Re: SSTC-556 timer IC malfunction

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>" <taylorss-at-rose-hulman.edu>

There should be no problem checking the 556 . . . it's just a dual 555 (i'm
pretty sure of this).  The CMOS part is dependant on the preceeding numbers,
such as LM555 or LM556.  An oscilloscope probe is very high impedance
compared to anything else in the circuit, 1 MegOhm for a standard probe, or
10 MegOhms for a 10X probe.  You might want to try testing the 556 alone,
without the 2904/2906 hooked up, check it's operation and voltage swing,
then check it with the transistors, and then check the output of the
transistors.  At the FET gate, is should be going as low as .5 to 1 V, and
as high as Vcc-.5 or 1 V, so 11 or 11.5 V for the 12 V supply.
-Sean Taylor


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, January 15, 2002 10:00 AM
Subject: RE: SSTC-556 timer IC malfunction


> Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<pkomen-at-zianet-dot-com>
>
> Matt,
>
> I had a 555 pop part of the plastic case off when I bypassed the output
> current limiting resister.  The first 555 circuit I built was a simple
> astable multi-vibrator with LEDs on the output and 9V battery power.  It's
> easy to pop in a 555 for a limited check.
>
> Can you check the 556 with an oscilloscope?  Don't forget that the CMOS
> version can's sink or supply as much current as the standard 555.
>
> I don't know the possible failure modes of a 555, but from your
description,
> I would guess that something has shorted.
>
> Pete Komen  (I'm no expert on 555s)
>
> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Monday, January 14, 2002 6:06 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: SSTC-556 timer IC malfunction
>
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <Fucian-at-aol-dot-com>
>
> In a message dated 1/14/02 6:48:00 PM Eastern Standard Time,
> tesla-at-pupman-dot-com
> writes:
>
> << Your problem with the 555 and driver circuit is a combination of not
>  enough current supply, and not having transistors that can handle the
>  current.  Your power supply should be able to supply at least 5 amps,
>  I would say 10.  Ignition coils can suck up a lot of current.  If you
>  don't have a large enough power supply, you could use a car battery,
>  or get a gel-cel from a hobby store.  Your output transistors also
>  need to be able to handle a large current, and be able to dissipate a
>  decent amount of power.  The FET will help, but don't be surprised if
>  it blows out - I'm surprised that the transistors you have right now
>  are still alive.  You could get a decent powered FET from Raido Shack,
>  or order one from a supplier like Maxim.  If you use a FET, it won't
>  have to dissipate much power, so a large heat sink wont be necesary.
>  Just get an N-channel, enhancement mode FET, with a continuous current
>  rating of at _least_ 5 amps, I would say 10 A.  However, FETs are very
>  sensitive to static electricity, and high voltages.  BJTs are more
>  robust.  I would use a 2n3055, which is an NPN type.  It will handle
>  15 Amps continuous, but will dissipate more power than the FET, it
>  will need a heat sink.  If you would like a schematic to hook any of
>  this up, let me know, I will be happy to provide one for you.
>  -Sean Taylor >>
>
> The 556 CMOS draws very little current.So does the transistors.I think you
> got me mixed up here.I have a seperate supply for the coil.The transistor
> pair is there to deliver more current to the FET gate.I have a totally
> seperate supply to power the coil and MOSFET. I have had NO timer probs
> before.It just all of a sudden happened.Is it possible for a timer chip to
> "Lock up"?
>
> Matt
>
>
>
>
>
>