Negative Voltage Supply

9,539

42

18

About: I've always liked pulling things apart - it's the putting back together again that I have some issues with.

Most who play around with electronics would have come across an audio circuit that uses a duel rail power supply. The first time I came across this it totally baffled me – how do I get a negative charge out of a power supply? Isn’t one positive and the other ground? For some reason I had never thought that a power source such as a battery has equal negative and positive charge!

Most of the time the negative charge is grounded and not used but in some builds such as audio projects like amps and synths, you need to use the negative charge along with the positive.

Robin Mitchellover at “All About Circuits” has published a very elegant and easy way to create negative charge using only a handful of common parts that most people who play around with circuits will have in their parts bins.

I won’t go into how this works as Robin has explained it excellently in his article which can be found here.

The circuit itself is made up of a 555 timer (is there anything it can’t do!), a few caps and diodes. I wanted to make mine variable voltage controlled and portable to use and test on future projects so I included a buck booster in the design.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Parts and Tools

Negative Voltage Circuit Parts

1. 555 Timer – eBay 100 under $5!

2. 5.6K Resistor. Buy these as assorted on eBay

3. 47K resistor

4. 100nf Cap – Buy these as assorted on eBay

5. 10nf Cap

6. Diode 1N194 – eBay

7. 10uf Cap – Buy these as assorted on eBay

8. 100uf Cap

9. Prototype board - eBay

10. Assorted wires


To make it portable

1. Case – This one would work fine from eBay. Mines an old garage door opener I found somewhere.

2. Male and Female banana plugs – eBay

3. Various wires

4. Switch - ebay

5. 9V battery

6. 9V battery holder - eBay

7. Voltage regulator – eBay

8. Knob for a potentiometer – eBay

9. 10K Pot - eBay

10. Voltage Meter - eBay


Tools:

1. Soldering Iron

2. Pliers

3. Wire cutters

4. Hot Glue

5. Drill

6. Cone stepper drill piece (always comes in handy for drilling holes into things)

Step 2: Breadboard It First

I know this this might be self-evident but I highly recommend that you breadboard this circuit (or any that you build) first. It will ensure that the circuit has been tested and works and is like a first run through which helps you get a better understanding of the circuit and how it is put together.

Once you have built it, test with a multi meter and ensure that the voltage being supplied from the circuit is negative.

Step 3: Making the Circuit - Part 1

The circuit is a really interesting one and uses a clever array of diodes and capacitors to achieve negative voltage on a capacitor plate. Check out this link if you want further details. If you haven’t made any circuits before then check out this Instructable which will show you the ropes.

I have included the original circuit schematic along with the modified one which includes the voltage module and a momentary switch connected to the capacitor. This switch can short the cap and discharge the voltage inside. I had to add this as the cap held the voltage supplied from the voltage regulator and if it was high, say 12v's and I reduced the voltage to 6v's, then the negative voltage will stay at 12V and slowly come down. The reset button discharges the cap and brings it in line with the positive voltage.

Steps:

1. First thing to do is to work out how big (or small) you need to make the circuit board. As I was putting mine inside the old garage door opener, I needed to make it as small as possible.

2. Trim the prototype board to size

3. Add a socket IC holder to the board. This will allow you to change out the IC if faulty for any reason.

4. Connect pin 1 to the ground bus strip on the prototype board, pin 4 and 8 to the positive bus strip.

5. Add a 10nf cap to pin 2 and ground

6. Add a 100nf cap to pin 5 and ground

Step 4: Making the Circuit - Part 2

To make the circuit as small as possible, I utilized the bottom of the circuit as well.

Steps:

1. Connect pins 2 and 6 together. I use a resistor leg to do this

2. Connect pins 2 and 7 together with a 47K resistor

3. Add the positive leg of a 10uf cap to pin 3 and the negative leg to a blank spot on the prototype board.

4. Add a diode (making sure it is connected the right way) to the negative leg of the cap and ground

5. Add another diode (checking again that it is correctly connected) to the negative leg of the 10uf cap and the other leg to a blank spot on the prototype board.

