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Crowbar vs Transorb (TVS)
- Thread starter aerhed
- Start date
And if the TransZorb breakdown voltage isn't exceeded? Errors in the control circuitry could result in legal field voltage and current, but excessive output voltage from the alternator under light to medium loads. Just throwing a clamp across the field circuit isn't any kind of properly designed OV solution.
A proper solution measures the output voltage, and opens or crowbars the field circuit when the output is illegal.
A proper solution measures the output voltage, and opens or crowbars the field circuit when the output is illegal.
Isn't it the bus output voltage you're regulating? Why would I care about voltage upstream of the bus? If breakdown voltage isn't exceeded at the bus, isn't everything okay? Won't tripping the field always take it off-line? Why clamp the output instead of the field? Don't just tell me I'm wrong, please explain.
As I understand it, the denso type alternators have a transistor that can fail in the regulator output which full fields the alternator with no provision to actually turn off the field in this condition as the field + is derived from the B+ terminal internally. The "switch" wire turns on the regulator but the field positive is already there looking for a ground through the previously mentioned transistor. The modifiers, i.e. Plane Power, reroute the positive field brush from B+ to the "switch" wire giving positive control of the field. They also put a crowbar circuit (sensing output) to short the "switch" wire which trips the field breaker and takes the field out. Now, for like the 3rd time: Why wouldn't a 16V TVS diode from the dowstream side of the field breaker to ground instantly trip the field breaker the same as a crowbar circuit with a 16V zener. Isn't a crowbar exactly the same function as a TVS except it includes a cap to soak up transient spikes or high ripples?
The problem with not speaking directly to a schematic is that the communication will be constantly subject to errors of assumption and misinterpretation.
If the TransZorb was at the breaker, rather than at the field, a number of faults might be caught. But not all of them. And this design might only work once. Breakers take long enough to trip, even with a large overload, that the TransZorb may reach it's thermal limit and die. They are designed to catch very short glitches, and they don't have the ability to shed watts for long.
If the regulator fails in a way that shorts measured output to the field, the scheme above will be useless. The alternator will then be essentially self powered.
A more foolproof method of OV protection would have a circuit monitoring the output voltage that can directly block the field current, i.e. is downstream from the regulator. Of course, these circuits are properly designed to work with the regulator, as it can simplify the design, and makes sure one does not destroy the other. This circuit can also be tailored to ignore high speed glitches, and thus reduce nuisance trips.
Which is why I assumed that's where you wanted to put the TransZorb.
TVS
The crowbar, which makes use of an SCR to short out the field suppy and take out its breaker, is triggered by an overvoltage from the alternator output, not an overvoltage on the field. If you used a 16V TVS at the field supply. all it would do is clamp the field voltage, but not reduce it. If the field voltage is already that high at high rpm, it is already an indication that the regulator has lost its ability to control the alternator output. In this situation, the alternator output is on its way to very high voltage, and even with a 1500W or 3000W TVS, the TVS will be driven beyond its capability and burn up immediately, and may go open, in which case the field breaker won't pop. In checking out circuits that had a uni-directional 1500W TVS on its power input, the Bench Power Supply was generating transients as its voltage was raised, due to some internal relay switching, and it was taking out the TVS. They're good devices when properly applied, but look at their little DO-18 or DO-201 diode-style package; they won't take much power for long! I made my own PWM regulator with a series mosfet that would open if the voltage went beyond 14.7V. It has a fold-back circuit that will restore the output when it drops back down through 12V.
The crowbar, which makes use of an SCR to short out the field suppy and take out its breaker, is triggered by an overvoltage from the alternator output, not an overvoltage on the field. If you used a 16V TVS at the field supply. all it would do is clamp the field voltage, but not reduce it. If the field voltage is already that high at high rpm, it is already an indication that the regulator has lost its ability to control the alternator output. In this situation, the alternator output is on its way to very high voltage, and even with a 1500W or 3000W TVS, the TVS will be driven beyond its capability and burn up immediately, and may go open, in which case the field breaker won't pop. In checking out circuits that had a uni-directional 1500W TVS on its power input, the Bench Power Supply was generating transients as its voltage was raised, due to some internal relay switching, and it was taking out the TVS. They're good devices when properly applied, but look at their little DO-18 or DO-201 diode-style package; they won't take much power for long! I made my own PWM regulator with a series mosfet that would open if the voltage went beyond 14.7V. It has a fold-back circuit that will restore the output when it drops back down through 12V.
