# Extremely fast on delay timers?

#### KevinTOC

Mar 4, 2019
2
Is anyone aware of any extremely fast and precise on delay timers?
I'm specifically looking for one that could stop 400VDC going through for ~0.007 seconds

I'm just wondering if there is a delay timer/relay that fast.

Or, perhaps some way to calculate how fast it takes for a capacitor to discharge depending on the resistance after the capacitor.

Last edited:

#### AnalogKid

Jun 10, 2015
2,660
7 ms probably is too short for an electromechanical relay to reproduce reliably, but very easy for a solid state circuit.

How is it that you have this as a homework problem, but don't know about (let alone how to look up) the equation for R-C networks? What is the course you are in, and what is the assignment?

g..o..o..g..l..e.........?

ak

#### BobK

Jan 5, 2010
7,682
Voltages do not go through anything. Current goes through something.

What is the relationship between the two questions?

Bob

#### KevinTOC

Mar 4, 2019
2
7 ms probably is too short for an electromechanical relay to reproduce reliably, but very easy for a solid state circuit.

How is it that you have this as a homework problem, but don't know about (let alone how to look up) the equation for R-C networks? What is the course you are in, and what is the assignment?

g..o..o..g..l..e.........?

ak
We had an assignment known as YFF (YrkesFaglig Fordypning) in Elektrofag. (Electro-subject when translated literally.) We had to come up with something that's at a level for 3rd-year students. I know how to look up stuff, but I couldn't find much that I understood.

Voltages do not go through anything. Current goes through something.

What is the relationship between the two questions?

Bob
I'm aware Voltages don't go anywhere, that was a mistake on my part.

#### davenn

Moderator
Sep 5, 2009
14,011
I will move it to general discussion

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,723
There are plenty of circuits available that will block the application of 400 VDC, typically using a MOSFET gated OFF until a "trigger" is received. The trigger, and associated logic, then allows the 400 VDC to be applied to subsequent circuit loads seven milliseconds (0.007s) after the trigger occurs. It is your design choice as to whether this occurs after the leading edge, or after the trailing edge, or at a particular voltage level of the trigger.

If the source of the 400 VDC is a capacitor, and the subsequent circuit load is a resistance, then the voltage across the resistance increases abruptly to the capacitor voltage, 400 VDC, at the end of the 7 ms delay period when the MOSFET is gated ON, and decreases exponentially after that according to this equation:

Eload = (400 V) (e^-(t/RC) )

where Eload is the voltage across the resistance load as well as the capacitor voltage, e is the base of natural logarithms or approximately 2.718281828459. ..., the caret ^ represents exponentiation, t is the elapsed time in seconds after the capacitor is connected to the load resistance, R is the load resistance in ohms, and C is the capacitance value in farads.

Neglected in this analysis is the small, decreasing, voltage drop from drain-to-source across the MOSFET when it is turned ON after the 7 ms delay.

Normally, a capacitor is considered to be completely discharged after six time constants, which is the product of R and C in seconds. Thus, if R = 1 megohm and C = 1 microfarad, the RC time constant is one second and the capacitor is considered discharged at the end of six seconds. This is nonsense of course, because the exponential factor never does equal zero, no matter how long t is. However, after a sufficiently long period of time, the only voltage remaining on the capacitor is random thermal noise.

#### Petkan

Feb 9, 2011
19
Is anyone aware of any extremely fast and precise on delay timers?
I'm specifically looking for one that could stop 400VDC going through for ~0.007 seconds

I'm just wondering if there is a delay timer/relay that fast.

Or, perhaps some way to calculate how fast it takes for a capacitor to discharge depending on the resistance after the capacitor.
What do you mean by on delay timer? That it delays application of power by say a couple of ms?
What is the value in it? I guess you need a leading edge delay. The high voltage could be handled by respective mosfet. It cannot operate in linear mode (voltage or current limiting ) for long time. There are plenty of timers - look at linear.com (now analog.com for TimerBlox family. They have leading edge delay, trailing edge dlay, both edges delay. What you likely need is power conditioner. Linear Technology has a family of such chips with external MOSFETs

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