A Low Cost High Tension Lead Tester By Thomas K. Eismin, Professor Aviation Technology Purdue University West Lafayette, IN 47907
testing defective leads (i.e., the current has traveled through the low resistance of the lead's defective insulation). Unit Construction
Have you priced a Bendix ignition harness high tension lead tester lately? If you have, you know why I was determined to produce a low cost substitute. The theory of operation of any ignition lead tester is simple. The tester must produce a high voltage low current, DC signal, send that signal to the ignition lead positive conductor and detect any current leakage from the lead positive conductor to the lead ground or shielding. My initial thought was to study the Bendix lead tester and build a similar "black box". This sounds simple; however, the Bendix tester utilizes two specially made transformers which I found virtually impossible to purchase. I knew there had to be a better solution, and there was. A solution so simple the entire lead tester can be built for under $30. My lead tester was built using a typical automotive ignition coil to generate a 6000 volt output. The output of a typical production model high tension lead tester was determined to be between 5500 and 6000 volts. Therefore, the automotive coil was perfect. The ignition coil can be powered directly from a self-contained battery, the onboard aircraft battery, or from a 120 volt AC source. I chose the 120 volt AC source for two major reasons: (1) the self-contained batteries (8-D cells), although simplest, required replacement much too often, and (2) the use of the aircraft battery was eliminated because aircraft are often 24 volt, and often leads are tested after the battery has been removed from the aircraft. Thus, the solution is a 120 volt AC power source. Similar to production models, my design uses a 7mm electrode gap to allow a maximum voltage of approximately 5600 volts available for lead testing. The high voltage generated in the tester has a choice of two current paths: (1) across the electrode gap, or (2) through the ignition lead insulation. Since current always takes the path of least resistance, a spark will appear across the electrode gap when testing good ignition leads. No spark will appear when 42 JULY 1985
Mechanical: The electrode gap is definitely the most critical part of the lead tester. The electrodes I recommend are constructed from two 1-1/2", number 10 brass machine screws filed to a point. The screws are then mounted to a clear plastic housing and adjusted to a 7mm gap (see Detail A). The electrode assembly was then placed in the tester case near a 1" punched hole. This placement allows the operator to view the electrode gap during lead testing. The tester case can be made of virtually any material, although plastic and aluminum boxes are commercially available. An 8" x 6" x 4-1/2" case will allow sufficient room for easy assembly of all components. If a metal case is used, I recommend mounting all com-
ponents to a 1/4" plywood base which will later be placed inside the case. This plywood mounting will help eliminate the possibility of short circuits. For final case assembly see Detail B. Electrical: As indicated by the schematic and parts lists in Detail C, the unit's electrical system is quite simple, and all components are readily available. The 120 volt AC power is directed through a transformer and a full wave rectifier to allow 12 volt DC power to be used by the ignition coil. A master switch is used to control the AC current to the lead tester. A second normally open switch, the lead test switch, controls the current to the ignition coil primary winding. This switch is manually operated during lead testing. The secondary circuit of the ignition coil is directly connected to the 7mm electrode gap in parallel with the test lead alligator clips. It should be noted that high voltage wire must be used in the secondary circuit to prevent unwanted arcing.
Detail A
I I'M'LP"
TOP VIEW
#10 Brass nut, 4 ea.
7 mm gap
4 I-eri'iiM-i.
•iii'in'i'fn> #10 Brass washer, 6 ea
1V 2 "#10
/~ brass machine >' screw, 2 ea.
\_ electrical crimp connectors, 2ea.
I — — 3/ie" or
thicker plexiglass
FRONT VIEW
DetaMB A.C. power cord grommet
f.-\
T, - Step down —— transformer
-f I — I
J
coil
W
R, - Rectifier assembly
w **y
-»//Tb
(4 diodes &
1 capacitor)
^—~^« ^ofnnHiri/ rvi*^iti\/P
^
Electrode
assembly
S, - Master switch
v
t ————
Primary & secondary S~ negative connection Primary positive t- __ / / connection
— si *.
,
connection —— C2 - Capacitor
*]
^ 1\——
Test lead grommets
S2 - Test switch
TOP VIEW
/
Negative r master Test Lead
viewing window
^\ \v test N
^ Positive Test Lead
FRONT VIEW
Detail C
C»2
Parts List
P, - A.C. power plug, 3A S, - SPST 120 V.A.C., 5A switch T, - 120 V.A.C. input, 3 AMP 12 volt output,
transformer R, - Rectifier Assembly —
4 Diodes, 3 AMP 50 volt;
Diodes are mounted to an Experimenter Gird Board 2%" x 3%"
C, - Capacitor 35V, 1000 uF
S2 - Normally open SPST 5A D.C. switch C2 - Typical automotive ignition capacitor 12 V.D.C., 1000 uF T2 - Typical automotive 12 volt ignition coil W, & W2 - 35 KV, 18 gage stranded wire (all other wires are stranded 20 gage wire) G, - 7mm electrode gap A, & A2 - Alligator dip
Unit Operation: The basic operation of this lead tester is similar to that of production models. To test the insulation of high tension leads: (1) Attach one test lead alligator clip to the aircraft ignition lead ferrule (i.e. the outer wire of the ignition lead). (2) Attach the other test lead alligator clip to the ignition lead positive wire (i.e., the inner wire of the ignition lead). (3) Connect the test instrument as a 120 volt AC source and turn on the unit's master switch. (4) Depress the spring loaded lead test switch and release rapidly. This will activate the high voltage source and test the ignition lead. (5) While releasing the lead test switch, view the electrode gap through the case viewing window. If a spark occurs across the gap, the lead insulation is airworthy. If no spark is viewed across the electrode gap, the ignition lead should be rejected. (6) Repeat steps one through five for each ignition lead to be tested. Other Operation Notes: (1) Always check your lead tester prior to operation. This can be done by separating the alligator clips approximately one inch, then depress and rapidly release the lead test switch. For a correctly operating tester, a spark should appear across the electrode gap. Next, connect the alligator clips together and repeat the test. No spark should appear across the electrode gap. If this sequence does not occur, retest and identify any problems prior to ignition lead testing. (2) When testing leads which are installed on the engine, be sure to keep the tester alligator clips and test leads well Jear of any grounded metal parts of the aircraft. If the test leads and alligator clips come in physical contact with grounded metal parts, the tester may falsely reject good leads. (3) On some engines, leakage may occur through the magneto distributor to the magneto coil. Therefore, before rejecting any leads which are tested while connected to the magneto, turn the engine (magneto) slightly and repeat the test. (4) Remember, insulation leakage is not the only cause of defective leads. Each lead should also be tested for continuity of the positive conductor. A typical ohmmeter or continuity light may be used for this test. This high tension lead tester will offer many years of faithful service to any technician. But keep in mind, the quality of any tool is only as good as the workmanship used during construction. Please take your time and assemble your lead tester carefully and accurately. If you have any questions concerning this item, please feel free to contact me. SPORT AVIATION 43
