Door Bell Transformer as Charger/Inverter

Korishan

Korishan

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I was in Lowes the other day, and wondered into the electrical section and started looking at the yellow tags. I came across a Door Bell Transformer

Doorbell Transformer
[img=300x300]https://mobileimages.lowes.com/product/converted/016963/016963125018.jpg[/img]

So, I was thinking, since the transformer outputs 8V @ 10A, you could use it to charge a 2s (or 3s if LiFePo4), being about 6.4 -8.2V. Though, the 8.2V would be higher, so this probably would not work. However, if you use two diodes in series (or one with the right value), you could have the forward voltage drop low enough for 4.2V to charge 1 cell.
But, if we go with th 16V @ 10A, then we can do the 4s setup (12.8V - 16.4V). Again, we are slightly needing higher than 16 for full charge, but if we did 4.0V then it's doable. A 3s setup (9.6 - 12.6V) is doable with 2 diodes in series.
And then again for the 24V windings, we could do 6s.

I know the voltages are actually slightly lower than what we need, but we could use this transformer with a buck/boost converter and get good results.
Now, there's another option. Instead of using it to "charge" the cells, how about using it to "power" other devices.

So, we have a 24V battery (22.4 - 29.4V) and we want to power a 120V AC device. This would make a crude inverter. Get a few FETs, resistors, and crystals that'll drive the FETs at 50/60 Hz, and we have a modified sine wave inverter. Fairly cheap, maybe a bit noisy, but definitely something to play with and learn from :)
 
With a transformer intended for this eg door bell use, it might only be spec'd for short duty cycle, so not suitable for longer term charging?
 
Hrmm, good point. It might not be very good quality ferrous material making up the plates. That's a good point. And possibly cause a lot of noise with a non pure sine wave. Hrmm.

Was just curious what others thought as they are relatively inexpensive for the "rated" amps it can handle.
 
mike said:
I have several of similar transformers from when RadioShack closed. I went and hoarded as many parts as I could grab when it was $40 for whatever you can fit in these massive Santa sack-sized bags.
Click to expand...

I was there duringclose-out too but most of good stuff was already gone.

I did scorea digital radio scanner on the cheap, and some antennas masts that were marked down 80% which I can use for some other projects.
 
Korishan said:
I was in Lowes the other day, and wondered into the electrical section and started looking at the yellow tags. I came across a Door Bell Transformer

Doorbell Transformer
[img=300x300]https://mobileimages.lowes.com/product/converted/016963/016963125018.jpg[/img]

So, I was thinking, since the transformer outputs 8V @ 10A, you could use it to charge a 2s (or 3s if LiFePo4), being about 6.4 -8.2V. Though, the 8.2V would be higher, so this probably would not work. However, if you use two diodes in series (or one with the right value), you could have the forward voltage drop low enough for 4.2V to charge 1 cell.
But, if we go with th 16V @ 10A, then we can do the 4s setup (12.8V - 16.4V). Again, we are slightly needing higher than 16 for full charge, but if we did 4.0V then it's doable. A 3s setup (9.6 - 12.6V) is doable with 2 diodes in series.
And then again for the 24V windings, we could do 6s.
Click to expand...
Sorry, thats wrong.
10VA @8 V is about 1,25 A
10VA @16 v is about 0,6 A.
Additionally, a doorbell transformer (at our ones) is magnetically somewhat strange. The coupling is loose, so that there is no high shortcut current, getting it away from a "CV" behaviour, but making it easier for unspezified overloading - for short times.
In summary: high(er) losses, weak voltage, low Power.
 
Well dratcola, Cherry67! Just rain down on my party :p

Yeah, was an idea and I got back what I was asking. Information about the viability of it all. It's not viable at all! So, for anyone else would may come across this thread before making one, they'll know that it's bad idea to use these types of transformers.
 
Korishan said:
Well dratcola, Cherry67! Just rain down on my party :p

Yeah, was an idea and I got back what I was asking. Information about the viability of it all. It's not viable at all! So, for anyone else would may come across this thread before making one, they'll know that it's bad idea to use these types of transformers.
Click to expand...

Approximately 20 to 10 years ago, I used these transformers for all sorts of projects such as a power supply for hot wire cutting foam for foam wings for RC planes. I made a crude charger for NiCad and NiMh batteries.......yes almost exactly as Korishan described, except for the voltage per cell being about half as what we use for state of the art processes. The methods andprocess wasn't very efficient. Even though the process isn't very efficient, and there are better ways to do all these things, the transformers are cheap, and the secondary voltages are relatively harmless. Even though any circuit that is made probably won't be that efficient it still is a good way to study transformers,AC, Rectification, Inductance, and all sorts of other things. Just make sure you respect the primary voltage and current, and try not to be too upset if you overload the transformer and let the smoke out.

