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Re: Yet another DIY substrate heating cable question

At 15:48 -0400 6/10/98, bickford at black-hole_com (Jay Bickford) wrote:

>Subject: Re: Yet another DIY substrate heating cable question
>
>On Tuesday, 6/9/98 Paul Nicholson wrote in response to my question quoted
>in part
>below:
>
>> >According to my calculations (using the formulas set forth in the Resler /
>> >Behle
>> >article in the March 1995 issue of Aquarium Fish Magazine) I need a 12
>> >Volt, 4.5 AMP
>> >transformer rated to handle at least 65 watts.  So far, so good.
>>
>> P = I * E, P = 12 * 5, P = 60 watts.
>>
>> Something is wrong with your calculations as a 12 volt, 4.5  amp
>> transformer is only good for 12 * 4.5 = 54 watts.
>
>Paul,
>
>I can see how you would think that my calculations are off, but I just
>took the
>advise of the Resler/Behle article which said, "...buy a transformer that
>is rated
>to handle a 20 to 30 percent higher wattage than you need."   The actual
>wattage
>needed for my system is 54 watts, but adding 20 to 30 percent brings that
>to 64.8
>to 70.2 watts, thus my 65 watt figure.On Tuesday, 6/9/98 Dave Gomberg also
>wrote
>in response to the same question quoted in part below:

You said you needed a 65 watt, 12 volt 4.5 amp transformer. That's like
saying you need a 2x4 that is 4x8. Get a 12 volt transformer that puts out
a bit more than 4.5 amps. That will be OK

>> At 03:48 AM 6/9/98 -0400, Jay Bickford suggested:
>> > I don't want to build a system that is unsafe
>>
>> That's a good point, Jay.  If you don't have the knowledge to pick a
>> transformer with the help of a vendor, how in the heck are you going to
>> ensure it is safely designed, installed and used?
>
>Dave,
>
>To be sure, the same question has crossed my mind, and that is why I am
>doing this
>research here in the APD, as well as through other sources both on and off the
>internet.  One of the things I love to do is to learn new things, so I am
>trying
>to learn about this subject as well. But believe me, if I can't get to a point
>where I understand this and feel confident that I can build a safe system,
>I won't
>do it.  However, thanks for the concern.

It's not too hard. Just make sure that you have the AC lines well insulated
and properly fastened with strain reliefs, etc. No bubble gum and tape is
allowed with 115 volts AC. As a safety precautiion, use a fuse on the AC
input side. For a 65 watt load, I would use a 1 or 2 amp slow blow fuse.
Make sure your transformer runs cool enough to hold your hand on it
indefinetly with no discomfort. You can run as many amps as you want
through a transformer until it overheats. If the transformer is too small
or your wire resistance is too low, your transformer will overheat and
fail. If it runs cool, you are OK.

Paul

Paul Nicholson               Electronic Imaging Systems, Inc.
TEL 805 532 1068             5148 Commerce Avenue, Unit F
FAX 805 532 1065             Moorpark, CA 93021 U.S.A.
paul at eisusa_com		     http://www.eisusa.com