Re: High Voltage leaky diode (or any diode like device)

On Mar 29, 9:18 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:
MooseFET wrote:
On Mar 29, 8:16 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:

MooseFET wrote:

Another option may be to use a depletion mode MOSFET could be used as
a constant current source in place of the resistor. Normally I would
suggest a Supertex product but the last time I checked their web site,
it appeared that they no longer make them.

That would be the absolute best way, but you can still get passable
performance with transistors:
View in a fixed-width font such as
Courier.

.
.
.
. .--------------.
. | |
. IN >----+ |
. | |/
. '-[160k]-+---| Q1 KSC5603D
. | |>
. - |
. D1 v |
. - [180]
. | |
. '-|>|-+---->OUT
. D2 |
. |
. ---/
. // \ 1N4085
. ---
. |
. ---
.

For the OP's need that won't really work because the circuit needs
some voltage before it conducts at all.

His minimum input is 100V so I don't think that is a consideration.

He was concerned about there being a tempco on the difference between
the input and output voltages. I think the intent was to protect some
sort of measurement circuit.




This circuit won't work
either but is better:

Fixed font:
Vin R1
-----+--------/\/\-----
! !
! !/ e
-->!-->!--+----! PNP
D1 D1 ! !\
/ !
R2 \ +---- Vout
/ !
! /-/ Zener
GND ^
!
GND

There is a resistance and a saturated transistor in the path up to the
point where the zener conducts and pulls the transistor out of
saturation. The current from R2 flowing in R1 scales the Vin down a
bit but the EB drop of the PNP is mostly taken out of the picture by
the fact that the voltages are so large. This makes the tempco small.

That's not bad but it does have its drawbacks compared to the NPN
circuit: 1) low voltage saturation does not bode well for immunity to
transients,

Yes, the could be a problem if the high voltage arrives quickly. The
zener's current would be very large as the PNP came out of saturation.


and 2) the output impedance is about 2x. That's a big price
to pay for a few less millivolts drop.

I don't see why the impedance would be lower. Your design has a non-
saturated NPN in the current path during the "normal" operation. Mine
has a saturated PNP which should be a much lower impedance.


Then are the HV pnp as readily available as the NPN, and same HFEs. He
can dip into the ultra-cheap HV NPN deflection amps for the NPNs. That
KSC5603D is $0.71 in unit quantity from Mouser.

Finding HV PNP is hard but not completely out of the question. It is,
I admit, going to be trouble at 600V.


I had a couple of other interesting ideas:

Does anyone make "Static Induction Transistors". In theory, they
would be a good part for this sort of circuit but nobody seems to make
them.

For the OPs purposes a variation of your circuit may work better:


-------+---------------
! !
! !/
--/\/\--+----! NPN
! !\ e
! !
! +------- OP's output
\! !
NPN !----+
e /! !
! \
! /
! \
! !
+------
!
/-/
^
!
GND

.

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