"CROWBAR" O VE R-VO LTAC E PROTECTION

By BOB NUCKOLLS . . once triggered, an SCR remains in a following reasons: EAA 205021 conducting mode until power is re- 1. Using a zener in this application The AeroElectric Connection moved. Momentarily opening the depends on qualities not specified by the 6936 Bainbridge Rd. alternator field switch allows a relay to zener manufacturer. The zener is de- Wichita, KS 67226-1008 "drop out" thus resetting the over-volt- signed to be a , not an age circuit. Crowbar module operation "anti-." To have some level of confi- Every airplane with an electrical sys- is almost identical to the over-voltage re- dence in this unspecified operating tem should be fitted with some means lay except the SCR is much larger (50 mode, hundreds of zeners from various for dealing comfortably with an over- amp). When triggered by an over-volt- manufacturers would have to be tested. voltage condition. If you are flying a age sensor, it forms a "dead short" across All would have to (1) fail consistently in small, 8-12 amp, permanent magnet al- the line. For this application, the SCR a shorted mode and (2) pop the field ternator, the alternator's capability to must be rated for peak currents of up to breaker without blowing apart! drive system voltages up quickly is lim- 300 amps or so ... that may seem like a 2. Once the zener is shorted and the ited. For these aircraft, an over-voltage lot but it lasts only for milliseconds, un- alternator is taken off line, the system warning light is sufficient announce- til the breaker opens. cannot be reset. If a regulator has failed, ment prompting a pilot to shut off a causing an over-voltage condition, then failed system. The battery will soak up Caution over-voltage "protection" has been ac- excess charging current until the pilot complished by deliberately failing The breaker between the bus bar and can react. For systems which carry another device which must also be re- wound-field alternators of 35 amps or crowbar over-voltage module MUST be placed. more, some form of automatic over-volt- in good condition and KNOWN to work! This technique has obviously passed age protection is required. Breakers seldom operate on any air- muster with the FAA and I will concede The majority of over-voltage protec- plane. Unless they've been cycled at that it probably provides adequate pro- tion devices are called "over-voltage least once you have no way to know if tection for Grumman's airplanes. Using relays." This is because most certified the thing really works! I have installed a zener for "anti-fuse," crowbar o.v. pro- aircraft does indeed use a relay for dis- crowbar over-voltage protection mod- tection is on shaky unless one is connecting a faulted alternator system. ules downstream of a bad ready to exactly duplicate all parameters and later managed to burn some wires Their operation is quite simple: Refer- of the Grumman electrical system. 'Nuf ring to Figure 1, a sensor and precision when the over-voltage module tripped said. voltage reference monitors bus voltage. and the breaker failed to operate. When installing a crowbar over-voltage protec- If bus voltage exceeds 15.5 to 16.0 volts Rolling Your Own (on a 14-volt airplane), the relay is ener- tion module, check the upstream breaker for proper operation. Use a jumper wire For those who are interested in build- gized and latched. When the relay is ing a crowbar over-voltage module, I will energized, the alternator field circuit is with alligator clips to power up a heavy broken and alternator output falls to load (landing light) through the alterna- supply a schematic with a list of materi- tor field circuit breaker. A landing light als to any who send a #10, zero. represents a 1.5 to 2.5 times over-load self-addressed, stamped envelope to me Relays are electro-mechanical devices on a 5 amp breaker; it may take 20-30 at the address below. The information subject to ravages of age exacerbated by sheet also describes proper incorpora- vibration, heat, moisture and corrosive seconds to trip it. The important thing to check is that IT DOES OPEN. The tion of the low cost, replacement "Ford" atmosphere. For the past 10 years or so, 200-300 amp load of a triggered over- voltage regulators into airplanes. solid-state technology for over-voltage voltage module will open it in protection has been evolving in elec- milliseconds. If your alternator field Bibliography tronic products, most notably supply breaker is old and/or of unknown computers. It has been dubbed "crow- Written coverage plus real-time con- bar" over-voltage protection in reference condition, consider purchasing a new, sulting services by the author on this to its mode of operation. During an quality breaker to replace it. and other electrical systems topics are over-voltage condition, the system delib- available from the AeroElectric Connec- erately pops a supply circuit breaker or Some Notes On The tion, 6936 Bainbridge Rd., Wichita, KS fuse which disconnects the protected "Grumman Alternative" 67226-1008. $42 USD buys all materi- circuit. Grumman Aircraft has used a form of als in print (13 chapters, lots of The crowbar device is a silicon con- crowbar over-voltage protection on some illustrations, 5 appendices, 200 + pages) trolled rectifier (SCR). This small device production models. Their technique plus one year's subscription to newslet- is inexpensive, compact and totally im- uses a 1-watt, 16-volt con- ters and the next issue of chapters on mune to conditions which age or destroy nected from field supply breaker to specific topics. Overseas subscribers relays. Figure 1 shows that an over-volt- ground. The common failure mode for add $20 USD for airmail postage. Mas- age relay simply breaks the power feed the zener is to SHORT when over loaded. terCard and Visa are accepted. The line to an offending voltage regulator. The idea is rather clever and you can't author may be contacted directly at Latching is achieved by using a small beat it for simplicity! I do not recom- 316/685-8617 or Compuserve (100 milliamp) SCR to control the relay . mend duplication of this scheme for the 72770,552.

68 DECEMBER 1993 TO ALTERNATOR FIELD

ITTY-BITTY SCRTO LATCH RELAY

PRECISION VOLTAGE REFERENCE

TO ALTERNATOR FIELD

"CROW BAR" O.V. PROTECTION

BIG FAT SCRTO POP BREAKER PRECISION VOLTAGE REFERENCE

TO ALTERNATOR FIELD

GRUMMAN "CROW BAR" 16 VOLT O.V. PROTECTION 1 WATT NOT RECOMMENDED GLASS ZENER

FIGURE 1

SPORT AVIATION 69