HV inverter assembly

HV Inverter Assembly

Read the following in its entirety! This assumes the basic circuit using a small flyback and input voltage of 12 VDC or less. Some modifications may be needed when using larger flybacks and higher input voltages.

1. Obtain flyback transformer with known good HV secondary winding. primary may be left intact if it is known to be in good condition - non shorted. A flyback removed due to failure may be used if it was the primary that failed and the primary turns can be removed without damaging the HV secondary or losing the secondary return connection! Flybacks fail in both ways (primary and secondary).
2. Locate the return for the high voltage winding. This may be a different color wire than the low voltage winding or may exit from the potted part of the flyback in a different place. It is not possible to use an ohmmeter to locate the return for the high voltage winding if your flyback has a built-in HV rectifier or multiplier as the forward voltage drop of the rectifier diodes is much greater than the battery voltage used in your multimeter. However, a winding connection that has infinite resistance to every other terminal is likely to be the HV return. On flybacks with no HV rectifier or multiplier, the return is easily located by measuring resistance between the HV output and all other terminals. The HV winding will have a resistance of 100s-1000s of ohms compared to single digit readings or less for all the other windings.
3. Wind 10 turn center tapped drive winding and 4 turn centertapped feedback winding using #16 to 20 gauge insulated wire. Make sure both halves of each coil are wound in same direction. Connect centertap in each case at the winding - do not bring out a loop. Insulate well with electrical tape.
4. Vcc should typically be in the range 12 to 24 volts at a couple of amps. Circuit should start oscillating at around a Vcc of 5 V or so. If you do not get any HV out, interchange the connections to the transistor bases. Heat sinks are advised for the transistors. Be aware of the capability of your flyback (B/W monitors up to 15 kV, color up to 30 kV). You risk destroying the secondary windings and/or HV rectifier if you get carried away. Running this on 24 volts will probably cause an internal arc-over in a small flyback, at which point you start over with more caution and a new flyback.
5. Actual output will depend on turns ratio of the flyback you have.