From: Allen Mulvey <allen@amulvey.com>
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Date: Mon, 13 Sep 2021 05:57:07 -0700
Subject: Re: [EE] LM1084 regulator question
Thread-Topic: [EE] LM1084 regulator question
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I have an access control system that uses 12v for magnetic door strikes=20
and holding magnets but 5v for the logic. They used a linear regulator=20
to tap 5v off the 12v supply. They put a small value, large wattage=20
resistor in series with the input pin on the regulator. It worked in=20
that the regulator did not get excessively hot but the resistor was too=20
hot to touch. I cut out the resistor and put in a switching device like=20
Justin mentioned below. Now nothing gets hot and it has been working for=20
a decade without any problems.

Allen

On 9/13/2021 2:13 AM, Justin Richards wrote:
> Hi Bill,
>
> if you are looking for cheap, I use  something like "Mini360 3A DC-DC
> Voltage Step Down Converter Buck 4.75-23V to 1-17V" fom ebay for ~$3
>
> Not sure they are good for the rated 3A, but if this is something you wou=
ld
> consider using i would be happy to load one up and see which point it fai=
ls.
>
> Justin
>
> On Mon, Sep 13, 2021 at 12:54 PM William Couture <bcouture@gmail.com> wro=
te:
>
>> As a reply to everyone:
>>
>> The output capacitor is a 22uF electrolytic, thought it would be enough.
>>
>> But, from the other comments, it's a thermal problem -- too much power
>> (heat)
>> is being dissipated going from 24V (needed elsewhere in the system) to 5=
V.
>>
>> I'll look at switching supplies and see what I can do there.
>>
>> Thanks!
>>     Bill
>>
>> On Sun, Sep 12, 2021 at 10:33 AM William Couture <bcouture@gmail.com>
>> wrote:
>>
>>> Hi!
>>>
>>> I need about 3A of 5V, with a 24V supply.  I'm trying to use a LM1084 i=
n
>> a
>>> TO-220, which is
>>> rated at 5A output.
>>>
>>> However, when I try to pull more than about 250mA, the regulator become=
s
>>> very hot and I
>>> start getting dropouts.  A heatsink does not help.
>>>
>>> I have an electrolytic capacitor on the output, not a tantalum like in
>> the
>>> datasheet, could that
>>> be the problem?
>>>
>>> I tried putting a 7805 in the circuit (with appropriate pinout changes)
>>> and it's fine up to 1A,
>>> so the problem is the LM1084 itself.
>>>
>>> If the LM1084 will not work, can anyone suggest a low-cost circuit to g=
et
>>> 3A or so of 5V
>>> from 24V?
>>>
>>> Thanks,
>>>     Bill
>>>
>>> --
>>> Psst...  Hey, you... Buddy...  Want a kitten?
>> straycatblues.petfinder.org
>>
>> --
>> Psst...  Hey, you... Buddy...  Want a kitten?  straycatblues.petfinder.o=
rg
>> --
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