Re: LED Charge Indicator

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 08:13 AM 10/9/2005, you wrote:
> Original poster: BunnyKiller <bunikllr@xxxxxxx>
>
> I would say that the voltages are a bit too high, the required 
> resistance to keep the LED lit at upper end voltages would be in the 
> near 1 megaohm rating. When the voltage drops in the cap ( but at 
> still dangerous levels)  the resistor will reduce the needed current 
> to keep the led lit thus giving you a false sence of security 
> thinking the cap is discharged.....
>
> for example   lets say the LED you chose needs .030 A to be lit at 
> full power and turns off at a minimum of .008A.
>
> 15KV NST can easily charge a cap to 21KV
> 21,000/.030 = 700Kohm resistor needed to keep led alive at 
> 21KV  (given that the resistor wont arc over)

Also contemplate the power dissipation in that 700K resistor.. a mere 
600W (21kV * 30mA)

> .008X 700,000 = 5600 V   will still be in cap when LED turns off
>
> and lets not forget that the cap can hold a charge that is reversed 
> of what the LED may need ( since the cap is an AC style cap) and the 
> LED wont even light up in this senerio.
>
> Scot D


In systems where I wanted a "HV present" indicator, I used a high 
impedance voltage divider, driving a threshold detector which then 
drove the relay for the light.

Here's the problem, though... I couldn't ever find a *good* way to 
make it failsafe, so it performs merely as eye candy (in that you 
couldn't trust it.. the light being off didn't guarantee no HV)