rule.

>> No.432921

couldn't a transformer also

>> No.432941

>>432903
good luck calculating the components with a load.
>>432921
ac only

>> No.432942

>>432896
the cheapest one for sale can only give whole integer value voltages.

the application requires an exact voltage.

>>432898
Its a DC voltage of 11.7 Volts to be exact.

Wouldnt a resistor in series use some of the voltage so the remaining voltage would be slightly less?

>> No.432943
File: 24 KB, 644x346, why-do-you-need-to-be-0.3-v-off.jpg [View same] [iqdb] [saucenao] [google]
432943

>>432941
I mean use voltage divider in same way you use it to get a floating ground.

>> No.432944

>>432942

voltage division has to do with the ratios of the resistances of the two loads.

V1 = (R1*V)/R

R1 is your resistor value
V1 is the voltage drop across said resistor
V is the supply voltage (12V in this case)
R is the total resistance, ie. your resistor + your load.

you can see from the equation that the resistor won't have the same voltage drop every time if its counterpart is changing. if your load is constant like he said, you can work it out pretty easily.

buy a better supply lol - sif something designed for 11.7 cant take 12 anyway

>> No.432951

>>432942

You can use a resistor if your load takes constant current.
However, I suspect that you're attempting something stupid, like feeding LEDs.

>> No.432956

>>432951

I know he's trying to work with LEDs. 11.7V is the forward voltage on the MK-R LED and there's not many other devices with such a specific input voltage.

OP, if you are trying to drive the MK-R, it won't matter if it's 11.7 or 12v. Once the diode hits 11.7v, the other .3v gets dropped. This is called a voltage drop, and is how LEDs work.

>> No.432961

>>432942
Why are you buying a variable power supply if you only want 11.7 volts. ebay a 12 volt power supply that has a potentiometer for adjustments and calibrate it. Why do you need exactly 11.7 volts anyway and how many amps are you using?

>> No.432967

>>432943
You can't put much load on a voltage divider like this at all. It'll smoke.

>> No.432975

>>432961

If it's what I suspect/know OP is doing, 11.7V @ 1250mA, Cree MK-R LED.

OP, if you really don't know how electricity works, you shouldn't even be trying. You're going to either fry some expensive engineering samples you got sent to you, or you're going to fry yourself, or burn your house down.

>> No.432976

>>432967
Use a 300 ohm 1 watt resistor(b=1000). Even if the load is a short, at 12V the power will only supply 40mA, which is 0.48W power dissipated across the resistor. It won't smoke.

>> No.432977

>>432976
Ah, alright then.

>> No.432982

>>432956
>>432951
>>432961
>>432975

its for a charge controller circuit. the voltages are exact because im dealing with a 12v battery.

when the charge of the battery drops to 11.7 then the circuit switches a relay to charging mode

when the voltage gets to 14v then the circuit switches the relay into dumping mode.

>> No.432998

>>432982
What.....you can't charge a 12 volt lead acid battery with 11.7 volts. Am I misunderstanding, are you trying to make a reference voltage for a comparator or something?

>> No.433000

>>432998
Isn't that the trickle charge voltage?

>> No.433003

>>433000

You can't trickle charge a higher-voltage source with a lower voltage. It doesn't work.

>> No.433005

>>433003
Ah no, of course not.

>> No.433009

>>433000
Think of electricity as water in buckets on a shelf, voltage is the height of the buckets. Water from a bucket 11.7 feet high will never flow higher to another bucket 12 feet high. A 12v lead acid battery is fully charged at 12.8 volts and mostly discharged at 12v, your charger will do nothing, no current will flow into the battery. That said it could be useful if you wanted to keep the battery in long term storage to prevent it from fully discharging.

>> No.433011

>>432888

power supplies generally just provide... power. relatively bulk power.

if you have a circuit that needs actual precision, then yo umake a precision circuit.

all the anons suggestion voltage dividers are wrong -- that's a textbook solution, or on ethat works practically when you are not drawing any current from the divider (eg. an opamp input, etc)

OP, STATE YOUR APPLICATION, EXACTLY. why are you making us guess?

you can use a very inexpensive "brick" power supply you can get from a thrift store, with a simple regulator. TELL US WHAT YOU ARE DOING.

>> No.433017

>>433009
A 12V lead-acid battery that has a terminal voltage of 11.7V *is* discharged.

>> No.433056
File: 1.84 MB, 1280x998, series r.png [View same] [iqdb] [saucenao] [google]
433056

I just went ahead and used loads of resistors in series.

i didnt have to buy anything, i found an old appliance charger rated at 15V DC (my multimeter reads it as 16V ??)

stripped it and attached resistors. using trial and error iv worked out i need 7K ohm to bring it from 16V to 14.9 V (good) and 36K ohm to bring it from 16V to 11.9 V (good).

my total circuit is for solar electricity. theres a solar input, battery and dummy load (for when the battery is full) all connected to a relay and some circuitry. Baisically using potentiometres and the circuitry the relay switches from the battery to the dummy load depending on how full the battery is.

