# Inductive power coupling?

#### Doktor Jones

Joined Oct 5, 2011
73
I'm looking to make an in-cabinet lighting system for my brother-in-law's house. Since he rents, drilling holes to run power into the cabinets is not an option, so I got the idea to make a power coupling pad that could transfer power through the countertop (it's particle board/laminate). Would this schematic be a decent place to start? The values need to be tweaked of course... I was thinking maybe C2 = 100pF and R1 = 4.7K... that should give me a range of ~50-70kHz. With the increased frequency, I'd need to tweak C4 and L1 too, right? How do I determine the diameter and # of turns of the coils?

The shelves are removable, so once I have power inside the cabinet I can run wires to the LED lighting and that won't be a problem.

#### ronv

Joined Nov 12, 2008
3,770
I'm looking to make an in-cabinet lighting system for my brother-in-law's house. Since he rents, drilling holes to run power into the cabinets is not an option, so I got the idea to make a power coupling pad that could transfer power through the countertop (it's particle board/laminate). Would this schematic be a decent place to start? The values need to be tweaked of course... I was thinking maybe C2 = 100pF and R1 = 4.7K... that should give me a range of ~50-70kHz. With the increased frequency, I'd need to tweak C4 and L1 too, right? How do I determine the diameter and # of turns of the coils?

The shelves are removable, so once I have power inside the cabinet I can run wires to the LED lighting and that won't be a problem.
I think I would look for a different way to do it. Maybe battery operated lights.
While wireless sounds cool it is not very efficient and it gets worse as the space between coils increase.
For a home brew one you will need a scope to match the transmit and receive frequencies.
Here is a write up on one.

#12

#### Alec_t

Joined Sep 17, 2013
13,337
Inductive coupling through 2cm or so of countertop will be disappointingly inefficient. For every Watt of lighting you might need 10 Watts or more of input power. I've seen something similar done many years ago as a party trick, with the primary of a 50Hz mains transformer fixed below a ~3/4" thick table top and the secondary on top of the table mysteriously driving a low wattage lamp. As I recall, it could run only briefly before the primary got dangerously hot!
If it were me I'd look hard for a non-damaging way of getting a wire discreetly into the cabinet or, failing that, power the light from a rechargeable battery in the cabinet.

#12

#### OBW0549

Joined Mar 2, 2015
3,566
In addition to the comments above, I'd point out that at the power levels this thing is going to have to put out, you'll also be emitting huge amounts of EMI to any nearby electronics-- especially any of those so-called "atomic clocks" that get their time updates from WWVB, which broadcasts on 60 kHz. You might even get in trouble with the FCC, depending on how vigilant they are in tracking down troublesome emissions.

#### #12

Joined Nov 30, 2010
18,224
The first thing to do is math out a power budget.
If you want to power a 20 watt device, you will need at least 2000 watts in the sending coil (and I think it's worse than that for a 3/4 inch gap). 2000 watts isn't legal for a 20 amp circuit breaker. You're going to need (at least) a dedicated 30 amp breaker with 10 gauge wire to run a 20 watt load.
There is no way 2 transistors are going to produce this much power, and you have to allow the extra 1980 watts of heat to escape.
You can't even buy a 2000 watt microwave oven. Where are you going to put all that heat?

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#### tcmtech

Joined Nov 4, 2013
2,867
If it was me, I would either go with low voltage lighting and some clever routing of light power cords like what's on a common low voltage power pack or fine enameled copper wire routed through seams or drill some tiny holes in inconspicuous areas.

An 1/8" hole in a location that is not direct and obvious line of site will never get noticed and even if it did they couldn't prove it wasn't a factory screw hole or an installation related one that was there from the day it was built or not.

#### Sensacell

Joined Jun 19, 2012
3,142
Wireless power transfer = another techno fad thing that provides very little real value.

#### Alec_t

Joined Sep 17, 2013
13,337
It would have real value for, say, recharging the battery of a pacemaker or other implanted device without having to open up the patient or provide trans-dermal electrodes.

#### #12

Joined Nov 30, 2010
18,224
You're right. It is useful for milliwatts at 1/4 inch.

#### OBW0549

Joined Mar 2, 2015
3,566
You're right. It is useful for milliwatts at 1/4 inch.
Ummm... perhaps a bit more than that: Linear Tech's LTC4120 is good for a 1-2 watts at an efficiency of >45% across a 0.4" gap.

