Protected Voltage Indicator Circuit

Introduction: Protected Voltage Indicator Circuit

There are times when working with electricity when you may want to use a high voltage circuit. While this may be essential to the functionality of whatever you're building, coming into contact with the energized circuit could cause serious bodily harm. You might find it useful to hook up a voltage indicator (a light, a noise, or anything else you can notice from a distance) so you can so you can tell whether your system is on, doing what it should, or is safe to work with.

This Instructable will walk you through the steps to set up and simulate a voltage indicator separate from whatever high voltage circuit you're working with. This allows for the indicator to be on a lower voltage circuit, increasing safety.

Before we begin:

  • We will be using mostly analog components, for simplicity.
  • Since this setup is made to complement an existing circuit, these instructions assume you have at least some knowledge of electrical theory. Only more complicated concepts will be defined.
  • You must have and know how to use Spice circuit simulation software to proceed from here. You may download LTspice for free from this link:
    • It is recommended you play around with the software and get an understanding of it if this is your first time working with Spice, but the steps ahead will walk you through some of the program.


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Step 1: What You'll Need


Once again, Spice software is required to proceed. Any kind will work, but these instructions will be using LTspice.

Components for Physical Design

You'll need to determine the proper ratings for each of these components, since these are general instructions intended to work with whatever input voltage you have. Short descriptions for components you aren't expected to know are included.

1. 1 bipolar junction transistor (BJT)

  • a device that conducts current only if the voltages at its source and gate are high enough. In this circuit, it will be used to protect the optocoupler and your output circuit.

2. 1 light emitting diode (LED)

3. 1 optocoupler

  • Analog component that consists of an LED and a phototransistor. All you need to know about it is that when it receives enough voltage and current, the LED inside it turns on, causing the circuit connected to the phototransistor to conduct. These circuits are physically disassociated from each other, which is why an opto is being used.

4. 1 power supply

5. 1 zener diode

  • Only allows current to flow after the voltage on the gate side reaches a certain threshold. The voltage allowed through remains constant, but current will go up as gate side voltage increases. The zener is essential to having the indicator turn on only at a certain voltage.

6. 2+ resistors

Step 2: Open LTspice and Gather Your Components

  • Open LTspice and gather the materials previously listed. Refer to the images above for what they should look like.

  • Click on the highlighted symbol to open the components menu, and you should be able to find most of them easily.
    • Optocouplers can be found under "optos" and an NPN BJT is being used here.

Step 3: Choose Your Zener Breakdown Voltage

  • Right click the zener diode you chose and click "Pick New Diode."
    • A list of different types of diodes with different breakdown voltages can be found (a breakdown voltage is the value at which voltage becomes high enough to overcome the natural flow of current across a diode).

  • Choose a rating for which the indicator should turn on.
    • For instance, in the figure above, the 1N5371B begins conducting at 60 V, so the indicator would turn on when the source voltage reaches that potential difference.

Step 4: Build Your Protective Circuit

  • Set up your circuit as it is seen in the image. This will be the input circuit we're building. It is recommended you place ground on the bottom wire.

The following steps include reasons for why components are oriented the way they are. It is recommended you read them so you can decide what values to choose and why they're there in the first place. Feel free to shift between this step and the next so LTspice can do the math instead of you.

  • Choose a BJT by the same right clicking method with which you chose your zener.
    • It matters less which one you choose, just make a choice so the program has a model to go off of.
    • The purpose of this BJT is to redirect current away from the optocoupler (and the LED inside it) once the voltage at the base (side pointed toward the opto) of it is high enough.
    • This prevents the octocoupler from blowing out by managing current coming through the zener.
  • R1 is essential to protecting the zener diode and the rest of the input circuit.
    • The zener can only handle a certain number of watts across it before it is destroyed, and R1 lowers current and thus lowers power (W=R*I^2).
    • Most resistors can only handle half a watt of power, so choose a high enough R1 that the current across it won't destroy it. If your input voltage is high enough, you may want to buy resistors rated for higher wattage.
  • R2 can be much smaller, and protects the BJT.
  • OPTIONAL: If you want to try a more advanced design after getting the values you need, consider putting an appropriately rated reversed diode in parallel with the BJT (and a fuse on the upper line, but you can't do that in spice).
    • In case the zener breaks, the voltage on the upper wire should go to ground across the diode and save the rest of the input, and the fuse will blow.

When your resistor values are chosen, the current you read across your resistors should be around 15 mA to protect the LED in the opto.

Step 5: Simulate the Circuit

Now that you've chosen values for your input circuit, you need to test them so you can be sure that they're accurate. This is what makes LTspice an extremely useful tool. The best way to do this is with a DC sweep.

  1. Click the "Simulate" tab in spice and pan down to "Edit Simulation CMD."
  2. Choose DC sweep.
  3. Refer to the attached image, sweeping from a suitably low value to one above where you'd like your indicator to turn on.
  4. Going back to "Simulate" click run.
  5. Now that you're running the simulation, you can view electrical values anywhere in the circuit. If you mouse over different wires and components, you can take voltage readings and current readings, respectively. Furthermore, if you hold the Alt key while clicking on components, you can see the power across them.
    • Ex: See the attached waveform image. You may notice that values on the right side of the zener diode remain 0 until the input voltage reaches 60, which the zener diode is rated for.

Use the values you find in the simulation to tweak component values until you start seeing what you want to see.

Step 6: Output Circuit

Your indicator circuit could be anything, whether it be a simple light or something more complicated. However, whatever you do, you'll need to have some small voltage source to power it.

  • Add a voltage source in a manner shown in the figure.
    • Once the voltage reaches the threshold you've set on the input side of the circuit, the optocoupler should complete the output circuit and power whatever you put at Vout.
  • Test your circuit again to make sure your indicator turns on.

Assuming your simulation met all the expectations listed, you should now order parts and build your circuit.

Step 7: Build What You've Designed!

Great work! You've made a voltage indicator that should work and is safe. You're now ready to build. I won't give you instructions on how to do that, because if you needed to build this circuit in the first place you probably have some experience with hands on electronics. Good luck purchasing parts and putting them together, and DON'T BUILD WITH THE POWER ON.

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    3 years ago

    Thanks for sharing :)