EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: nsled on May 07, 2023, 11:34:32 am
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Hello, I'm trying to create a slight delay in the voltage rise when a switch is closed, I can see how this would be done if the load and power supply are separate, just put a capacitor in parallel, but how can it be done when the power supply is the load?
The device is a flash unit, with a wired "switch". I want a very slight delay when its triggered.
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Do you really want the delay when you activate the switch by hand? If you use some other mean to activate the switch then have the delay before the switch.
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The switch is symbolic, but it is physically triggered just not by hand. The delay cannot be before the triggering of the switch.
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If I understand correctly, maybe a capacitor in series to the switch with a bleeder resistor?
As soon as you close the switch, it starts charging, which will add some delay to the rise.
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Basically, you are asking to "debounce" the switch with a 500 us delay.
http://www.ganssle.com/debouncing.pdf (http://www.ganssle.com/debouncing.pdf)
The image below is from Grassle. There are other versions. Just charge a capacitor to hold the flash in standby, then discharge it through a resistor and your switch to ground to make it flash.
500 us is a very short time for debouncing.
EDIT: Attachment #2 shows what I tried to describe.
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The switch is symbolic, but it is physically triggered just not by hand. The delay cannot be before the triggering of the switch.
You can use a transistor or SCR as a switch and then you delay the base or gate signal that coming from the triggering device like a photocell or camera sync contact.
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Depending on the exact circuit of the flash there may be a substantial voltage across those open contacts. Possibly enough to kill a typical "small signal" transistor.
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The voltage across the switch is shown as 3.3V.
I suspected something like a pull-low to flash switch. That is, the positive terminal of the switch does not have a direct connection to VCC. Voltage may be high, but current capability might be quite low. In a way, that is good as one would only need to pull to the appropriate "low" (e.g.,TTL or CMOS). On the other hand, charging of the capacitor could be quite slow. I use a MAX16054 to turn on a circuit (https://www.analog.com/media/en/technical-documentation/data-sheets/max16054.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/max16054.pdf)). The pullup is about 100k. If the TS uses a 100nF capacitor and a 10K resistor, the time constant is 1 ms. The RC time constant for charging would be 10 ms (i.e., 3.3V/100k = 33 uA), which is probably OK for slow repetitive flash speeds.
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The voltage across the switch is shown as 3.3V.
I suspected something like a pull-low to flash switch. That is, the positive terminal of the switch does not have a direct connection to VCC. Voltage may be high, but current capability might be quite low. In a way, that is good as one would only need to pull to the appropriate "low" (e.g.,TTL or CMOS). On the other hand, charging of the capacitor could be quite slow. I use a MAX16054 to turn on a circuit (https://www.analog.com/media/en/technical-documentation/data-sheets/max16054.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/max16054.pdf)). The pullup is about 100k. If the TS uses a 100nF capacitor and a 10K resistor, the time constant is 1 ms. The RC time constant for charging would be 10 ms (i.e., 3.3V/100k = 33 uA), which is probably OK for slow repetitive flash speeds.
I can interpret the opening post as indicating that the unit is powered from 3.3 Volts, not definitively stating that the switch has 3.3 Volts across it. I can also interpret "The device is a flash unit, with a wired "switch". I want a very slight delay when its triggered." as indicating that the switch is not a power on-off switch but is the switch that triggers the flash.
The device is referred to as a "flash unit". Although the built-in flash in mobile phones is invariably a LED, I think a flash tube is much more common in add-on units for cameras.
Need clarification.
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The switch has 3.3v across it, it is a >=0.5us LED high speed strobe. Once a bullet trips the switch i want a short delay so the bullet can be photographed away from the switch. 500us is just round abouts in practice it may be 200us etc. In my head I was picturing changing the cap value to change delay. So is what Shay suggested viable? just a cap in series with switch? I know there's certain specifics that need to be done experimentally like what voltage it actually fires at, what inherent delay it already has etc.
I know this can be done any number of ways but I'm specifically after the bullet acting on the switch mechanically.