Voltage inverter

Discussion in 'General Electronics Chat' started by gdrag, Aug 16, 2008.

  1. gdrag

    Thread Starter New Member

    Aug 16, 2008
    3
    0
    I am a naive hobbyist and I am looking for a simple voltage inverter that will take a varying voltage input and simply output that same negative voltage. It not be heavily loaded (probably less than 1 amp) and I am really looking for the simplist easiest to implemnent soluytion and am really hoping there is a regulator that I can utilize with no or few external parts to the actual component solution...does any one have any suggestions?
    Thanks
    Guy
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Welcome to the forums, Guy.

    What is the minimum and maximum voltages that the input signal might have?

    What kind of frequency range is expected for the input signal?

    Do you wish to actually invert the waveform, or just offset it from another voltage?

    1 Ampere is actually a good bit of current. This rather limits various options. It's still do-able, but it will help us recommend less expensive solutions if your actual current requirements were considerably less.
     
  3. gdrag

    Thread Starter New Member

    Aug 16, 2008
    3
    0
    Thanks for the greeting...I appreciate the respose.
    The voltage will vary from around +3 to +12v in (0 to -12v out)...it will also be a dc input voltage if that helps. Basically I am having to convert and input voltage to a color mixing circuit for strip LEDs from a unit that switches the positive side RGB (turns on leds by applying positive voltage to RGB led with a common negative) to switch RGB from the neg (turns on RGB led by appying negative side to turn on to a common +). If anyone has a suggestion as to a better way to get there I am certainly open as well.
    Thanks again
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK. The first thing to clear up is that LEDs are current devices, not voltage.
    LEDs have a Vf (forward voltage) that is relatively constant over a fairly wide range of current. If you try to vary the voltage across an LED, you will get a tremendous change in current for a very small change in voltage.

    A typical way to control the intensity of LEDs is to supply them a constant current, but modulate that current using PWM (switch the current on for a period of time, then off for a period of time) so that one can efficiently control the apparent brightness of the LEDs.

    It is very likely that each LED contained within the RGB LED has a different Vf, which complicates the control somewhat. The red portion might be 1.7v to 2.4v, the green portion might be 2.0v to 3.0v, and the blue may be 3.4v to 3.9v. It is likely that they all will have a similar current rating.

    The RGB LEDs with a common negative are called common cathode.
    The RGB LEDs with a common + are called common anode.

    You cannot run LEDs in parallel without matching their Vf. For RGB LED's, this would be practically impossible. The best way to balance them out would be to run them in a multiplexed array, but that's a bit advanced, and I'd hate to confuse you so soon.

    So, you are basically "stuck" with using multiple resistors for each LED - unless they already have the resistors built in, or someone has wired them up? If the LEDs already have resistors limiting their current, then they actually CAN be controlled using a voltage (since the Vf across each LED will remain constant, but the voltage across the resistors will vary considerably)

    Do you have a schematic for how the LEDs are wired up?

    How many LEDs are there?

    Do you know their ratings? Do you have a datasheet?
     
  5. gdrag

    Thread Starter New Member

    Aug 16, 2008
    3
    0
    Essentially I already have a controller for these strip LEDs. The only problem is that (taking into account your insight into my previous post) the controller is meant to control a cathode? (+) strip and I ended up with an anode? (-) RGB strip led. So I figured that I could just invert the DC voltage coming out of the controller and use it (it is a nice one with an IR remote and a multi program multi mix capability).
    Does this make sense or am I off base in thinking I can easily just reverse the voltages that the current controller sends to the red/green/blue leds and use this one on the strip leds that are + common (cathode ?)
    Thanks again for all of the help...
    Guy
    PS the strip leds have 4 wires to each flexible circuit strip that has about 150 multicolor (all colors on each individual led) chips with red-/green-/blue-/white+ wires to input power for the entire strip of multicolor leds. On the strip I need to drive the plus side is common (as detailed above) but the controller that also has 4 wires uses the minus side as common (red+/green+blue+/white-)...ie the problem I have...
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, have a look at the attached schematic.
    It's an excerpt from National Semiconductor's LM675 datasheet, which you should download from their website.
    http://www.national.com
    Newark and Digikey both carry this opamp for under $6. The opamp can put out up to 3A.

    You will need to supply the opamp with both +12V and -12V, with sufficient current to drive the LEDs. If you need a supply, you might check out http://www.mpja.com
    You could use a 24v single supply, and create a virtual ground as shown in the LM675 datasheet.
    24v 2.5a supply:
    http://www.mpja.com/prodinfo.asp?number=16008+PS

    For R1, you could use 10 Ohms, for C you could use somewhere around 0.1uF.
    For R2, you could use 1k, 3.3k, 4.7k, 7.5k, 10k... basically anything from 1k to 10k just as long as they were very close to the same resistance. I suggest metal film resistors. They are usually blue in color, where carbon film resistors are usually light brown or tan. The metal film resistors are normally 1% tolerance, are less noisy and much more stable over temperature than carbon film resistors are.
     
Loading...