Still, it falls far short of what the TS needs.

#12

#### crutschow

Joined Mar 14, 2008
31,581
Although the LTC4120 states you don't need a core, I would think the efficiency and power handling would be significantly improved if two ferrite pot cores where used to concentrate the magnetic field across the gap.

#### ci139

Joined Jul 11, 2016
1,873
But what about sound energy transfer . . . never done such . . . it's less dangerous - /!\ i guess /!\ - in specific the possible metal pins in the walls won't get up to high temperature - but as for general - i don't know - as for oil fields - i don't know

chk list ::
• the capacitive energy transfer (the capacitors also heat up)
• the energy to light (50% loss) light to energy (76% down to 29% loss geometric mean apx. 50% loss) total 75% loss -- but as i get it not an option -- if i get this (thread) right -- the room should be hermetic ??? so you can't have anything holes/windows in the walls ???
• there are hydrogen - fuel cell generators - ??? as i get it they put out the water vapour ??? making the hermetic room pressurized
• misc ordinary batteries - may also build up air pressure - by emitting gas

● ● ● the "only" option -- some sort of 100% sealed batteries - that have no possible explosive-failure ??? or this -- http://www.instructables.com/id/SERVO-TORCH/ -- you need the spiral spring from 200ya city-hall clock (and an extension handle wrench to rewind it)

#### Doktor Jones

Joined Oct 5, 2011
73
Well the battery idea is a non-starter; the issue here is that my brother-in-law is low vision (he's legally blind -- totally blind in one eye, and 10% vision in the other)... so the idea here is to not have to periodically fiddle with things, especially since the cabinet doors are older and don't always close right. A manual switch has the potential to be accidentally left on, and any auto switch might not engage if the door doesn't close (and due to his vision issues, he won't necessarily be able to tell at a glance that the door hasn't closed all the way and the light's still on). With a wall-supplied circuit, if it gets left on, the worst that happens is his electric bill is a couple pennies higher... with batteries, it means the hassle of him having to find the batteries, take out the old ones (and not drop them when the springs in the casings like to occasionally force-eject them when you're pulling on them), figure out the polarity on the new ones, and get them installed in the proper direction.

The enameled wire idea is interesting... what sorts of temperatures can enameled wire usually tolerate? There is a hole in the countertop where the water pipe runs to the upstairs radiators, but there's only about a 1/8" or so gap around it, and that pipe gets hot enough to almost instantly burn you when the furnace is running (it's a really old house, ca. 1900, so there are lots of retrofits that have been made since its construction, many back in the 30's-50's that wouldn't necessarily meet today's codes). It makes me wonder about silicone wire though -- isn't that usually very temperature resistant? I can get a short loop of that stuff (~5ft per conductor) for like 6 bucks on Amazon, and put an inline connector on the end above the countertop to take a normal DC adapter plug, and a connector underneath to connect to the under-the-counter wiring. It only needs to be temperature resistant within an inch or two of the pipe anyways.

#### OBW0549

Joined Mar 2, 2015
3,566
The enameled wire idea is interesting... what sorts of temperatures can enameled wire usually tolerate? There is a hole in the countertop where the water pipe runs to the upstairs radiators, but there's only about a 1/8" or so gap around it, and that pipe gets hot enough to almost instantly burn you when the furnace is running (it's a really old house, ca. 1900, so there are lots of retrofits that have been made since its construction, many back in the 30's-50's that wouldn't necessarily meet today's codes). It makes me wonder about silicone wire though -- isn't that usually very temperature resistant?
You don't need to resort to silicone insulated wire for that; there are many types of insulation which will handle 105 °C and up. Here is a good place to start. Scroll over to the "Operating Temperature" column and take your pick.

#### Doktor Jones

Joined Oct 5, 2011
73
Is there any reason not to use silicone insulated wire, when a 2-conductor 5' length can be had for \$6?

#### OBW0549

Joined Mar 2, 2015
3,566
No, provided it's the right gauge for carrying the current you need...

#### #12

Joined Nov 30, 2010
18,224
Useless hype from 8 years ago.
I think you're right.
"Future induction systems will be able to transmit power over a distance of several feet with efficiency of 50 percent or more."
And no results have become available on the retail market, here in 2016.