Re: Lightbulbs and Power Resistors?

If you want to drain your cap's you need a high load (for a light)
LED's are typicaly rated for 10 to 20mA with a little longer life
(and dimmer) if you de-rate them. brighter if you pulse them, but
I'm going astraty.

Incansescent lights ie:filiment bulbs, have a resistance that changes
with heat. not a problem here. if you use a 20mA LED it will take
many minutes to let the power drain, and then there is a voltage at
which the LED will no longer light but you will still have power in
the circuit.

A filaement bulb would offer a lower resitance and faster drain.
Check out pilot lights. even illuminated switches. Be cautions some
are neon and will not offer a closed circuit! You can wire an
illuminated swtich so it is always lit.

another way is to have a relay coil of some sort that closes on
power. That coil would be a nice bleed until the power dropped.


Dave

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> Chris and Dee wrote:
>
> >So I built my power supply and it outputs 63v. Great! Except
that
> >once its charged, even if I shut down the p/s, I can't hookup
> >anything to the connector I have coming off of the capacitor
unless I
> >creatively and safely discharge the capacitor (we all know how
much
> >DC voltage likes to ARC). I also don't have anything but my
> >multimeter to tell me if the capacitor still has a charge. I
tried
> >using a power resistor from Rad shack on the capacitor so that
when
> >the AC power was removed the cap would discharge somewhat
quicker.
> >However, they don't have any that are rated high enough (or I'm
too
> >electronically challenged to know what one to buy or how to hook
it
> >up). I also wanted to hookup 4 lightbulbs (or LEDS, I don't care)
to
> >show me when the cap has a charge and for each of my 3 drivers to
> >show when power is going to the driver. The only problem with
this
> >is that I can't find any bulbs rated for that voltage does rad
shack
> >carry any voltage regulators for that voltage (not to mention that
I
> >would prefer not to have to solder voltage regulators to each
bulb).
> >
> >Can anybody tell me what resistor to buy and how to hook it up
safely
> >(I thought it just went across the leads of the cap, but maybe
this
> >is wrong). Also, any ideas on how to install bulbs or LEDs to
tell
> >me when the cap and drivers have voltage?
> >
> >
> If you can find a modest light bulb with a 24 V rating (48 would be
even
> better) and can get the
> design current rating for that bulb, you can then figure out a
resistor
> to use for it.
>
> Let's say you have a 24 V, .020 Amp bulb. Power supply voltage is
63.
> So, the series resistor
> needs to drop 63-24=39 V at 20 mA. R=V/I = 39/0.020 = 1950 Ohms.
So,
> you want a 2 K Ohm
> resistor. now, for the power, P=I^2 * R. So, P = (.020)^2 * 2000
= .8
> Watts. So, a 1 W
> resistor will work, but will run somewhat hot. A 2 W resistor
would be
> good, if you can find it.
> Otherwise, four 510 Ohm, 1/2 W resistors in series will do as well.
>
> If you use an LED, the red ones drop 1.6 V at all reasonable
currents,
> the green ones are about 2 V.
> You can run just about all LEDs on 20 mA, and some of the bigger
ones
> will tolerate 50 mA
> pretty well. With the LEDs, the resistors therefore have to
dissipate
> practically all the power.
>
> If your cap bank is really large, you may find it takes many
minutes to
> discharge the caps this
> way. You could rig a 120 V relay with a normally closed contact to
> connect a larger power
> resistor across the cap bank when the AC power goes off. You could
make
> a bigger power
> resistor from a string of smaller ones. You can calculate current
with
> I = V/R.
>
> Jon