I think these doorbell transformers are the ultimate vampire load for houses. 99.999 percent of the time they are "on" but not being used for power, then for the .0001 percent of the time they are "on" it is almost a direct short to the rectified chime circuit.

Anyway I hope people still get to blow stuff up with door bell transformers.
Ryan
 
Riplash said:
Anyway I hope people still get to blow stuff up with door bell transformers.
Click to expand...

We look forward to it! :D (as long as it's safe) and make sure it's on video! ;)
 
Korishan said:
Riplash said:
Anyway I hope people still get to blow stuff up with door bell transformers.
Click to expand...

We look forward to it! :D (as long as it's safe) and make sure it's on video! ;)
Click to expand...

It's always safe :p

Unfortunately not always on video!
 
This thread reminds of off the time I wanted to charge my packs using the "free" power from phone lines haha
 
Crimp Daddy said:
This thread reminds of off the time I wanted to charge my packs using the "free" power from phone lines haha
Click to expand...

Well, you're still with us, so it must of turned out, umm, well(?). Altho, I'm sure there were fireworks involved
 
I never actually did it, but it was a thought I had. It would probably take forever to suck down any note worth watt hours but I am sure it could be done.
 
Crimp Daddy said:
I never actually did it, but it was a thought I had. It would probably take forever to suck down any note worth watt hours but I am sure it could be done.
Click to expand...

Ohhh, were you trying to grab the magnetic field and convert that into usable power? That'd be neat. Just need to get a field coil close enough but not too close (zap!)

Hahah, you could take a 1:1 split transformer and place it around the hot wires above the meter. Then run that into a separate panel as you don't want to mix the outputs before/after the meter together.
Just don't get caught doing it, though. They'd have a fit and a field day
 
Phone lines have a 50V supply and are measured in REN I think. Typical line would support 6 REN and each phone was something like 2 REN. Overall fused at about 50mA or something small, enough for a 25mA @ 50V x 24x7x365 = 76kWh per year....

Best option is just to take a very large coil out in the car and park under a HV line and fast charge.... At night take a couple of flourescent tubes with you, hold them vertical above your head under a 400kV line to light up what your doing. Awesome and really weird at the same time...

There was a commercial greenhouse near where I grew up that was under a 132kV line and hung hundreds of tubes in the greenhouse to provide light 24x7. They ended up getting prosecuted as they were the only greenhouse with no power supply that was lit up 24x7.. hahah.

Just had a quick read... 4 Ren maximum ring loading (12.5mA each) at 50V for UK standard, 48V US. This is with the old style metal bell ring phones not the new digital phones...
 
What kind of person would actually get near a 10kV+ line?

That would be a death sentence.
 
completelycharged said:
...
There was a commercial greenhouse near where I grew up that was under a 132kV line and hung hundreds of tubes in the greenhouse to provide light 24x7. They ended up getting prosecuted as they were the only greenhouse with no power supply that was lit up 24x7.. hahah
...
Click to expand...

That's a bit rude. It's not like they stole the electricity. It simply 'leaked' out of the lines.

No way on earth would I muck around with coils of wire under a HV line. Unless you really know what you are doing, what if you did actually manage to energize a coil. Would it be 1v, 10v, or 132kv?

BlueSwordM said:
What kind of person would actually get near a 10kV+ line?

That would be a death sentence.
Click to expand...

You can get a Fluro light to glow from the ground. It is a very safe experiment. Google Furos Glowing Under HV Line.
 
Geek said:
what if you did actually manage to energize a coil. Would it be 1v, 10v, or 132kv?
Click to expand...

Lots and lots of coils on the secondary windings ;) There's simple math to calculate it.
 
Korishan said:
Geek said:
what if you did actually manage to energize a coil. Would it be 1v, 10v, or 132kv?
Click to expand...

Lots and lots of coils on the secondary windings ;) There's simple math to calculate it.
Click to expand...

I wonder how big it would have to be to actually be effective. You would probably get some funny looks parked under high voltage lines with a giant transformer on your roof.
 
It was quite unnerving to stand in the back of a pickup truck with a 6ft tube under a 400kV line buzzing overhead, even though I knew I would need to be about 50ft higher off the ground to get anywhere close to flashover. That was lot less unnerving compared to crawling over HV poowrer cables under a switchroom, some hot, some vibrating to the touch with the high current....

Coils under lines are great and yes they do add to line losses as you are just creating a massive air gap transformer so it is classed as theft (unfortunately) rather than just hysterisis losses in the ground and surroundings. When I was younger I had a coil (1000's of windings) and used to walk around with it attachec to a volt meter to see where all the magnetic fields were strongest.

If the line is 3 phase the field distribution is spread out and a lot cancels out so you have to have a coil wither in a small area or a very large area (parallel to the outside width of the overhead lines). The field current is along the lines to the majority of the "coil" has to be parallel to the lines so more rectangular in shape.. Think of a basketball court with a few km of wire running around the edge...
 
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