Capacitor question

Tumbleweed

Hi,
I'm working on a project where I've got a stand-alone instrument that puts out a TTL word that I want to multiplex into 16 discrete DO lines. I've got the multiplexer working, but I'm worried about my instrument sending the 'word' too slow and getting a false signal to one of the other 15 lines as the word builds.
I have the option of using an additional line as a latch, but that would complicate my life greatly. (plus I really don't want to lose one of my lines for this)

I'm using the TTL DO to trigger a relay that then does the task. What I'd like to do is build a delay into the circuit so that any chatter is eliminated while the 'word' out signal stabilizes. I have some timer relays that would work, but I was wondering if there was a better (cheaper, simpler) way to accomplish the same thing with a capacitor.

Just something that waits 50msecs would be plenty.

Does anyone have any suggestions or recommendation as to where I can get some more info?

Thanks,
Lance

Skywalker49

Lance,

a scheme is worth more than thousand words.

Ed

Mike Green

You can put a capacitor across the relay and use a series resistor between the capacitor/relay and the DO driver and the RC time delay for charging the capacitor through the resistor will slow down the relay's closing. The series resistor needs to be small enough to conduct the relay's current, yet the RC time constant needs to be long enough for the time delay.

Tumbleweed

Cool. Thanks, that worked. (well, there is a delay I just don't know how long it is)

I have a question though...How do I calculate the delay?

My stamps manual says t=RC. Do I understand this correctly to be the time it takes to charge the capacitor and then trigger the relay?

For testing I'm using an LED as my "relay" and my 'series' resistor is 470 ohm. I figure it will be easier to control the timer with the capacitor as well as protecting the LED.

So if I have a 470 ohm resistor and a 3300 microF capacitor that would give me a t=1.551

Is that the delay?

From observation it takes about a second for the LED to reach full intensity, but it seems to light up almost immediately.

Thanks,
Lance

Mike Green

The calculation is complicated by the fact that the relay or LED take some current that would otherwise charge the capacitor. Also, the LED doesn't really conduct significantly until the voltage across it reaches a threshold that depends on the type and color of the LED. For most red ones, it's around 1.2V. The LED will conduct whatever current is available until it either burns out or the voltage across it drops below the threshold. A relay will conduct whenever there's a voltage across it with a current determined by Ohm's law and the coil resistance and won't "pull in" until the current through it reaches some threshold that you can find from the relay's datasheet.

Tumbleweed

Thanks Mike.

I'll have to do some research and find the datasheet for the relay in question.

Lance

Mike Green

One thing you could try is to get a copy of an analog circuit modelling program like Spice. Others may have better suggestions, but you could put this simple circuit into it and get a graph of current through the relay coil over time and see just when the relay would start to pull in.