Step 5: Making the Circuit - Part 3

Steps:

1. Add the negative leg from a 100uf capacitor to the end of the diode

2. Add the positive leg to ground.

3. If you used similar prototype board as me you will need to connect the ground and positive bus strips together. Solder a couple of small wires to connect these

4. At this stage I always like to check and make sure that the circuit will fit inside my case. There wasn’t much room inside the garage door remote I used and the circuit fitted just. I did remove a small amount of the prototype board to make it fit a little better.

Step 6: Adding Wires to the Circuit

Next thing to do is to add a bunch of wires to the circuit. Once you have added these you can test it to see if it is working

Steps:

1. First add 2 wires (make all wires longer then needed) to the positive bus strip. One will be joined to the positive output on the voltage regulator and the other to a female banana plug

2. Add another 2 wires to the negative bus strip. One will be connected to ground on the voltage regulator and the other to a female banana plug

3. Lastly, add a wire to the negative leg of the 100uf cap. This will be connected to negative voltage banana plug

Step 7: Adding the Banana Plugs and Switches

There wasn’t much room in my case so I had to think carefully where each of the parts were going to go, especially the banana plugs and switch.

Not shown here as it was something I did later was another momentary switch which you will also need to add. This switch will be later connected to each leg on the 100uf cap to discharge any voltage it may be holding.

Steps:

1. First, drill a hole each for the 3 female banana plugs

2. Secure the banana plugs to the case. I went with from left to right, red – negative, black – ground, and red – positive. Seemed like the most logical way to set them up

3. Drill another hole for the SPDT switch and attached this as well.

4. Drill another hole and ad the momentary switch.

5. Lastly, drill a hole in the top of the case for the wire on the voltage meter. Push the wires through and secure the voltage meter to the case with some hot glue. As I didn't have much room I had to stick the meter on the top of the case. The better way is to cut out a section of the case that the meter will fit into. It's a cleaner finish.

Step 8: Modding the Voltage Regulator

I’m not going to go through this in a lot of detail as I have already provided details on how to do it in this ‘ible. I have also included a diagram which will help you visualise the wiring

Steps:

1. Remove the pot that is on the regulator by carefully de-soldering it

2. Grab your 10k pot and place the legs on the solder points. Re-heat them and push the legs into place.

3. Add a little solder if necessary to the solder points on the circuit board.

Step 9: Adding the Parts to the Case and Wiring-up

You can see in the images below, I really didn’t have much room to play around with!

Steps:

1. First, secure the voltage regulator in place. Make sure that you can get at the solder points easily. If not, then don’t secure into place until you have done all of the soldering

2. Next add the negative voltage circuit to the case

3. Connect ground and positive from the circuit board to the outputs on the voltage regulator

4. Solder the wires from the voltage meter also to the output of the voltage regulator.

5. Solder the positive wire from the battery holder to the switch and another wire from the switch to the input positive solder point on the voltage regulator

6. Solder the ground wire form the battery holder to the ground input solder point on the regulator

7. Now connect the negative wire from the circuit to the negative banana plug. Do the same for ground and positive

8. Now you should be able to add a battery and test whether it is working

Step 10: Testing and Using

The first thing that you want to know is whether your voltage regulator is working ok.

Steps:

1. Turn it on and check that the voltage meter is working by adjusting the potentiometer. The voltage should move up or down.

2. Next, test to see if the negative voltage is working by using a multi meter. Please the positive wire from the multi meter into the negative banana plug and the ground into the ground banana plug.

3. Check to see if the multi meter shows a negative voltage. If it doesn’t, check over your circuit and make sure everything is correctly soldered and there are no shorts.

4. Lastly, you should check the capacitor discharge button. Turn up the voltage meter (don’t go too high or you could fry the 555 timer) and then bring the voltage down. Check the negative voltage with the multi-meter. It will show higher then what is being displayed on the voltage meter. This is because the cap is charged to the last voltage that the regulator was at. To discharge, push the momentary button.

5. Check the multi-meter again. It should show close to the voltage meter

Be the First to Share

    Recommendations

    • CNC Contest

      CNC Contest
    • Make it Move

      Make it Move
    • Teacher Contest

      Teacher Contest

    18 Discussions

    0
    None
    Hairy Bear

    6 days ago on Step 10

    I tend to use Black for Ground (0 volts), Green for earth, Red for positive, Blue for negative.
    But then you could always use two batteries if you don't need variable voltages.

