A
Anonymous
Guest
what about 20 diodes in series?
"John Fields" <jfields@austininstruments.com> wrote in message
news:749bovc98ph8r1b6p8070aoo6hv8oenqvi@4ax.com...
"John Fields" <jfields@austininstruments.com> wrote in message
news:749bovc98ph8r1b6p8070aoo6hv8oenqvi@4ax.com...
On Thu, 09 Oct 2003 07:49:08 -0700, Lizard Blizzard <NOSPAM@rsccd.org
wrote:
John Fields wrote:
On Wed, 08 Oct 2003 20:58:04 GMT, "Ryan Kremser"
news@rkremser.endjunk.com> wrote:
Hello, hopefully someone can help me. i'm not thinking straight so this
question is probably quite simple. I have a 30vac momentary switch
mechanically triggered and need to be able to close another circut
through a
relay while also using the 30 volts to drive a solonoid. the relay I
wanted
to use is rated at 5 volt dc for the coil and i wasn't sure how to drop
the
30vac down to 5dc.
---
___
30VAC>----O O--+-------+
| |
| | +---[<DIODE]---+
| | +-----+ | |
| +--|~ +|--[R]-+-[RELAY COIL]-+
[SOLENOID] | | |
| +--|~ -|---------------------+
| | +-----+
| |
| |
30VAC>---------+-------+
The value of R will be equal to the supply voltage minus the relay coil
voltage multiplied by the relay coil current. For example, if the
relay
coil currenr is 100mA,
E 30V - 5V 25V
R = --- = -------- = ----- = 250 ohms
I 0.1A 0.1A
The power the resistor will dissipate will be
P = IE = 0.1A * 25V = 2.5W
However, if the switch closure is only going to be momentary, the
resistor will only dissipate power while the switch is closed, so the
power rating of the resistor could probably be made much smaller. I'd
use 270 ohms at 1 watt, which you can easily make out of 270 ohm 1/4
watt resistors, like this:
---+------+
| |
[270] [270]
| |
[270] [270]
| |
---+------+
I would say that without a filter cap after the bridge, the relay will
chatter with every pulse of the DC.
---
Depends on how hard you bang it.
I tried an old 6V 52 ohm coil Allied Control T154-CC-CC with a 24VA
control transformer, a bridge made from 1N4004's, and a 150 ohm resistor
in series with the coil and it draws 113mA and works perfectly. No
chatter, no nothing.
Note that 6V/52 ohms = 115mA, so with 113mA through the coil, it's
coasting!
What's happening is that once the armature mates with the
electromagnet's pole piece it doesn't take much of a magnetic field to
keep it there, and at 120PPM (DC) it just doesn't have time to get away.
Also, since it's DC it's not like the magnetic field has to take the
time to completely reverse polarity and allow the armature to escape.
--
John Fields