This page: original date:8 June 2002
Last update (24 June 2002 (Page in work)



GENERAL: Ballast Coils are inductors used in series with the primary winding of low impedance power transformers in Tesla coils. See Figure 1 on the right. They limit the transformer T1 current during spark gap firings and reflect a high inductive reactance to T1 secondary during the charging of the primary circuit capacitor Cp.

B1 is the ballast coil. It is in series with the primary of T1. I referred to T1 as a low impedance transformer because the coupling between its primary and secondary windings is very close to unity. Short circuiting the secondary winding of T1 will reflect a short circuit to its primary winding which results in the 240V 50Hz appearing across B1. The impedance (reactance and resistance) of B1 limits the primary circuit current accordingly ( I = 240V / Z ). L1 and L2 offer essentially zero impedance to the circuit at 50Hz, so they can be ignored when the spark gap fires. Furthermore, the spark gap resistance during firings is approximately 2 ohms and so it can be ignored also.

As stated above, the ballast B in the primary reflects a high inductive reactanceto the secondary of the power transformer. This is shown in Figure 2 on the right where circuit a is equivalent to circuit b. If you short circuit the secondary in Figure 2a, the secondary current will be only 105mA!! There is nothing strange about this though. Follow this line of reasoning below and you will understand why:

(i) When the secondary winding is short circuited, there will be zero voltage across it. Then there will be zero voltage across the primary winding. That means that the 240V must appear across the ballast B inductor. See Figure 2a

(ii) At 50Hz the inductive reactance of B is 55 ohms.

(iii) Primary current is then 240/55 = 4.365A.

(iv) The transformer turns ratio is 1:41.7

(v) The primary current (4.365A) is stepped down to 0.1047A (104.7mA)

(vi) The reflected Z to the secondary is then 10kV/0.1047A= 95.5k ohms

(vii) The 95.5k ohms works out to 304 Henries See figure 2b

I have omitted the resistive components in the transformer windings and the ballast for simplicity. The actual resistances in the primary circuit is less than 10 ohms, and in the secondary circuit less than a 1000 ohms. These values are a small fraction of the inductive reactances, so have little effect on the calculated results.


In Tesla Coil practice one would have a low impedance power transformer (pole pig), so an effective amount of secondary inductance would be required depending on the value of Cp used. Such values range in the hundreds of Henries, and to make such inductors would be difficult and expensive. (Lots and lots of turns with very high voltage insulation would be required) Therefore a ballast in the primary circuit as described above is usually considered. In my situation, I am using a 35nF capacitor for Cp in Tesla Coil primary circuit, so a 304H secondary inductor would appears to work best with it at 200 bangs per second. This was determined using Microsim8 software to simulate the circuit. To get to the point, I simply divided 304H by the square of the transformer turns ratio to get the value needed for the primary circuit ballast. 304/1736 - 175mH. Then I proceeded to make one.

BALLAST COIL CONSTRUCTION: Construction of a ballast coil calls for insulated wire and a laminated magnetic core. Since the ballast coil would have to take about 4.65 Amperes when 240 Volts is applied, I estimated that the laminated iron core salvaged from a transformer rated at 1200VA or more would have enough iron do the trick. I found one rated at 450VA continuous and 1800VA intermitantly. It weighed 14kg. so I felt it should be adequate. A U. K. Tesla Coil builder, Richie Burnett, who did a lot of work on ballasts, advised me to stack the "I's" and "E" separately to allow for gaps in the core magnetic circuits, that at least 250 turns of wire would be required for the coil, and that the winding should fill the space within the core. (There is a link to Richie's web site on the home page) With a bit of calculation, 300 turns of PVC insulated wire having an OD of 3mm would just about fill the core space and provide sufficient inductance. I obtained 100m of 32/0.2mm PVC covered wire having a maximum current rating of 6A (10A commercial) which was slightly under 3mm OD.

Salvaging the Laminated Iron Core: The first objective was to retrieve the transformer "I's" and "E's". This required removing the wire and coil former from the transformer and then dissamble the "E's" and "I's). I decided fish the secondary coil wire through the core first as it contained four layers of 4mm diametre copper wire, each layer containing 22 turns. I then cut through the smaller primary coil wire and former to remove them. This left me with the bare transformer core. The next step was to separate the "I's" and "E's" which were lightly stuck together by the original varnish. This was a delicate operation as the laminations were very soft iron. Being careful not to bend and damage the laminations, I removed them one by one separating them one by one using a Stanley knife and small wooden hammer.

Winding the Coil: This required considerable care also, so I made a special fixture and a former for the wire. Click here to enlarge picture on the right. The wire was tightly and evenly coiled on to the former and subsequently fitted on to the stacked "E"'s". This effort was tedious and time consuming, but the results made it all worth while.

The pictures below show further stages of construction: (click on them to enlarge)



AMP / Z / L

AMPS / Z / L

25 0.34 / 64.7 / 206mH 0.44 / 56.8 / 181mH 0.56 / 44.6 / 142mH/  
50 0.66 / 75.8 / 241mH 0.89 / 56.2 / 179mH 1.00 / 50.0 / 159mH*  
75 0.96 / 78.1 / 249mH* 1.25 / 60.0 / 191mH* 1.50 / 50.0 / 159mH  
100 1.20 / 82.0 / 261mH 1.70 / 58.8 / 187mH 2.00 / 50.0 / 159mH  
125 1.55 / 80.6 / 257mH 2.20 / 56.8 / 187mH 2.50 / 50.0 / 159mH  
150 1.80 / 83.3 / 265mH 2.60 / 57.7 /184mH 3.00 / 50.0 / 159mH  
175 2.05 / 85.4 / 271mH 3.05 / 57.4 / 183mH 3.50 / 50.0 / 159mH  
200 2.40 / 83.3 / 265mH 3.50 / 57.1 / 182mH 4.00 / 50.0 / 159mH  
225 2.70 / 83.3 / 265mH 4.00 / 56.3 / 179mH 4.50 / 50.0 / 159mH  
250   4.45 / 56.2 / 179mH 5.00 / 50.0 / 159mH  
ave. L




* |||Changed Ammeter Scale from 0 - 1A to 0 - 3A
**|||Readng taken using an AVO Analog multimeter
***|Inductance appears to be about +/- 10% repeatable when spacers are removed and replaced
****Voltage source 240V 50Hz via 0 - 260V Variac


Additional pictures including the finished Ballast in a frame and showing the spacers for the magnetic gap. Also test data is planned.