Re: High Voltage leaky diode (or any diode like device)
- From: mike <spamme9@xxxxxxxxx>
- Date: Fri, 28 Mar 2008 10:12:49 GMT
Mook Johnson wrote:
I have a circuit that is essentially a resistor and zener. The zener is a 200V low TC zener (1n4085) and the resistor feeding is is 300K. The supply voltage will vary in an nominal range of 100 to 300V and nust deal with 5 second transients up to 600V (once/hour). To keep the peak power dissipation below the ~1/2 the diodes rated power of 2.5W, the current limiting resistor has to be 32K.Can you be more specific about what you're trying to accomplish?
In this circuit I need the zener to be a follower (Vcathode = Vin) when the device is below 200V and a clamper at 200V when Vin ia above 200V. Odviously the zener has a knee that is somewhat soft but acceptable. My problem is at 100V the zener has leakage that is proportional with temperature. This will cause an error when the sener is in vollower mode that gets worse with temperature.
I'm thinking that if I can find a 800-1000V diode that leaks in a manner that more that compensates for the zener leakage below Vz, and hang that in parallel with the 32K resistor, that would fix this problem.
What I really need is a say a 1mA 800 to 1000V rated current source that is fairly constant over a wide temperature range and doesn't require more than 1V to turn it on.
any ideas?
The statement "Vcathode = Vin" makes no sense in engineering terms.
How closely does it have to match over what range of source voltages
and load currents and whatever else is in the circuit. The devil is in
the DETAILS.
Trying to match leakage of one device with a different type of device
with WIDELY different voltages over temperature
is often futile. If you're building one test fixture, it might be worth the time
to select something that works...but probably cheaper to use more
parts and less time. If you're designing a production device,
you won't (shouldn't) get past the first design review. NEVER, EVER
design using a parameter that is not tested/guaranteed and expected to
stay the same when purchasing second-sources the part...or the vendor
second-sources the raw material...or
And I must not understand your reasoning. If you expect Vcathode = Vin,
that defines the voltage across the resistor to be ~zero...which means
that there can't be any leakage in a parallel connected diode???
Pesky details again...
If the leakage is your only problem and the current design is
close to working, you may be able to tweak it by using a two-stage
zener setup. The first zener doesn't have to be very precise,
just heavy enough to burn the power required to get the source resistance
down to what your leakage can tolerate. You can also do some
interesting stuff with PTC resistors and/or incandescent light bulbs.
If diode leakage across 32K is a problem, you can't be delivering
much current to the load. Maybe should just bite the bullet and
build a voltage controlled voltage source to get what you need.
All depends on what you're trying to accomplish.
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