Subject: Re: 24 VAC to 9VDC supply
From: email@example.com (roma)
References: <firstname.lastname@example.org> <email@example.com>
Date: Fri, 13 Dec 2002 06:54:17 GMT
NNTP-Posting-Date: Thu, 12 Dec 2002 23:54:17 MST
firstname.lastname@example.org (Mark Lloyd) wrote in
>On Mon, 09 Dec 2002 21:24:05 GMT, Marc_F_Hult
>>On 4 Dec 2002 15:29:08 -0600, Mark Lloyd wrote in
>>>On 4 Dec 2002 02:30:38 -0000, Steve W.EE
>>>>What is the quickest, dirtiest, cheapest way to get
>>>>9 volts DC from a 24VAC source.
>>>You won't need a transformer, but can connect the 24VAC directly to a
>>>> Need about 150ma
>>>>of current but the DC does not need to be regulated.
>>>>Half-wave rectifying will give too much DC voltage,
>>>>it would burn up a regulator since I don't have a
>>>>place to heatsink one.
>>>I doubt it. The power dissipated in the regulator is:
>>>Where Vi is the input voltage, Vo is the output voltage, and I is
>>>current. In your case.
>>>(Vi-Vo)*I = (24V-5V)*.15A = 19V*.15A = 2.85W
>>For a full wave rectifier + capacitor, the voltage drop for the 5 vdc
>>case Mark Lloyd analyses is:
>>(24VAC x 1.4VDC/VAC)-(2 x 0.65VDC) - 5VDC = 27.3VDC not "19V" as Mark
>>Furthermore, 27.3 VDC * 0.15A = 4.1 watts which is not "2.85W" as Mark
>>>Less than 3 watts isn't that much. You could even use one of the
>>>low-power regulators (78L05). These handle up to 150mA, are in TO-92
>>>cases, and don't use heatsinks.
>>The National 78L05 is rated for 100ma, not 150max.
>It was a faulty memory that made the 78L05 seem suitable.
>>Furthermore, the power dissipation is limited to between 1 and 0.5 watt
>>depending on the length of the leads and ambient temperature. See
>>http://www.national.com/ds/LM/LM78L05.pdf so "3 watts" really is *way*
>>Furthermore, why use a TO-92 package when T0-220 and TO3 devices are
>>What Mark Lloyd recommends is _guaranteed_ to shut-down or self
>>destruct and is incorrectly analyzed, poorly informed, bad advice.
>16 days until the winter solstice celebration
>"There's no more proof for the existence of God than there is for the
>existence of the Easter Bunny. That's right. The Easter Bunny."
Does'nt take much to get you guys going and screw up a group