(P6KE or 1.5KE) will not survive the high-energy discharge of a “load dump”. Special automotive suppressors must be used to use up the 20A to 30A peak currents being shunted. Several manufacturers, such as Motorola, Harris and Seimens, manufacture suppressors specifically for automotive applications. Some devices provide “zener diode” style protection, while others provide “back to back zener diode” bidirectional protection. Each type has advantages, but unless they are used correctly, they will fail to protect the electronics. Ratings on the transient suppressors can be confusing. A suppressor with an avalanche voltage of 24V to 32V will have a clamp off voltage of over 40V. In addition, ambient temperature can vary from –40C to 70C and can result in the avalanche voltage being several volts lower at –40C and a clampoff several volts higher at 70C.
Not all vehicles have 12V battery systems. Some trucks use 24V batteries, aircraft use 28V and trains from 45V to 85V. Transient suppressors for aircraft cannot use the 12V system automotive components. Instead, a suppressor with an avalanche rating of 35V is needed to allow for low ambient temperature compensation, but this results in clamp off of over 70V. Tri-M Engineering’s High Efficiency PC/104 Vehicle Power Supply, employs a Diode Inc. (part#5KP43A), allowing an input voltage range of 6V to 40V. If a high clamp off voltage cannot be tolerated, other techniques must be used. A series device such as a MOSFET can act as a pre-regulator, but it also must be selected to withstand transients. In addition a series device adds to in-efficiency and creates a heat dissipation problem, especially at high ambients.
“Load dumps” occur infrequently in a vehicle’s lifetime, but any electronics wishing to survive in this environment must be designed to withstand the assaults. “Load dumps” co-operate slightly through, their worst-case voltage does not typically occur with worst-case source impedance. In fact, although the total energy of a “load dump” may be 500 joules, a transient suppressor capable of 70 joules typically will be adequate because of the distributed electronics in the vehicles. That is, provided the suppressor ratings are the same or larger than other suppressors throughout the vehicle. A quick thinking engineer can take advantage of this and design his power supply to withstand higher voltages and thus let others’ transient suppressors do the work.
APPENDIX 5
BC104 Battery Charger and PM104 Power Management Units
1) Description
When the BC104 and PM104 units are both installed on either the V104 or HE104 (hereafter referred to as PSU), as universal battery charger can be setup and the PSU unit made into an UPS (uninterruptible power supply).
The BC104 is a constant current “buck” switching regulator with an adjustable “float” voltage. The float voltage is adjusted via a potentiometer. The PM104 is programmed by the user using a “control basic” called Pbasic. A sample program is suppli ed to show a typical NiCd charging control. Before using the BC104 and the PM104 the battery charging program must be set up for the intended battery pack. The sample program has separate settings for normal charge current and trickle charge current. In addition, the charge termination methods should be set, including
Tri-M Engineering | Tel: | 800.665.5600, 604.945.9565 | 24 |
1407 Kebet Way, Unit 100 | Fax: | 604.945.9566 | |
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