"CROWBAR" O VE R-VO LTAC E PROTECTION By BOB NUCKOLLS EAA 205021 The AeroElectric Connection 6936 Bainbridge Rd. Wichita, KS 67226-1008

. . once triggered, an SCR remains in a conducting mode until power is removed. Momentarily opening the alternator field switch allows a relay to "drop out" thus resetting the over-voltage circuit. Crowbar module operation is almost identical to the over-voltage relay except the SCR is much larger (50 amp). When triggered by an over-voltage sensor, it forms a "dead short" across the line. For this application, the SCR must be rated for peak currents of up to

following reasons: 1. Using a zener in this application depends on qualities not specified by the zener manufacturer. The zener is designed to be a voltage regulator, not an "anti-fuse." To have some level of confiEvery airplane with an electrical sysdence in this unspecified operating tem should be fitted with some means mode, hundreds of zeners from various for dealing comfortably with an overmanufacturers would have to be tested. voltage condition. If you are flying a All would have to (1) fail consistently in small, 8-12 amp, permanent magnet ala shorted mode and (2) pop the field ternator, the alternator's capability 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 acexcess 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 recrowbar over-voltage module MUST be placed. wound-field alternators of 35 amps or more, some form of automatic over-volt- in good condition and KNOWN to work! This technique has obviously passed Breakers seldom operate on any airage protection is required. muster with the FAA and I will concede Unless they've been cycled at that it probably The majority of over-voltage protec- plane. provides adequate protion 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. proaircraft does indeed use a relay for dis- crowbar over-voltage protection mod- tection is on shaky ground unless one is downstream of a bad circuit breaker ready to exactly duplicate all parameters connecting a faulted alternator system. ules and later managed to burn some wires Their operation is quite simple: Referof the Grumman electrical system. 'Nuf when the over-voltage module tripped said. ring to Figure 1, a sensor and precision voltage reference monitors bus voltage. and the breaker failed to operate. When a crowbar over-voltage protecRolling Your Own If bus voltage exceeds 15.5 to 16.0 volts installing tion module, check the upstream breaker (on a 14-volt airplane), the relay is enerfor proper operation. Use a jumper wire For those who are interested in buildgized and latched. When the relay is ing a crowbar over-voltage module, I will with alligator clips to power up a heavy energized, the alternator field circuit is load (landing light) through the alterna- supply a schematic with a list of materibroken and alternator output falls to 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 a 5 amp breaker; it may take 20-30 at the address below. The information subject to ravages of age exacerbated by on seconds to trip it. The important thing sheet also describes proper incorporavibration, heat, moisture and corrosive 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 "crowWritten coverage plus real-time conconsider purchasing a new, sulting services by the author on this bar" over-voltage protection in reference condition, to its mode of operation. During an quality breaker to replace it. and other electrical systems topics are over-voltage condition, the system delibavailable from the AeroElectric ConnecSome Notes On The erately pops a supply circuit breaker or tion, 6936 Bainbridge Rd., Wichita, KS "Grumman Alternative" 67226-1008. $42 USD buys all materifuse which disconnects the protected 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 newsletis inexpensive, compact and totally im- uses a 1-watt, 16-volt zener diode 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. Masage 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" O.V. PROTECTION NOT RECOMMENDED

16 VOLT 1 WATT GLASS ZENER

FIGURE 1 SPORT AVIATION 69