# Help designing a safety indicator for a high current transformer

I'm working with a neon sign transformer and want an indicator (preferably a light) on the high/output side to give me a visual cue when there is current coming out of the transformer. I'm not 100% sure how to do this, and given the output voltage, I don't want to make something by trial and error. I've been having a hard time finding resources or parts and I suspect I'm googling the question properly.

I've attached a very rough schematic of what I'm trying to do. Transformer Input is 120VAC/0.9A, grounded on the supply side and coming from standard 120V wall power. Transformer Output is 10kV/30mA I want a lightbulb indicator on the 10kV side.

This link is the closest I've been able to find, but it's not quite what I need.

Do I need to get a transformer to get the output current back down to 120V for a bulb? Or is there some indicator device that can sit on the 10kV line?

What should I be looking for and asking for as I hunt for this device or design the circuit?

• If you feed 120 V 0.9 A to the transformer you get only 10.8 mA at 10 kV. For 30 mA at 10 kV you need 2.5 A at 120 V.
– Uwe
Commented May 8, 2022 at 19:24
• Have you considered a current transformer on the secondary side? A 10:1 ratio transformer with a 6V bulb would supply 100mA to the bulb with a 60V drop on the primary side, which may be acceptable. If you need a lower drop you could use a fixed burden resistor with a Couple of back-to-back LEDs. Using LEDs directly in the 10kV path is technically possible but unwise from a safety perspective.
– Frog
Commented May 8, 2022 at 19:52
• Are you wanting to know about current? Or just knowing if their is a voltage difference? (The current in a circuit with 10 kV potential difference is likely pretty low.) (I gather this is AC from the drawing.)
– jonk
Commented May 8, 2022 at 20:36
• Isn't the 10kV side also constant-current? Because 10kV@30mA = 300 VA, which is 2.5A on the primary. Methinks 30mA is the flow rate and 10kV is a maximum. Commented May 9, 2022 at 0:39
• do you want to know if the output oif the transformer is live (voltage detector), or if it is being used? (current detector) Commented May 9, 2022 at 5:15

The most common indicator for a safety no-voltage/current is a low voltage Neon Indicator with 220k current limiting resistor or as desired. https://www.intl-lighttech.com/instrumentation-sensor-light-sources/neon-lamps. Many suppliers also put these in panel-mount brackets.

On secondary side it is difficult because the insulation. Much easier it is on the primary side. No voltage on primary side => no voltage secondary.

• I believe the OP is asking about current rather than voltage
– Frog
Commented May 8, 2022 at 19:53
• Note that primary side current measurement is difficult, because this type of transformer design has significant primary and leakage inductance. A simple current transformer might not do. A current transformer around a HV wire would be okay though. Commented May 8, 2022 at 20:03
• Usually you measure the bar current if there is a bridge. Place a third coil on the core.
– Bill
Commented May 8, 2022 at 20:07

There are several companies that make current sensing indicators using a mini CT and LED. These provide electrical isolation up to some point. Wiring it in the return wire on a grounded HV system would avoid stressing the insulation. Otherwise using adequately rated HV cable should ensure your safety.

One such current transformer with the manufacturer's name deviously obscured.

Knowing if there's any current flow in the secondary (or the primary) doesn't really help much from a safety standpoint. Suppose there's nothing attached to the output, there would be no current flow to turn on an indicator but there would still be 10,000 V on it. Of course if you touched the wires some current would flow, but having a lamp that lights when you're being electrocuted isn't much of a safety feature.

The only time the output is going to be safe is when there is no voltage on the primary side, a simple pilot lamp on the primary side after the power switch would tell you when the transformer has power to it. You would need something reliable though, such as a neon lamp as Tony pointed out in his answer. You could even use a second one for redundancy so that if one indicator fails there's still another one.

The transformer primary current will be quite small if there is no load on the secondary (high voltage) end. Therefore it is sufficient (and much safer) to monitor the primary (120 V) current.

So now, you just need to detect presence or absence of ~ 2.5 A on the primary. That's a little more difficult to do without electronics.

Use a current-sensing relay -e.g. this which will work if the load is at least 1 A. You would switch a normal 120 V indicator via the outputs.

If the load is actually less than 1 A, you can thread the sensing wire multiple times through the relay to increase sensitivity (e.g. two times would increase sensitivity to 0.5 A).