    0
    None
    vk2gwk

    4 weeks ago

    Suggestion: use a different colour for the negative banana socket (like green) so you don't mix up positive and negative. This circuit won't tolerate much load on the negative. What you do is create AC with the 555 and rectify that to negative. The output power of the NE555 is not high - about 30 - 50 mA at most - and the rectified negative voltage is not stabilised so will drop on load. So this circuit is only suitable for very low load circuits.

    2 replies
    0
    None
    lonesoulsurfervk2gwk

    Reply 4 weeks ago

    you are right - it won't take a lot of load but for small projects Op amps etc it works well. I've got another build I'm doing for -12v which uses AC.

    0
    None
    Hairy Bearlonesoulsurfer

    Reply 6 days ago

    I tend to use Black for Ground (0 volts), Green for earth, Red for positive, Blue for negative.
    But let's face it, a PP9 battery won't supply much current for long anyway.

    0
    None
    MarPok

    24 days ago

    A tip about negative voltage... there is a good IC "ICL7760". Works in the same way as the circuit with the 555 (see charge pump), but it can give a bigger current without voltage drop. I'm using it with 79L05 to get precisely -5 V.

    1 reply
    0
    None
    lonesoulsurferMarPok

    Reply 22 days ago

    Great tip. Will look to get a few of these IC’s and give them a try

    Add a resistor in series with the push button to short the output cap, your 5.6k will do, anything over 50 ohms will be fine. Without the the resistor the momentary current through the switch will be very high and will eventually burn the switch contacts. The resistor limits this current though it does slightly slow the cap discharge (t=RC).

    1 reply
    0
    None
    maxhirez

    4 weeks ago

    Nice work! Maybe I read through this too quickly, but did I miss the link to the original AAC article somewhere?

    3 replies
    0
    None
    lonesoulsurfermaxhirez

    Reply 4 weeks ago

    opps! I have added the link to the original in the intro.

    0
    None
    maxhirezlonesoulsurfer

    Reply 4 weeks ago

    Thanks dude-this is awesome! I may try the Schmitt trigger version too! Have you tried this on any Eurorack modules?

    0
    None
    lonesoulsurfermaxhirez

    Reply 24 days ago

    Nah - it's not really powerful enough to work on a eurorack. I am working on one however that should work a treat

    0
    None
    abomin

    Question 4 weeks ago

    Hello! What current it could deliver?

    1 answer
    1
    None
    JohnC430abomin

    Answer 4 weeks ago

    the 555 circuit with 1N914 diodes and the puny 9V battery will realistically deliver about 80mA max before the diodes get hot or the battery gets run down. in this circuit, the positive input voltage is the positive for the load (audio amp) and the output from the negative side will be the negative to the load (again the audio amp). I did not see a complete schematic in his i'ble, but i am assuming that the boost circuit will be the one to supply the LM555 circuit.
    The whole idea is ok for a few opamp circuits and not for an audio power amp. but you can get much more power if you throw away the 555 and connect the 10uF cap at the output of the 555 that goes to the 1N914 directly to the boost transistor Drain. the boost is capable of much higher current. Change the 1N914 to some higher current faster diodes, e.g. https://www.ebay.com/itm/10-PCS-SR5200-SB5200-MBR5...
    these are 5 amp/200V Schottky diodes, (I just checked the spec sheet) although 45Volt diodes would be better. the boost module is good for about an amp or so and this mod will give you a lot more power. make sure the 10uF cap is connected in the correct polarity, or use a 10uF ceramic cap which will work a lot better.

    0
    None
    yrralguthrie

    4 weeks ago

    Very good work on the writing. But any power supply would have worked just as well. Just reverse the connections.
    This statement, "...Most of the time the negative charge is grounded and not used but in some builds such as audio projects like amps and synths, you need to use the negative charge along with the positive...." is a gross misunderstanding.

    1 reply
    0
    None
    bardenbobyrralguthrie

    Reply 4 weeks ago

    Only power supplies with outputs truly floating with respect to the AC line power (such as this battery operated one) are able to have either terminal connected to ground. Many switching supply converters getting their power from 110 VAC have one of their output terminals (usually the negative) tied to the 3rd wire ground for safety purposes. If you then ground the positive terminal to try to get an output negative with respect to ground you will short out the supply.