так. жизнь продолжается!
что-то тема эта заглохла...
вот что дружеские бусурманы предложили по поводу защиты от напряжения, поданного ВО ВРЕМЯ измерения (когда реле подключило щупы):
System 1
1. Your original design uses less than 0.6v for the test - so the test can be performed in-circuit without causing conduction in the semiconductors in-circuit with the capacitor under test. The protection circuit must therefore be sensitive enough to detect any condition which causes the voltage to rise above the level where any semiconductors in the circuit under test can start conducting and therefore affect the test - while being robust enough to withstand the voltage produced if a charged capacitor is connected to the circuit while the pet is in its ESR or capacitor-value-sensing mode.
2. Design a circuit that monitors for current through the back-to-back protection diodes on the input - which happens when you have enough voltage in the circuit under test for the diodes to conduct. If you have sufficient voltage to achieve conduction in the protection diodes - open the circuit to the pet's input and restart the cap-discharge process.
You do this by adding a small resistance in series with the protection diodes. Monitor the voltage drop across this resistor. Should the resistor start producing a voltage drop - it indicates that one or the other of the protection diodes is conducting.
Having a differential op-amp input across the resistor and adjusting the gain suitably, with a comparison against a Vref - where the circuit is polarity independent - will produce an op-amp output that indicates when either one of the protection diodes is conducting, but ignores any leakage current in the before-threshold-is-reached condition. (Typical polarity-independent Schmitt Trigger situation.)
The output of the op-amp when current is flowing through either protection diode toggles the relay to open the connection to the pet's input and begins the capacitor-discharge cycle again.
3. The existing silicon diodes used for the protection circuit must be sized to survive the current through one or the other of the protection diodes for the short time it takes for the protection circuit to disconnect the pet from the input leads. You will have to experiment to see how much dissipation will actually occur in the few-milliseconds transition timeframe - and adjust the size and ratings of your silicon protection diodes and the wattage of the current-sensing-resistor accordingly.
Note: Relays react in timeframes thousands to millions of times longer than those in electronic circuits - but that relay is required in your discharge circuit in order to get sufficient current flow in discharge mode to discharge a highly-charged large-value-capacitor in a reasonable amount of time. Therefore, things that happen in the transition time between the sensing of a voltage that should not be present on the input to the pet - and the time it takes for the relay to open the circuit to the pet's input - must be accounted for.
System 2
Monitor the voltage across the existing back-to-back diode-pair. Look for the *presence* of a too-high voltage across the diode-pair, indicating that either diode is conducting during the measurement phase. If the voltage drop increases (indicating that either diode has been biased into conduction to the point where it is carrying substantial current) the op-amp causes the relay to break the connection to the pet and begin the cap-discharge process again.
This circuit is cranky. Forward conduction voltage-drop across a silicon diode
does vary with current. However, the voltage variation is very small - and the conduction-threshold-voltage varies with temperature. Production variations in batches of diodes can also affect the conduction-threshold voltage - such that the diodes will have to be matched, you need a temperature compensation circuit, and the op-amp's threshold voltages must be fine-tuned to the particular set of diodes installed in the pet. And if either diode in a diode set fails, another matched set must be installed and calibrated before the pet is safe to use again. More trouble than it's worth IMO.
System 3 - Further protection for either System 1 or System 2:
1. For the capacitor-discharge circuit to protect against someone putting a protected pet in-circuit with the power turned on - you can use another set of op-amp inputs as a ripple-voltage detector. Should the protection circuit detect the presence of ripple - this means the circuit under test is not powered off and the discharge function should not be performed at all. This is the very first test performed - and it protects the circuit under test from the near-short-circuit condition that occurs when a supposedly-unpowered capacitor is being discharged. This requires two relays - one which keeps the pet disconnected from the test leads until a safe condition is detected, and another which will not energize the cap-discharge-relay until the source of ripple is removed.
IMO, this would be a great feature that would differentiate your unit from the Atlas ESR Plus. I suggest that this circuit be installed regardless of whether System 1 or System 2 is employed. However, System 3 is optional.
And if a user is trying to check a capacitor far enough downstream of the rectifiers or switching transistors that there is insufficient ripple to cause the ripple-detector to activate - then you'll just have to admit that you can't protect against that level of stupidity.
Hope this helps.
twixt is offline Report Post Reply With Quote
