Repurpose An Old CRT Computer Monitor As A High Voltage Science Project Power Supply

High Voltage Monitor Power Supply Conversion

Finally somebody has found a good use for all those old CRT computer monitors finding their way to the landfills. [Steven Dufresne] from steps us through a very simple conversion of a CRT computer monitor into a high-voltage power supply. Sure you can make a few small sparks but this conversion is also useful for many science projects. [Steve] uses the monitor power supply to demonstrate powering an ionocraft in his video, a classic science experiment using high voltage.

The conversion is just as simple as you would think. You need to safely discharge the TV tube, cut the cup off the high voltage anode cable and reroute it to a mounting bracket outside the monitor. The system needs to be earth grounded so [Steve] connects up a couple of ground cables. One ground cable for the project and one for a safety discharge rod. It’s really that simple and once wired up to a science project you have 25kV volts at your disposal by simply turning on the monitor. You don’t want to produce a lot of large sparks with this conversion because it will destroy the parts inside the monitor. The 240K Ohm 2 watt resistor [Steve] added will help keep those discharges to a minimum and protect the monitor from being destroyed.

Yes this is dangerous but when you’re working with high-voltage science experiments danger is something you deal with correctly. This isn’t the safest way to get high-voltage but if you have to hack something together for a project this will get you there and [Steve] is quite cautious including warning people of the dangers and how to safely discharge your experiment and the power supply after every use. This isn’t the first high-voltage power supply that [Steve] has constructed; we featured his home-built 30kV power supply in the past, which is a more conventional way to build a HV power supply using a doubler or tripler circuit. Join us after the break to watch the video.

17 thoughts on “Repurpose An Old CRT Computer Monitor As A High Voltage Science Project Power Supply

  1. This brings back memories…

    Once, for the science fair, some friends and I decided to build some electrostatic loudspeakers. We got to the point where we needed the high voltage bias supply for the diaphragm, but didn’t have the parts on hand to build it properly. Instead of waiting, we cannibalized a mac classic and were able to get things going until we could build something from scratch.

    On an related note, this was also the project where I learned why bleeder resistors in power supplies are a good idea :)

  2. be careful doing this with sensitive electronics on the same circuit. I once borked a computer that was on the other end of a large living room when using my tv for this.

  3. Shouldn’t that return be through neutral and not Gnd/Earth? – That must produce some differential current – evidently not enough to trip the RCD though…

      1. Soitenly, & it’s entirely possible that i’m talking crap here – power systems is not my area…

        RCDs are basically a differential protection scheme (obviously on a trip curve but I believe most (for homes anyway) are around 30mA instantaneous as well) with the differential current detected between the live and neutral… So the earth should be entirely dead while the system is working properly to avoid tripping the RCD/// Any current through the earth line will cause a differential (live/neutral) and cause the RCD to trip as intended.

        Obviously at the voltages out of the flyback the current will be small which is why the RCD won’t trip… I was basically asking if this should ‘technically’ use the houses neutral instead of earth? I wasn’t sure but I think that when the flyback is used as intended (to shoot electrons at the screen) then the return would be through neutral and not earth???

        1. @signeduptocomplain, I think you’re mostly right. You should never intend to run current through ground. That’s what neutral’s for. Running current through ground is a bad idea, since the total ground system isn’t sized for it, isn’t protected against overcurrent, generates ground loops, etc.
          This works for him because either his neutral is grounded inside the TV and no current is flowing though the mains earth conductor, or he has no RCD.

          FYI, here in the States, we usually don’t know what an RCD is. We use GFCIs. A class A GFCI is for residential use, and trips at 5mA. A class B GFCI (or I think GFPE) is 20 or 30mA, and is more for equipment protection, and would be found in an industrial environment. I believe the European RCDs fall somewhere between the in terms of sensitivity.

        2. Ah, RCD = GFCI here in Canada as wernicke said. Makes sense. Our building is so old that we don’t have any GFCI at all. The current is too small to trip while the lifter is running and when there’s a spark, the current is likely high enough to trip but too short a time, somewhere in the microseconds. But I agree technically it should go through the neutral.

