# Obtaining multiple voltage levels from 12volts 3.5ampere switchmode adapter

hi

topic says it all,,

i have a switch mode 12volts 3ampere power adapter,, for the sake of necessity and fun i want it o have many output voltage levels say 9V , 6V, 5V, 4V,3.3V,3V,1.5V so how can i use it ,,?

also i have spare wire from another adapter with many leads so i will be replacing the stock wire so i can connect different types of devices,,

so how should i progress,,,?

is there any voltage regulator available in the above voltage ranges in single ic ? so i will just make a simple case and wire the leads and a selector knob,,?

also some devices would be needing much lower current as well ,, i say 20mA o 50 - 100mA range at 3-4 Volts should i be concerned with my adapter? i think i should only be concerned with proper voltages and and make sure my adapter is capable of delivering the required amps,, which certainly is with it pumping 3amps,,

[quote=“jhagra, post:, topic:”]

i have a switch mode 12volts 3ampere power adapter, for the sake of necessity and fun i want it o have many output voltage levels say 9V , 6V, 5V, 4V,3.3V,3V,1.5V so how can i use it ,?

[/quote]

Do you need all these voltages simultaneously or you will be using any one voltage at one time ? If later is true, you are actually trying to make an adjustable voltage output power supply, in which case the solution is simpler.

I would suggest to use an adjusable output linear voltage regulator like LM317 (for upto 1.5A output) or LM338 (for upto 5Amp). You can attach a variable resistor with a knob to adjust the output voltage. Also you can put an analog type voltage meter to see how much the output voltage is.

However if you want multiple voltage outputs at the same time, then you need to have many circuits which will have a common input and separate outputs. AFAIK, 9V, 6V, 5V, 3.3V are standard, and you can find the voltage regulator chips for these values. for rest of values, you might need to use adjustable output regulators, just fix them at whatever voltage you want.

i want one voltage at one time..also is there any such pre prepaired item? which can give me the above results//? one with a analog scal would be much better,,

First decide how many voltages you want - let's say you want 3 different output voltages, 1.8V, 3.3V and 5V. Do you really want 3A for all these outputs? That may make things slightly complicated. I'll discuss the simpler issue - a 1.1A output @ 1.8, 3.3 and 5V.

You can use a LT3080 for this and use a 3-contact switch to switch the resistance that sets the voltage. Look at the following schematic

The voltage set at the SET pin determines your output voltage. The LT3080 outputs a current of 10uA at this pin. So, let's say you want a 3.3V output - 3.3/10uA = 330K. So you want Rset to be 330K. For 1.8, it would be 180K and for 5V it would be 500K.

All you need is a switch that can switch these resistances. I'm not sure how the device will behave in the small time when the switch is being switched and there is no resistance present between SET and ground. This is something that will probably have to be tested. This is all assuming you want FIXED voltages. If you want adjustable, use a good quality linear pot and it should work fine.

i am more inclined towards the ijaz solution that will be much usable,,

i will use a variable reistor (pot ?) ,, few resistors an analog voltage scale to fix the voltage required .. and amp scale to monitor/observe the current drawn,, ( should show fom 0-3amps)

now i will need circuit diagrams,, and will preferable make it myself..

but how will i implement this circuit? i mean will the homemade pcb be able to carry such a high currents?

[quote=“jhagra, post:5, topic:16614”]

i am more inclined towards the ijaz solution that will be much usable,

[/quote]

Actually we both are talking same, he is just referring to a modern LT3080 IC and I am referring to LM317 (the good old thing and easily available)

For schematic, have a look at the datasheet of LM317

http://www.national.com/ds/LM/LM117.pdf

On page 1, the schematic under “Typical Applications” is all what you need to implement. The ratio of R2 /R1 will set the output voltage. If you can not find a 5K pot, you can use 10K pot and adjust the value of R1 accordingly.

Remember to put a heat sink on LM317 and you can make it on strip board as well. Also buy LM317T (TO-220 package) which will be easy to handle for you.

The only catch with this type of regulator is that you can not get output voltage less than 1.25V

PS: The LM317 is only able to deliver 1.5A, if you need more just switch to bigger brother LM338 which is a 5A version.

^ i will be needing mostly under 1 amp so which regulator is good?

the 317 or 317t,,i think to have 317t (to 20) as it will provide more current if need arises..?

also this will be my first practical hands on circuit,,hope it goes well,,,

LM317T would be fine since this is TO-220 package and capable of delivering 1.5A

All the best for your first flight

how can i determine how many watts resistor i need?

suppose if i consider peak/extreme high case say 12v input 9 volts op at 1.5 A ? also for the pot.also if my input is 12volts will i be able to get anything greater then 12v at o/p?obviously if i can then my current delivered will be low..?

Is 1.5A flowing through the resistor? If so, the power dissipated is I2R in the resistor.

If you want to boost the output voltage to more than the input voltage, then you need to have a Boost Converter circuit.

[quote=“jhagra, post:9, topic:16614”]

how can i determine how many watts resistor i need?

suppose if i consider peak/extreme high case say 12v input 9 volts op at 1.5 A ? also for the pot.also if my input is 12volts will i be able to get anything greater then 12v at o/p?obviously if i can then my current delivered will be low…?

[/quote]

For this LM317, you will need 1/4 Watt resistors.

No matter whatever you do with the pot, you can not even get 12V at output. The output voltage will always be lower than the input voltage in case of a linear voltage regulator.

oh wow

it is interesting information for me

thank you very much for posting a interesting post on WP community