One of the first uses for electromagnets was for telegraphy and self winding clocks
(1 & 2),
doorbells, phone ringers, etc.. In these applications two
side by side electromagnets had their cores connected together
by an iron strap leaving two pole pieces exposed to attract an
armature. These typically had soft iron cores, it was only
later that Silicon steel laminations were used to reduce losses.
The key equation is B = u0 * k * I * (N/L)
B = field (Tesla)
u0 = 4π×10−7 (N·A−2)
I = Current (Amps)
N/L = Turns per meter.
The N/L term may assume a single layer coil construction
rather than a multiple layer coil. That's because the
first layer encloses some area, but the next layer encloses a
larger area and so on for more layers where each successive
layer generates a weaker and weaker field.
Using a smaller wire diameter makes for a stronger field
(N/L) increases, but smaller wires can carry less
current. The total length of wire and so the voltage
drop across the coil (and the power dissipated in the coil)
are related to the diameter of the coil, so a smaller diameter
is desirable, i.e. only as big as needed.
Tattoo Machine
While looking for low cost electromagnets I came across Tattoo
Machine Coils on eBay. The are labeled by the number of
"Wraps", which is Tattoo speak for what I'd call layers. I
ordered 8, 10 and 12 Wrap coils. Sold in pairs with a capacitor
for under $4 per pair, or under $2 per electromagnet.
Note that the capacitor is covered with black shrink tube so if
using the pair be sure to note polarity by means of the dimple
(see: Fig 2 and Fig 3).
| Wraps |
Ohms (2 Coils) |
Guess Ft of Wire per coil1 |
|
| 8 |
2.7 |
53 |
|
| 10 |
3.7 |
73 |
|
| 12 |
4.0 |
Note 1: Resistance divided by 0.02567 mΩ/ft (from wire
table) divided by 2 for one coil.
Metal Core: not removable. 9mm OD, Length 31.0 mm, Mounting Thread: TBD4x0.7mm,
Plastic Coil form: Flanges: 19 mm O.D., Height: 25mm, Winding height: 225mm
8 Wrap & 10 Wrap Dimensions:
Metal Core: 7.8mm OD, Length 32.0 mm, Mounting Thread: 4x0.7mm,
no groove for a circlip ( Wiki)
Plastic Coil form: Flanges: 18.5 mm O.D., Height: 27mm, Winding
height: 25.75mm, Core hole: 8mm I.D. (too small for the 8.6 mm Tefenol-D rods)
Coil: Typically wound with 24 AWG enamel wire (Wiki):
AWG Wire Table
| Diameter | Turns of wire, without insulation | Area | Copper wire | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Resistance/length[7] | Ampacity,[8]
at 20 °C insulation material temperature rating, or for single unbundled wires in equipment for 16 AWG and smaller[9] |
Fusing current[10][11] | ||||||||||||
| 60 °C | 75 °C | 90 °C | Preece[12][13][14][15] | Onderdonk[16][15] | ||||||||||
| (in) | (mm) | (per in) | (per cm) | (kcmil) | (mm2) | (mΩ/m[a]) | (mΩ/ft[b]) | (A) | ~10 s | 1 s | 32 ms | |||
| 24 | 0.0201 | 0.511 | 49.7 | 19.6 | 0.404 | 0.205 | 84.22 | 25.67 | 3.5 | 2.1 | - | 29 A | 62 A | 348 A |
Tattoo Coil Photos
Fig 1: 8 Wrap & 10 Wrap pairs. |
Fig 2 Note dimple on Capacitor = + End |
Fig 3 Note dimple on Capacitor = + End |
| Fig 4 12 Wrap - Capacitor polarity unknown. 22 uF, 35 V, but polarity no clearly marked.  |
Fig 5 Looks like the same size wire, but with a shorter winding length  |
I also ordered on eBay "Complete Tattoo Machine Kit - 2 Gun
Skull Set" for $12 w/free shipping. It came in a 1
cubic foot box with a bunch of stuff! Way lower cost than
buying parts separately.
In addition ordered "Digital Dual Machine Tattoo Power Supply
LCD Display W Clip Cord &ft Switch P025" which is a tattoo
power supply that displays Voltage, Frequency and Duty Cycle,
but not current (see Fig 4 below). about $40.
Tattoo Machine Kit Photos
Fig 1 Contents of Kit |
Contents: 2 multi-use tattoo machines with low-vibration frames, upgraded brass contacts, and dual 10-wrap coils; can be used for lining and shading 1 pro-quality analog power supply system (US-standard 120 volt power cord) 1 stainless steel foot pedal controller 1 silicone clip-cord with stainless contacts 1 white LED machine-mountable work light 1 high-grade resin skull ink holder, holds up to 7 medium-sized ink caps 4 round plastic tips in sizes 3R, 5R, 7R, 9R and 11R (1 of each size) 5 flat plastic tips 5F, 7F, 9F, 11F, and 13F (1 of each size) 3 diamond plastic tips 3D, 5D, and 7D (1 of each size) 20 round liner needles in sizes 3RL, 5RL, 7RL and 9RL (5 of each size) 20 round shader needles in sizes 3RS, 5RS, 7RS and 9RS (5 of each size) 5 flat shading needles in size 5FS 5 magnum shading needles in size 7M1 100 small (#9) ink caps 50 medium (#13) ink caps 7 full bottles of ink by Millennium Colorworks (MOM's) Inks in 1/2 ounce bottles of the following colors: Black Onyx Grey Hound Power White Blue Balls Hellow Yellow Ectoplasmic Monthly Red 1 plastic squeeze wash bottle, holding 8 ounces 1 dual-tip, disposable skin marking pen with fine and regular tips 75 tattoo machine anti-microbial, heat and UV resistant rubber bands 75 mixed-color anti-microbial, heat and UV resistant grommets 50 spare anti-microbial, heat and UV resistant o-rings 3 hex-wrenches for machine adjustment (1.93mm, 2.45mm (coil, lower electrical terminal & contactholder) & 2.95mm (armature spring) 5 wire cleaning brushes on a convenient hanging ring 1 single-use packet of tattoo aftercare ointment containing vitamins A and D 5 pairs of black nitrile, textured powder-free gloves 1 - 8 ounce bottle (COSCO Brand) Tincture of Green Soap 3 - 8.5" by 11" sheets of Spirit 4-ply thermal-style transfer paper 1 dual-sided and re-usable realistic practice skin Tattoo Basics instructional eBook |
Fig 2 Tattoo Machines top & bottom
views |
| Fig 3 The analog supply is labeled: 1.5 to 15 Volts 2 Amps  |
Fig 4 DT-P025 Power supply displaying: 16.5
Volts, 52% Duty Cycle & 100 CPS. Can be adjusted from 0.0 to 17.8 Volts in 0.1 Volt steps. Large knob is digital shaft encoder, not a pot. It takes many turns to cover full voltage range.  |
|