[SJRA-Members] Electrical Theory - Dimmable LEDs - RFI

[Bob Turner]

Bob,

You said everything I was thinking.  I wanted a sanity check.  Thank you.  It's dark now, the brightness at 60vac via Variac is perfect.  I can brighten or dim by adding or subtracting bulbs.  Once the strings are in series, it will be easy to have same number of bulbs in each string.

There is merit to the idea of a dimmer (RFI suppressed one).  It would have to go in basement either before light sensor or after it.  I should add a GFI while I'm at it.  That definitely needs to go before the light sensor, I already tried it the other way.   It’s a bigger project.  Not ready to start that.  But I'm going to check my allocation of old extension cords.

Thanks everyone.

7 3   Bob
N2SCJ

-----Original Message-----
From: Bob Beyer [mailto:bob_beyer at verizon.net] 
Sent: Monday, September 16, 2019 7:10 PM
To: Bob Turner <n2scj at outlook.com>
Cc: SJRA-Members at mailman.qth.net
Subject: Re: [SJRA-Members] Electrical Theory - Dimmable LEDs - RFI

Yes Bob, that should work. 
Each string would have half the total voltage across it. 

Each string is a set of 12 parallel bulbs. If you cut the plugs and sockets off and wire two strings in series with each other, you can treat each of the strings as if it’s a single bulb. 

We can then think of each string as a single resistor and in this case the resistances are equal. Since the current is the same anywhere in a series circuit, the voltage will be divided proportional to each resistance. Since the resistances are equal, the voltage will divided equally among them. In this case that comes to 60 volts across each string. 

One thing to note is that if you remove bulbs from one string but not the other, then the voltage distribution will become uneven. 

73,
Bob - KE2D

Sent from my iPad

On Sep 16, 2019, at 11:16 AM, Bob Turner <n2scj at outlook.com> wrote:

To replace a 12vdc yard light with built in switching noisy power supply that bothers my HF I'm trying something different.  I have the Harbor Freight (HFT) string lights that take an S14 bulb.  The smallest bulb with standard Edison socket I can find is a 1w LED.  They are still too bright.  I could use candelabra adapters and 7w incandescent night light bulbs as they have correct brightness.  I'd rather go low wattage LED.  I've tested my 1w LED bulbs and they do work with 60vac as provided by my Variac.  I need to wait for dark to get an idea as to the brightness.  Now for the question.

If I take two of these HFT strings, with same number of bulbs in each, and wire up an adapter to put them in series will that work out about the same as 60v from a Variac?

Bob
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