Re: [vox-tech] [no linux] High voltage circuits?

[jdnewMAPSmil@dcn.davis.ca.us]

On Tue, 20 Mar 2001, Peter Jay Salzman wrote:

> jeff,
> 
> that makes total sense -- an increase in kinetic motion causes an increase in
> resistivity of the filament.
> 
> i think adding a small inductance would also work, and has the benefit of not
> drawing power from your source.  the "back EMF" would kick in only when the
> current initially flows (and stops).

Two different time scales are at work here... the thermal filament warm-up
period, and the 60Hz ac time period.  Even if the thermal time constant is
much shorter than one 60Hz cycle (which I doubt), an inductor probably
won't be a practical solution.  Choosing a larger relay sounds more
appropriate.

> 
> pete
> 
> On Tue 20 Mar 01,  8:53 AM, Jeff DeFay said: 
> > Mark,
> > If you are using incandescent light bulbs, your current will be much higher
> > during the
> > brief time that the filaments are cold.  The 60 watt power consumption is
> > calculated
> > with the filaments at normal operating temperature;  introducing resistance
> > into the circuit
> > would also keep the effective resistance of the filament low by preventing it
> > from reaching
> > normal temperatures.
> > 
> > Regards,
> > Jeff DeFay
> > 
> > "Mark K. Kim" wrote:
> > 
> > > Anybody here ever work with high voltage circuits?  I'm creating a
> > > low-volage / high-voltage hybrid circuit for the first time; a digital
> > > circuit (low voltage, 5DCV) controls the logic portion, and uses two
> > > relays to control two light-bulbs (high voltage, 110ACV). And I'm
> > > wondering:
> > >
> > >    1. How much current usually goes through those 60W light bulbs?
> > >       How much current is required to make it look bright enough?
> > >
> > >       My guess would be:
> > >
> > >          110ACV * I = 60W
> > >          => I = 0.55A (max)

Standard voltage is 120+/-10%, so call it 0.5A.

Also, R = 120/0.5 = 240 ohms.

> > >
> > >       And I'm using a 1A relay rated at 120ACV, so my guess would be
> > >       that I can hook up this relay to the light bulbs directly.
> > >       But when I did that, once the light bulbs were turned on,
> > >       they stayed turned on (I couldn't turn off the relay connection
> > >       -- apparently the connections got fused together.)

What a great practical experiment.

> > >
> > >    2. So my idea was to force-limit the current.  I got new relays
> > >       (same kind) and bought 1W resistors (200 Ohms... actually, two 100
> > >       Ohm resistors soldered together.)  Once I got them in circuit,
> > >       it had two problems:
> > >
> > >       a. The light bulbs were too dark, and too slow to lighten up.
> > >          Apparently too much resistance?

The bulb expects 0.5 Amps, and you are only giving it 120/440=0.27A

> > >
> > >       b. I started seeing smokes come out around the resistors.
> > >          I don't know if this is because the resistors need to be
> > >          higher wattage, or if the current is melting the solder.
> > >          I'm pretty sure the electrical wires are good enough for
> > >          this project (it's rated at 700V), although if the solder
> > >          is melting (meaning it's at least 400 degrees Ferenheit)
> > >          then it could be also melting the electrical tape.
> > >          Ideas?

Voltage rating is for insulation breakdown.  Current rating is the
critical issue here, since small wires have higher resistance.and
dissipate more power.

> > > I need to know if I need to switch to higher rated relays, and if so, to
> > > what.

I haven't done it myself, so I can't say what the actual rating should
be.  You are a student... see if you can get a storage oscilloscope and
look at the current through the bulb as you light it. :)

Might also consider a 40W bulb? or a flourescent bulb?

> > >  Also I need to know if I need to use a different resistor and/or do
> > > not use solder.  I'd really appreciate any suggestions from anyone
> > > knowledgeable in this field.  Thanks!

I don't think the resistor is the right solution, since the bulb is
designed for 120V, and the resistor solution creates a voltage divider.

Compression connections are best for power transmission.

---------------------------------------------------------------------------
Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil@dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live Go...
Work:<JeffN@endecon.com>              Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...2k
---------------------------------------------------------------------------