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how to go from 5v to 12v

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wrx_pacman

New Member
Hey, I'm newto trying to step up and down voltages but i know what i want to do, so hopefully someone can help


i'm going to be getting a motoral 3bar map sensor (forget digikey part #)
but the map sensor has a 5.1v input and 4.9v output

what i want to do is go from 12v dc (automotive) to the 5.1v input of map sensor hooked up to a vacuum line that gives a varying output of -4.9 - +4.9

I want the map sensor to control a 12v lamp connected to it. Kind of like using the map sensor as a dimmer for the lamp

the lamp is .24 Amps (2.88 watts) but i may end up w/ a larger lamp with a high amperage rating.

is there a way (and how) to increase the +/-4.9 to 0.0 - 12.0 volts?
and have the ability to add more or larger lights to this circuit?

thanks
 

wrx_pacman

New Member
MPX4250AP is partnumber for my sensors (actually its an absolute manifold sensor) actually tho. I only want a dim glow on anything < 0 psi and anything 0psi < 13 psi I want a ramped elevation in power applied to lamp so that at 14+ psi it would be at full ramp
 

tansis

New Member
The application note pdf listed in my previous post is still valid , just ignore the LM 3914 Bargraph chip.
It's late here and I gotta work tomorrow, perhaps another member in a different time zone would be kind enough to post a link to an op-amp simulator.
 

wrx_pacman

New Member
ok so i would need to build an op-amp that runs off +-5v and steps the voltage up to 0v to +12v?

then from having a 0 to +12v output i could possibly adjust it so that outputs can be altered from 0v to +12v to say maybe +4v to +12v?
 

wrx_pacman

New Member
oh jeez i dunno where i pulled the - voltage out from then
oh wait yes i do i was confusing it w/ the gm sensor

ohhh wow thanks a LOT

so no wonder either nobody knew what i was talking or it wasn't feasible because i didn't know what i was talking about!

thanks for straightening that up and thanks for the data sheet

i think i could get this now w/ a op-amp

you've been more than helpful

thanks alot tansis
 

wrx_pacman

New Member
alright ya this is completely revised.

APPARENTLY (to me only coz i'm probably wrong) MC33272 is that part i need and i need 2 of them to get the voltage necessary for my application.

If someone can look brief and lemme know how far off i would be

I drop voltage from +12v to +5 volt with resistors
+5v input into a MPX2000 (i changed the sensor))
then from the MPX2000 run into 2 MC33272
from there I can power my device?

thanks

ps I realize i know alot lot lot less than i thought
 

tansis

New Member
You're getting there,

Would strongly recomend using a voltage regulator rather than just a resistor divider, the supply voltage in a car can go as high as 18volts when the engine is running.

Follow this link for some usefull stuff...

https://www.101science.com/Swelectronics.htm
 

wrx_pacman

New Member
wow Pspice is some powerful stuff :D i'm gonna have to get a book or something on it to flip through

but anyways

i have small dilema maybe you can help w/

on the op-amp (i got a quad coz that's all radioshack had)

and i assume that i'm only going to use 5 of the pins

pin 1 (1out) pin 2 (1in-) pin 3 (1in+) pin 11 (ground) and pin 4 (Vcc)

pin 11 obviously will go to my ground point (same as where my regulator and sensor will go)

pin 1 will obviosly go to my lamp or whatever i may have to but inline before the lamp such as a pots?

pin 2 would be ground? or is that for inversion?

pin 3 would come from my sensor's output (the .214 to 4.9v output)

and pin 4 would go to 12v supply? (would I need regulator even if the opamp can take +16vdc or +32vdc?)

thanks

sorry to keep posting what seems to be stupid questions (coz they prolly are)

here's what i got so far in jpg (can't really figure out pspice that quick)
 

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tansis

New Member
Have a read of this link,
**broken link removed**
 

wrx_pacman

New Member
hey,

i kind of don't really understand this

it appears that the "fan/light dimmer" is looping through the op-amp a second time... can you please explain to me the reasoning behind that

also the oscilator as i understand it is there to prevent the negative voltage kick from the motor... is that needed for a .24amp neon lamp?

and also would i need the comparator (being that i have no clue what it is?)

also why the mosfet (would i need)?

i can understand the caps tho
 

tansis

New Member
Allright let's walk you through this,

The op-amp you got from radio shack is not capable of powering a bulb by its self it needs help in the form of a power transistor or mosfet device.
It is designed to amplify the voltage level but cannot provide sufficient electrical current to drive a bulb, the transistor or mosfet take the current load.

Now the normall way to dim a bulb in a simple case , say with just a battery and bulb is to put a resistor in series with the circuit this works by reducing ther current flow and the bulb dims, the trade off however is that the resistor gets hot , same applies to transistors ( tis y big audio amps have large heatsinks)
Pulse width modulation on the other hand works by just switching the bulb on and off faster than the human eye can detect and this creates the illusion of the bulb dimming , kinda rough explination but the system works very well and gives good control of the lamp brightness.

Neon bulbs are not suitable for automotive use they require voltages upwards of 80v to function.

The motorola application circuit with the LM3914 bargraph display would be a simpler beginners project.

Ps Not been ignoring your post , it's the end of the month and business is frantic and I have deadlines to meet, would be nice if one of the 200+ people who have read this thread said Hi.

gotta go , family calls
 

wrx_pacman

New Member
ah ok thanks

i was looking at cold cathode neons... requires a
12v and a .24amp draw

so what i would do after the op amp is to add in the mosfet

i believe that the amperage (i'd have to check) coming from the op-amp is around .4 amps...


thanks
 

tansis

New Member
You might have a bit of a problem here, cold cathode tubes operate at around 120volts they use a step up voltage converter to get this from the 12volt automotive supply. Reducing the input voltage in order to dim/reduce the output for the neon lamp may cause the voltage converter to stop working. It all depends on how it's designed to work.

This adds another level of complexity to your design.
You can still use the PWM circuit , but instead of a power transistor or mosfet you will need a device designned for switching high voltage , the triac.
Instead of switching the 12v supply to the voltage converter this controls the output (high) voltage much like a houselhold mains lamp dimmer.
 

wrx_pacman

New Member
oh... now i'm really lost i think

use triac after the opamp rather than a mosfet?

i do still need to step the voltage down for the sensor... then back up for that to have control... I'm gonna try and get some books that may help me, but i'm getting more and more lost as i think i'm getting it ;D
 

wrx_pacman

New Member
ah the cathode tubes i was looking at came w/ sound triggers... Which i believe would dim the lamp (switch on and off as fast as bass was detected)

so I believe they would work... Also they are for 12v personal computer supplies... and run only off 12v input... so i don't know :/
 

wrx_pacman

New Member
well thanks for help tansis.. i think i am around good.

i'm not entirely sure on the project but i'm getting the lamp tomorrow and then the sensor in a week and if i blow stuff up oh well :D
 
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