11.9 V of battery charge is what causes the battery to charge and 14.9 volts is what causes the battery to stop charging.

>> No.433461

>>433056
what the bloody fuck am I looking at

nigger what are you doing

>> No.433496
File: 216 KB, 393x391, cheetah lion lizard hhhehehe computer laughing blue background.png [View same] [iqdb] [saucenao] [google]
433496

>>433056

>that image

>> No.434061

>>433056
OP please post pics when you've burned your house down, we'd all like to see.

>> No.434067

>>433056
Are you for real? Could you do yourself a favor and learn some soldering job?

>> No.434228

a nice way to do what OP wants is to use silicon diodes in series with the load. each one will drop about 0.65V, so, for example, to get 11.7V, you start with 13V, then use 2 diodes in series to drop 1.3V, and you end up with11.7V.

the advantage over resistors is that it doesnt depend on current flow, so no need for trial and error.

>> No.434230

>>433056
...and now I know why free form circuitry is no longer practiced, nor respected.

>> No.434240

>>433056
OP, you are not making much sence to be honest.

If it's about "solar electricity" than what does any of it has to do with power supply (that only produces integer voltages or whatever)?

>> No.434245

>>434240
Maybe he's using his photovoltaic cells as a hot plate?

>> No.434252
File: 83 KB, 1067x1024, oh-god-why.png [View same] [iqdb] [saucenao] [google]
434252

>>433056

>> No.434273

>>433056

you have absolutely no idea what you are doing, wont tell us what your project even is, and are just practicing voodoo.

that string of resistors performs no useful functiion in the real world.

>> No.434303
File: 100 KB, 533x515, 1365575743695.jpg [View same] [iqdb] [saucenao] [google]
434303

>>433056

>> No.434350

>>433056
oh god this is going on my wall

>> No.434352
File: 70 KB, 799x478, IMAG1043.jpg [View same] [iqdb] [saucenao] [google]
434352

For some reason I thought of this.

>> No.434353
File: 314 KB, 1058x900, 1365441172810.jpg [View same] [iqdb] [saucenao] [google]
434353

>>433056

>> No.434467

>>434245
>Maybe he's using his photovoltaic cells as a hot plate?
Took me a while to figure out how exactly you can use photovoltaic cells as a hot plate. It was then I remembered about the power supply…

At first I dissmissed the thought as a blasphemy. Then I looked at >>433056 again and started to have doubts.

>> No.434502

>>434352
What is that?

>> No.434530

>>434352
>>434502
google reverse image search,

https://sites.google.com/site/nakazoto/robot-rapture/my-robots/symet

>> No.434533

>>434530
http://www.beam-wiki.org/wiki/EZ_Solar_Engine

I'm still not sure what these really accomplish.

>> No.434547

>>434533

Apparently they first charge a capacitor, then run the motor with capacitor's charge. Rinse and repeat. This way you run the motor intermittently even when the solar cell isn't producing enough power to run the motor continuously.

>> No.434567

>>434533
A symet is a very early staple of BEAM robotics, wherein freeform circuitry is used to build kintetic circuitry that mimics biological behavior. Symets spin when light is intense enough, but other BEAM bots will walk, vibrate, or crawl, and even seek out light using basic components, and the occasional small IC.

>> No.434586

>>434567
Could they be used to help up the amps for charging a small battery if the solar cell does not output high enough amps?

>> No.434615

>>434586
They're typically designed so that the capacitors won't discharge until there is enough power to operate the motor in a short burst. In low light, symets tend to stagger around lazily without such a system.

https://www.youtube.com/watch?v=dYLg6dtblik&feature=youtube_gdata_player

>> No.434738

>>434615
Well, given the application these people are using this for I think this is the robotic equivalent of impressionistic finger painting.

What practical things could this be used for?

Large battery charging?
Glider-play hybrid thrust?
Transmission bursts for satellites and/or surveillance equipment?

>> No.434739

>>434738
>Glider-plane hybrid thrust?

fixed

>> No.434752

>>434738
Some solar panels are equipped with motors to turn to face the sun.

>> No.435231

If you just use a resistor the voltage output will vary with load.Use a series resistor and a reversed biased zener diode based on the voltage you want. This will make a crude voltage regulator. You could also use series connected rectifier diodes, each one will drop about 0.6V. 12V->11.4->10.8->10.2... Otherwise just use an LM317T and a pot.

>> No.435318

wait... WAIT. I think I understand now.
OP: Is this 11.7 volts just a reference voltage to indicate a discharged battery? So, you compare the battery voltage to this voltage, and when they become equal, that means the battery is discharged?
In that case, a simple voltage divider would suit your needs fine. Don't bother with the series string of resistors, it's arse. Just use a potentiometer wired as a voltage divider, and fine-tune the voltage to what you need.

>> No.435388

OP won't be coming back, he most likely burned his house down.

RIP in piece, goodnight sweet prince.

>> No.435606
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435606

>>433056