The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Spehro Pefhany"
Subject: Re: V regulator input cap size?
References: <0001HW.B9D991AD000C5917163F7590@news.covad.net> <firstname.lastname@example.org> <0001HW.B9D9CD54001A5D15163F7590@news.covad.net> <0001HW.B9DAE8B7003F6E14163F7590@news.covad.net>
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Thu, 31 Oct 2002 20:21:38 GMT
NNTP-Posting-Date: Thu, 31 Oct 2002 15:21:38 EST
In alt.engineering.electrical DaveC wrote:
> On Tue, 22 Oct 2002 8:10:08 -0700, Lizard Blizzard wrote
> (in message ):
> Circuit current requirements:
> 555s: 10mA each x2
> 4020: 1mA max including output drive current
> LEDs: 15mA each x2
> Relay coil: 25mA (I was a bit off on this figure!!)
> Total: 76mA
> Supply voltage measures at 28vac rms. (for some reason it's running lower
> than normal for HVAC) which will give ~ 37vdc(?). The max input for the 7812
> is 35v.
I think you may confused the normal HVAC tranformer voltage (24VAC RMS
nominal) with the filtered output DC voltage (around 35VDC). The unloaded
voltage that you measured of 28VAC certainly agrees with this.
Suggest you use a CMOS 555 pair, and reduce the current draw down to
about 2mA total (not including LED(s) or relay), use a simple
zener/resistor power supply for the logic, then run both
LEDs and the relay coil from the unregulated, with suitable resistors in
series as required. If the supply voltage is 24VAC you can use a 24VDC
coil by isolating the filter capacitor from the relay coil with an
additional 1N400x diode. This will draw minimal current and will be more
resistant to the kind of spikes typically seen in HVAC applications than
most of the previous suggestions. A single ULN2003 will drive your relay
and both LEDs, and will include the clamp diode for the relay coil.
"it's the network..." "The Journey is the reward"
email@example.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup