From: Mark Lloyd
Subject: Re: 24 VAC to 9VDC supply
Date: 9 Dec 2002 17:35:26 -0600
References: <email@example.com> <firstname.lastname@example.org>
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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 Lloyd
>Furthermore, 27.3 VDC * 0.15A = 4.1 watts which is not "2.85W" as Mark Lloyd
>>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* too
>Furthermore, why use a TO-92 package when T0-220 and TO3 devices are readily
>What Mark Lloyd recommends is _guaranteed_ to shut-down or self destruct and
>is incorrectly analyzed, poorly informed, bad advice.
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