Voltage Trigger

[Will]

Hi everyone,

I am trying to build a circuit that will trigger an SCR when a
capacitor is charged to, or above, 800 volts. In the past I would
have just started tying things until I got close but I'm not entirely
comfortable at the voltages and am apprehensive about trial and
error. Could I just add an 800V (or series adding up to) diode and
let it break down? Seems like there would have to be a better way to
do this but I have no idea what. In a perfect world, I would like to
place 2 or 3 of (whatever) in parallel for a little failure
protection. Any advice anyone can provide will be Greatly
appreciated.

W

 

[John Popelish]

Hi everyone,

I am trying to build a circuit that will trigger an SCR when a
capacitor is charged to, or above, 800 volts. In the past I would
have just started tying things until I got close but I'm not entirely
comfortable at the voltages and am apprehensive about trial and
error. Could I just add an 800V (or series adding up to) diode and
let it break down? Seems like there would have to be a better way to
do this but I have no idea what. In a perfect world, I would like to
place 2 or 3 of (whatever) in parallel for a little failure
protection. Any advice anyone can provide will be Greatly
appreciated.

If you can sacrifice a tiny current from the capacitor, you
could make a voltage divider with two resistors that scales
the capacitor voltage down to something more practical to
react to and compare that to a voltage reference somewhere
around 5 to 15 volts with a comparator. This assumes you
have a low voltage supply available for this comparator to
be powered by.

For instance, you could connect a 10 meg resistor (rated for
1000 volts) in series with a 127k resistor. When the total
voltage reaches 800 volts, the pair will be consuming 79 uA
and the drop across the 127k will be about 10 volts.

An integrated voltage reference like the LM4041
http://www.national.com/ds/LM/LM4041.pdf

Here is a dual comparator that might be used to tell you
when the divider output exceeds the reference voltage:
http://rocky.digikey.com/WebLib/Texas Instruments/Web data/LM193,293,293A,393,393A,2903,2903Q.pdf

Using the output to fire the SCR is a separate problem to
solve, that depends on where the SCR is in the output circuit.

 

[Jasen Betts]

Hi everyone,

I am trying to build a circuit that will trigger an SCR when a
capacitor is charged to, or above, 800 volts. In the past I would
have just started tying things until I got close but I'm not entirely
comfortable at the voltages and am apprehensive about trial and
error. Could I just add an 800V (or series adding up to) diode and
let it break down?

maybe use a PUT instead.

Seems like there would have to be a better way to
do this but I have no idea what. In a perfect world, I would like to
place 2 or 3 of (whatever) in parallel for a little failure
protection. Any advice anyone can provide will be Greatly
appreciated.

ONSemi has an excellent thyristor document "HBD855/D"

http://www.onsemi.com/pub/Collateral/HBD855-D.PDF

Bye.
Jasen

 

[Will]

Thanks guys. You've given me enough to keep me busy for a couple of
days. Thanks again.

 

[[email protected]]

Thanks guys. You've given me enough to keep me busy for a couple of
days. Thanks again.

There is also a way to use the LM4041 as the comparator, as well as
the reference. Divide the high voltage with two resistors, so that
the divider voltage just reaches 1.2 volts when the high voltage
reaches your target. Connect that divider output to the reference pin
of the LM4041, with the anode tied to zero volts. As the divider
voltage rises through 1.2 volts, the cathode current will rise very
rapidly fro, somethhing like 40 uA to a much larger current limited to
about 10 mA, with a saturation voltage of about 1.5 volts. You may be
able to figure out how to trigger your SCR off that current swing.