Re: 0.20 delay in 6v control circuit

From: Bob (bfraser1www_at_optonline.net)
Date: 11/09/04 

Date: Tue, 09 Nov 2004 03:21:06 GMT

On Mon, 08 Nov 2004 22:10:19 GMT, Robert Monsen
<rcsurname@comcast.net> wrote:

>(fixed top posting below. Please post replys after the main body of
>text. It's just convention here)
>
>Bob wrote:
>> On Mon, 08 Nov 2004 07:37:01 GMT, Robert Monsen
>> <rcsurname@comcast.net> wrote:
>>
>>
>>>Here is a slightly better circuit for you needs:
>>>
>>> 6V
>>> ----------------------------o--o
>>> | | |
>>> | - C|
>>> | o ^ C| Relay Coil
>>>|=| switch | C|
>>> | o | |
>>> | '--o
>>> | |
>>> | 10k |/
>>> o---/\/\/--o-----o-----------|
>>> | | | |>
>>> | | | |
>>> o----|<----| | + |
>>> | * --- 220uF |
>>> / --- |
>>> \ | |
>>> / 1k | |
>>> \ | |
>>> | | |
>>> | | |
>>> '----------------o-------------'
>>> GND
>>>
>>> * Schottky Diode
>>>
>>>created by Andy´s ASCII-Circuit v1.25.250804 www.tech-chat.de
>>>
>>>The base will drop immediately from 850mV to about 200mV when the switch
>>>is opened, due to the 1k resistor and the schottky diode. When the
>>>switch is closed again, the base will come up from 200mV to about 850mV
>>>before the transistor conducts, so
>>>
>>> V(t) = V ( 1 - exp(-t/RC) )
>>>
>>>thus
>>>
>>> t = -ln(1 - .65/6) * 10k * 220u
>>> = .25s += 0.05s
>>>
>>>Once the cap is charged up, the current will be about 500uA, so the
>>>current through the transistor can go up to 25mA (assuming a beta of 50).
>>
>>
> > Thanks again Robert,
> >
> > Do you have any idea on the specs for the diodes needed. I went to
> > pick up the Schottky diode and the one in parallel with the relay but
> > was not able to choose from the variety available. Thanks for the
> > design.
> >
> > Bob
> >
> >
>
>You can use any of 1N4002-1N4007 diodes for both of them. A Schottky
>diode would be nice where the * is (it doesn't have to carry any
>appreciable current, so any of them will do) but isn't necessary. It
>just makes the recovery time a bit better, because it has a lower
>forward voltage drop.

As you can probably tell, this is the first time I ever posted to a
news group. Anyway I built the circuit as you designed. What
actually occurs is the relay is energized for only .20 seconds instead
of a delay of .20 seconds then energized until the switch opens. (Did
you expect this? I checked the path of the circuit 3 times)

Bob

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