My war against dual rcd boards

The question that Graham asked was quite specific:

”However, if the lamp is damaged, and the user is being protected against accidental contact with live parts, say after the rectifier, would the type AC RCD operate is perhaps another?”

in that he said “after the rectifier”, so assume there are some diodes charging a capacitor, which discharges to supply the DC current, and the input current is 25 mA at 230 volts, what is the DC output current and voltage?

You tell me. 

The input is 6.44 W, so the product of the output current and voltage must be no more than that.

If there is a DC fault, there is no reason to believe that a Type AC RCD would trip, but it might. Graham will correct me if I have misunderstood.

LED car headlamps glow with a supply of nominally 12 V, so that would give potentially 0.5 A @ 12 V. The figure that I have in my memory for the resistance of a human body finger-to-toe is 2 kΩ, so now we are looking at a current of 12/2000 = 6 mA.

The figure that I have in my memory for the resistance of a human body finger-to-toe is 2 kΩ, so now we are looking at a current of 12/2000 = 6 mA.

Indeed such a figure is quite credible- and  a glancing dry contact with mains is not normally fatal - just painful. But, not always ! and certainly there are enough fatalities to justify the desire to take steps to protect against mains shocks.

That resistance falls if the shock current is sustained, as the skin chars at the point of contact, and the current then rises - the core of the body is much lower resistance, and damaged or wet skin removes most of that high resistance, allowing a far more dangerous current to flow.

Also be aware that a lot of the small LED lights use a series capacitor dropper to set the LED current, and the mains current is higher than the wattage may suggest, as the power factor is poor. If the LEDs total 100V and a the rest is capacities, then the current is more than double the wattage divided by 230.

Mike.

Chris Pearson said:

You tell me

I asked AI to explain it, because it's a long time since I did this at college! The DC voltage will be around 330 volts:

"In a simple diode-capacitor circuit (a half-wave rectifier with a smoothing capacitor), the capacitor charges to nearly the peak voltage (Vpeakcap V sub p e a k end-sub) of the AC input, which is about 1.414 times the input RMS voltage (Vrmscap V sub r m s end-sub), minus a small diode voltage drop (Vdcap V sub d). So, Vpeak≈(Vrms×1.414)−Vdcap V sub p e a k end-sub is approximately equal to open paren cap V sub r m s end-sub cross 1.414 close paren minus cap V sub d≈(×1.414)−, resulting in a DC output close to the peak value, not the RMS value, with some ripple.”.

mapj1 said:

Also be aware that a lot of the small LED lights use a series capacitor dropper to set the LED current, and the mains current is higher than the wattage may suggest, as the power factor is poor. If the LEDs total 100V and a the rest is capacities, then the current is more than double the wattage divided by 230.

Yes, but Sparkingchip specified the input current and voltage, so in this example, the wattage of the LED itself is less than 6.44 W, say 3 W.

I too have dim and distant memories of a simple rectifier circuit, but I do not think that applies to LEDs.

Without doubt, powerful LEDs need only 12 V (nominal) else my cars' headlamps would not work. That is why I used a value of 12 V. It may be that the voltage across an LED is lower, and the current correspondingly higher.

Of course there might be a string of LEDs in series with a higher voltage across them.

Filaments were so much simpler!

I asked AI about HID headlights, they are not something to mess about with!

Automobile HID (High-Intensity Discharge) headlights use a ballast to transform the car's 12V power, requiring a massive 20,000 to 26,000 volts for ignition, then settling to a steady 80-90 volts (or 42V for newer D3/D4 types) to operate the xenon gas arc, providing bright, efficient light. The ballast is crucial for both the high-voltage start-up and stable operation, making HID systems potentially dangerous if handled improperly.

I asked AI about HID headlights, they are not something to mess about with!

Automobile HID (High-Intensity Discharge) headlights use a ballast to transform the car's 12V power, requiring a massive 20,000 to 26,000 volts for ignition, then settling to a steady 80-90 volts (or 42V for newer D3/D4 types) to operate the xenon gas arc, providing bright, efficient light. The ballast is crucial for both the high-voltage start-up and stable operation, making HID systems potentially dangerous if handled improperly.

The gas discharge car lights are indeed a marvel of HV tech, but in a car, with enough petrol to put the driver into orbit, 25kV sparks to ignite that fuel, and red hot catalyst glowing in the exhaust, in a single skin metal box that does up to 70mph, (honestly officer), the additional risk is actually 'small beer'. If they were indoors, they'd probably be banned .

Mike.