Hey guys, I have a common 24VAC output transformer that's generally used in HVAC systems. I am converting the 24VAC to 5VDC. I am doing this by running the 24VAC through a bridge rectifier and capacitors (2x 10uF ceramic capacitors), which creates a 31VDC output (+-4V ripple). This is then ran into a switching voltage regulator to drop it down to a 5VDC.

One problem I am noticing, is that when I energize the transformer, there is a quick 65V voltage spike. Apparently this is common with 24VAC transformers, especially when drawing a small amount of current. This device typically draws around 50mA on the 5V rail (short spikes up to 200mA).

Here is a scope reading, taking the measurement at the 31VDC output:


And here is a schematic:


What is the best way for suppressing input voltage spikes like this? My switching voltage regulator has a max input rating of 45V. Should I just find a 36V or 39V Zener Diode, and pair it up with a series resistor?
If so, what is the best way to make that series resistor selection, and Zener Diode wattage selection?

Thanks and any help or advice is greatly appreciated!

 

Probably what is going on is that "leakage inductance" in the transformer secondary or just wiring inductance is resonating with the ceramic capacitors. This is one of the drawbacks to "all ceramic" designs. Ceramic caps perform very well at high frequency and will for a "high Q" resonant circuit that can lead to substantial voltage ringing. The easiest way to deal with this, and what is really necessary anyway with a 50 or 60 Hz transformer and rectifier is a reasonably large electrolytic capacitor. If you have no load other than your oscilloscope and have 4 V p-p ripple, you can imagine the ripple with a load of tens of milliamps (try adding a resistor of 10k across the caps and see what you get). The electrolytic will do a much better job of "smoothing" and it will also probably completely fix the big spike. Since your maximum load is going to be about 50 mA, a 100 µF capacitor is probably enough and won't be very big.

If the electrolytic cap doesn't fix the spike, a zener diode rated at 1 watt will probably be adequate. The 39 V part should be OK and will give you some margin in the event your AC line voltage increases occasionally. With your load you could use a low value resistor between the output of the bridge and the zener, but you probably don't really need one for a "one time" event that isn't very high in energy. A 22 ohm resistor would probably be OK and will only drop about 1 volt in normal operation at your expected maximum load. You might consider a zener-type transient suppressor and not bother with the resistor. Even the small ones will survive pretty substantial current spikes.