HV Isolation Transformer

Any device plugged in to the power mains should be isolated from direct connections by using a transformer. This provides safety from accidental contact, and limits the current in case of short-circuit conditions in the power supply.

The circuits shown below describe how an isolation transformer was selected to charge the HV capacitor.

Isolation Transformer

Although commercial “isolation transformer” exist and are readily available, several surplus transformers in like-new condition were at hand. We can achieve the same effect by connecting two identical transformers back-to-back, to obtain 110 vac. Note that this reduces the overall current capability compared to ratings, and so some testing is needed to confirm how they will operate.

Three transformer pairs were tested. Our goal is to charge the 12,000 TF capacitor in ten seconds or less.

The photo shows one each of the transformers; an identical set of transformers are available but not shown.

Each of the three transformer pairs were tested (see schematic and photo above) and their output measured. We recorded the voltage in loaded and unloaded conditions.

1. T1 = T2 = 25.2 vac ct, 450 mA
   Manufacturer: Radio Shack 273-1366A
   No load: V_out = 116 vac
   RL = 25 ohms: V_out = 10.1 vac
   Current = I = E/R = (10.1v) / (25 ohms) = 404 mA
   When connected to bridge rectifier and charging circuit we measured a charge rate of 30 sec to reach 150vdc, with no current-limiting resistor. This is much too slow.
   
   
2. T1 = T2 = 12.6 vac ct, 3-amp
   Manufacturer: Radio Shack 273-1511B
   No load: Vout = 116 vac
   RL = 25 ohms: Vout = 30.5 vac
   Current = I = E/R = (30.5v) / (25 ohms) = 1.22 amp
   When connected to bridge rectifier and charging circuit we measured a charge rate of 8 sec to reach 150vdc, with no current-limiting resistor. This almost meets requirements.
   
   
3. T1 = T2 = 48 vac ct, 3-amp
   Manufacturer: Unknown
   No load: Vout = 120 vac
   RL = 25 ohms: Vout = 64.7 vac
   Current = I = E/R = (64.7v) / (25Y) = 2.588 amp
   When connected to bridge rectifier and charging circuit we measured a charge rate of 3 sec to reach 150vdc, with no current-limiting resistor. This is quite good, and therefore this pair of transformers were chosen.