This article and video focusses on configuring and validating the Universal Driver (UD2.9) functions and reforming the DC bus capacitors. Starting up a new electronics circuit can be done with various degrees of precaution. The easy way would be to apply full voltage and all inputs / outputs connected and see that the circuit behaves as designed (or burn to the ground). A more careful approach would be to check power supplies, check single functions and add layers of complexity one at a time.
This is the 4th article / video in the series of designing and building a DRSSTC Tesla coil from scratch, with focus on being easy and cheap, for you to replicate. You can find the previous information in these articles:
- Designing A Complete DRSSTC Tesla Coil In 10 Minutes
- Building the 10 Minute DRSSTC Part 1: Finding the Components
- Building the 10 Minute DRSSTC Part 2: Construction and Assembly
Testing the UD2.9
We need some different tools like power supplies, function generator or signal generator, oscilloscope and multimeter. For testing all 4 outputs from GDT to the IGBTs, isolators or differential probes are needed. You could properly also make your own small 1:1 current transformers to isolate your ground clips from eachother.
There is 4 steps that we will go through in testing the UD2.9, but these principles really applies to almost all versions of Tesla coil universal drivers, just omit the tests where those sub-circuits are not present in your driver:
- Supply voltage tests at all voltage regulators, ICs, functions and transistors. Before inserting ICs.
- Interrupter phasing test and adjustment to correct phasing.
- Check feedback input and lockout input to the two AND gates that connects to MOSFET driver IC inputs ENA and ENB. Feedback signal should be a square wave signal at same frequency as the sine wave input on your FB CT connection.
- Check GDT phasing of the outputs connected to the full-bridge of IGBTs.
Typical jumper configuration:
- J6 Insert two jumpers top or bottom, they have to be moved in pair, to change polarity of GDT output stage.
- J7 UVLO Enable is for under voltage lockout feature with jumper in.
- J16 with jumper in is for additional flip-flop running a skip-pulse driving feature. Skip-pulse resets the driver faster than waiting for next interrupter pulse.
- J17 UVLO test measurement point. No jumpers.
- J9 is a OCD comparator test point. No jumpers.
- J14 with jumper disables reset time delay of flip-flop. No jumper for DRSSTC.
- J15 with jumper in left or right side, chooses polarity of optical interrupter input.
- J18 is FB CT measurement point. No jumpers.
- J4 terminal is to be shorted if no phase lead inductor is used. Feedback CT always goes to J3.
Capacitor Reforming
When a electrolytic capacitor is left unused for extended periods of time, years after years and to start with are used capacitors, reforming is recommended. There is a risk that the electrolyte is not completely covering the plates, that it being cold is taking up less space or oxidization layers have formed.
There is a general recommendation, that reforming has to be done over the same amount of hours, as the same amount of years, the component have been in storage. That translates from 5 years on the shelf, to 5 hours of reforming. There is two “methods”, either slowly increasing the voltage or stepping up voltage. We also need two multimeters to watch both the DC voltage across the capacitors and a multimeter capable of showing current in milliamps, to watch the charging current.
- Use a variac / variable transformer / auto-transformer with a full-bridge rectifier for slowly increasing the voltage during the process.
- Use steps of 100 VDC input voltage, with a current limiting resistor keeping current at a maximum of 100 mA.
An empty capacitor looks like a direct short-circuit to a power supply. This high inrush current can destroy an old and cold capacitor. Ensure your capacitors are at room temperature before this process begins. A cold capacitor subjected to a large inrush current is very likely to fail with a internal flashover from the sudden stress.
Expected capacitor life time in Tesla coils is normally not of great concern. 95% of all Tesla coils will most likely NEVER run for more than 10 hours. So we do not need perfectly reformed capacitors or we can even abuse them a bit from not properly overhead in ripple current capability.
Capacitor datasheets will normally state the allowed leakage current for a capacitor to be healthy. Once a voltage level is reached, we leave the capacitor at this level for a little while and see that no current is flowing.