    1. CRTs (probably) use flyback transformers, so the high voltage circuit will be at a different (higher) frequency. Since Neutral is just an AC voltage out of phase with Active, you can’t (or at least shouldn’t) use it to ground the HV circuit.

      Even if you assume the CRT was consuming the maximum possible load (10A at 240 V), shorting it to ground would only result in a current of 1.7 uA. According to wikipedia, even high sensitivity RCDs detect only currents greater than 6 mA.

      1. R, makes sense – in the tv circuit, is this still sent through ground or filtered and put back through neutral?

        wernicke – I wouldn’t be surprised to see similar classes in the Australian standards… Their mostly cobbled together from British standards, ANSI and/or IEEE though adherence to the standards is far more strict here than in the states (anecdotal) so it would be very very rare to see a building without RCDs|’GFCI’ here regardless of age.

  4. Fuzz Baker was a venerable old soul, an ancient guy that added to the shop’s electronics knowledge base by forcing examples to indelibly infect your mind. He, with two fingers of one hand, was good to within 10V on anything from 50 to 650V, meters proved him out regularly. Sucker used to grab the anode line of a crt then reach over and “touch” you. Thankfully he watched to see if you were grounded or not. The effect was still … hmmm… “impressive”. You could trust him completely, but never should have at all.

    I love electronics… but there’s just some things I don’t want to expose the newbies to till yr 5. .

  5. If your current needs aren’t very high, a much safer option is to simply put a sheet of aluminum foil over the screen and connect a lead to it. No modification needed (except maybe the addition of a ground wire, but you can get that from the power cord or video input).
    Related science-fair project idea: use an old CRT as a Williams tube.

  6. Well you knew someone was going to rant about safety so here it is –

    1) The pliers he used to lift the rubber anode cap may well be insulated but their insulation rating is probably about 500 Volts or less. It is a fallacy to believe these pliers offer any level of safety when you’re potentially dealing with tens of thousands of volts. Use a long plastic implement to lift the anode cap if you need to do so manually. A long plastic paint scraper from the hardware store for example.

    2) Discharge leads should be rated for the full anode voltage. The reason for this is that if your earth point fails then the tube will discharge through the person holding the discharge lead unless it is rated for use at this voltage. To make a cheep discharge lead cut the anode lead from a trashed monitor (that of course has been discharged) and very carefully solder an alligator clip to one end. Solder a long nail to the other end. Then glue the nail into a plastic tube like an old felt pen with the felt removed. This way you don’t have to lift the anode cap, the nail will slide under it so you don’t in any way have to get close to the high voltage before it is discharged. The glue you use should only be at the end where the nail is. Silastic is a good insulator to use as glue but it cures by absorbing moisture. Let it cure for a week before using.

    3) It is the tube that you are discharging so you need to connect to the tubes earth (Cathode). Using any other earth point is just inviting an earth failure to cause an electric shock! The tubes earth is a conductive coating you will see on the back of the screen. You will also notice it is kept away from the anode cap. The normal connection for this conductive coating is a bare wire that goes around the the back of the screen and is anchored to the four corners of the plastic case holding the screen and has a spring to keep tension on it to hold it against the conductive coating. Make sure it is taunt against the screen and connect to this wire to discharge the tube. Nowhere else!

    4) There are reference’s here to ‘TV’. This is NOT a TV, it is a Monitor. The earthing scheme in TV’s are different in some cases. Some TV’s have what is called a ‘hot chassis’. If you don’t know exactly what a ‘hot chassis’ is then don’t even think about using a TV or it may well kill you!

    I personally would never make something like this in the way it has been made and I am a qualified technician with many years of experience working on monitors and TV’s. However I realise that some will try this so please please be very careful. Don’t be anywhere near this when it is powered.

    One final note. When you are soldering the alligator clip onto the end of the discharge lead, solder it like your life may depend on it because … well … it does!

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