On Mon, 30 Jul 2018 15:52:29 -0400, you wrote:
The supply has to deal with (effectively) a ramp up of the input
voltage rather than an abrupt step. May confuse some chips that need
a minimum dv/dt, but I'd guess that depends on the design of the
regulator and the rest of the circuit.
Harvey
A snubber capacitor is a capacitor, used in conjunction
with a series resistor, to dampen, or eliminate the back EMF
transient caused when an inductor abruptly loses its
current source.
They are useless in capacitor input power supplies, like
used in the 2465 family of scopes, because they have no
appreciable inductance to make a back EMF transient.
However, capacitor input filter type power supplies have
their own switch eating problems, in that potentially,
they could draw extremely high inrush current when the
power is first applied to the discharged filter capacitor.
This inrush will quite nicely burn, or weld, the contacts
of an otherwise properly sized switch.
To cure this inrush problem, tektronix put a thermistor,
with a shunt resistor in series with the power switch.
The thermistor starts out at a high impedance, causing
much of the initial charging current to be limited by
its resistance. As the current passes through the
thermistor, I2R losses cause it to self-heat, which
because of its negative temperature coefficient (NTC),
will cause it to lower in resistance to a point where
it is near zero ohms. At that point, it self regulates,
never getting hotter, or cooler than is necessary to
supply the current to the scope.
Back to snubber capacitors: The snubber can be sized
such that it completely eats any back EMF transient.
To do this, it would be chosen to have a capacitive
reactance equal in absolute value to the inductive
reactance of the load it switches. Its resistance
would be chosen to equal to the DC resistance of the
inductive load.
This is rarely done, because, it is not necessary to
eliminate the back EMF peak voltage, only to dampen it
down to a manageable value that will not burn out the
switch. Usually 10 to 100x the load's resistance is
adequate. Similarly, the capacitance can be reduced
too... but either reducing the capacitance, or increasing
the series resistance will cause the EMF spike to rise.
-Chuck Harris
M Yachad wrote:
I have now seen two 2465 PCB blocks with a burnt-out on/off switch.
The damage is caused by the arcing across the now-separating terminals, when the machine is switched OFF.
Replacement is easy with NE182UEEP6AMP, but examining the circuit shows me that this failure could well be preventable with the addition of either a single ceramic snubber capacitor of between 3.3nF to 10nF (example Vishay¡¯s VY1 or VY2 series), OR an R+C snubber network ¨C I calculated a 120 ohm 2W ceramic comp in series with a 33nF X2 film cap, across the Tek switch¡¯s on-off terminals.
What are the considerations for selecting EITHER a single ceramic cap, OR making up a R+C snubber?
I¡¯m not looking at the apparent cost savings of installing a single cap vs constructing a R+C.
The objective is long-term reliability.
Also, how would one select the capacitance value of the single ceramic cap?
I¡¯m thinking of applications on vintage stereos.
I noticed that a new Yamaha hi-power stereo uses a 10nF, and a 20-year old CD-player uses a 3.3nF.
Is the current draw a factor?
Over to the experts¡.
Menahem