Sense voltage
The TVS is sensing the buss voltage through the field CB and the Field switch. The problem with a TVS is its power handling capability. A 1500W, 15V unit can handle a peak current of 72A for about 1 millisecond, but that drops to 7.2 A by 4 ms and shortly after that it will go belly-up. And that's only if it's attached to an infinite heatsink. It's only meant to clamp very short, microsecond to millisecond, duration transients to protect electronic circuits. It may or may not pop the CB in that short interval, but when the TVS goes out it can't be reset as can the crowbar. The SCR is usually in a bigger package that can handle more current. The reason why is that the SCR when turned on drops to about 1V, so even at 20A it's only dissipating 20W, whereas the 15V TVS will be dissipating 300W. Keep in mind the characteristics of the CB trip current, where it will handle small overloads for many seconds, minutes, or hours, and to get it to trip rapidly requires a big overload. The circuit I mentioned previously where the TVS popped because of power supply transients never tripped the 5A breaker!
The TVS is sensing the buss voltage through the field CB and the Field switch. The problem with a TVS is its power handling capability. A 1500W, 15V unit can handle a peak current of 72A for about 1 millisecond, but that drops to 7.2 A by 4 ms and shortly after that it will go belly-up. And that's only if it's attached to an infinite heatsink. It's only meant to clamp very short, microsecond to millisecond, duration transients to protect electronic circuits. It may or may not pop the CB in that short interval, but when the TVS goes out it can't be reset as can the crowbar. The SCR is usually in a bigger package that can handle more current. The reason why is that the SCR when turned on drops to about 1V, so even at 20A it's only dissipating 20W, whereas the 15V TVS will be dissipating 300W. Keep in mind the characteristics of the CB trip current, where it will handle small overloads for many seconds, minutes, or hours, and to get it to trip rapidly requires a big overload. The circuit I mentioned previously where the TVS popped because of power supply transients never tripped the 5A breaker!
Crowbar
I'm not familiar with the crowbar circuits used on aircraft, but in its simplist form it's an SCR with a resistor from the gate to the ground and a zener diode from the supply voltage to the gate. If you are familiar with electronics you should be able to make one for under $10. If the voltage exceeds the zener breakdown voltage rating, the SCR's gate is triggered and it turns on, shorting the field suppy CB and removing current from the field. Where I worked we had a crowbar circuit on the high voltage power supply to a magnetron. Another thing I hadn't mentioned with the TVS is that once the field of the alternator has built up to some level of magnetism, it doesn't collapse immediately so that even after the current source is removed, the alternator will continue to put out a decaying output, depending on the R-L-C of the field circuit.
I'm not familiar with the crowbar circuits used on aircraft, but in its simplist form it's an SCR with a resistor from the gate to the ground and a zener diode from the supply voltage to the gate. If you are familiar with electronics you should be able to make one for under $10. If the voltage exceeds the zener breakdown voltage rating, the SCR's gate is triggered and it turns on, shorting the field suppy CB and removing current from the field. Where I worked we had a crowbar circuit on the high voltage power supply to a magnetron. Another thing I hadn't mentioned with the TVS is that once the field of the alternator has built up to some level of magnetism, it doesn't collapse immediately so that even after the current source is removed, the alternator will continue to put out a decaying output, depending on the R-L-C of the field circuit.
From what I'm finding online, crowbarring the field seems to "the way" nowdays. I'm definitely not an EE guy. When I went to school all my electronics lecturers and lab instructors were guys with mideast accents and as a backwoods kinda boy I could hardly understand them. TVS's are new to me so... Didn't the old gen setups use an overvoltage relay? Been awhile since I saw a carbon pile that wasn't on a shelf.
