My SPICE program is telling me that even when I remove the GND supply from a 556, the 556 still outp

[Mr. J D]

I wanted to turn on-off the 556 by cutting off its ground supply, using
a NPN transistor in a common emitter configuration. But when I turn off
the supply to ground, by turning on the transistor, the 556 still
outputs the same exact signal as when the GND supply is attached. Whats
going on?

 

[Eeyore]

I wanted to turn on-off the 556 by cutting off its ground supply, using
a NPN transistor in a common emitter configuration. But when I turn off
the supply to ground, by turning on the transistor, the 556 still
outputs the same exact signal as when the GND supply is attached. Whats
going on?

When the transistor is *ON* the 556 will get power ! You need to turn it off.
I'm doubtful this is a great configuration to turn off power though. Where do
the related Rs and Cs connect to 'ground' ? At the transistor collector or the
power supply 0V ?

Graham

 

[colin]

I wanted to turn on-off the 556 by cutting off its ground supply, using
a NPN transistor in a common emitter configuration. But when I turn off
the supply to ground, by turning on the transistor, the 556 still
outputs the same exact signal as when the GND supply is attached. Whats
going on?

some models internally connect to gnd (node 0)

Colin =^.^=

 

[Mr. J D]

When the transistor is *ON* the 556 will get power ! You need to turn it off.
I'm doubtful this is a great configuration to turn off power though. Where do
the related Rs and Cs connect to 'ground' ? At the transistor collector or the
power supply 0V ?

Graham

Thanks for answering, i meant when I turn the transistor off. I had a
suggestion from some other people to use the Rs, reset pin, instead of
the ground, because of internal groun connections, etc.
However I am having a hard time realizing how to use a transistor to
control the Rs pin. I could use an inverter connected to a PNP, but i
am looking for something more simpler.

 

[Mr. J D]

When the transistor is *ON* the 556 will get power ! You need to turn it off.
I'm doubtful this is a great configuration to turn off power though. Where do
the related Rs and Cs connect to 'ground' ? At the transistor collector or the
power supply 0V ?

Graham

Hmmm. How much amperage does the reset pin need? does it just need a
logic level signal? if it does, than I guess I could just directly
connect it to my digital circuitry.

 

[Eeyore]

Hmmm. How much amperage does the reset pin need? does it just need a
logic level signal? if it does, than I guess I could just directly
connect it to my digital circuitry.

I didn't mean the reset specifically. I meant all the resistors and capacitors
needed to make it function that normally go to ground. They would have to be
connected to the npn transistor's collector along with V- on the 556.

There's no reason you can't switch power to V+ with an npn emitter follower though.

Graham

 

[Kevin Aylward]

I wanted to turn on-off the 556 by cutting off its ground supply,
using a NPN transistor in a common emitter configuration. But when I
turn off the supply to ground, by turning on the transistor, the 556
still outputs the same exact signal as when the GND supply is
attached. Whats going on?

In general, many/most spice models do not model power supply effects by
design. Unless a model specifically says what supply effects it
includes, assume it don't.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

 

[Rich Grise]

I didn't mean the reset specifically. I meant all the resistors and capacitors
needed to make it function that normally go to ground. They would have to be
connected to the npn transistor's collector along with V- on the 556.

There's no reason you can't switch power to V+ with an npn emitter follower though.

Well, yes, there are several, albeit not why you "can't", but why you
_shouldn't_.

But they're not as bad, or as numerous, as the reasons why you shouldn't
open the ground pin. J D's idea of the reset pin is ideal. J D, drive the
reset pin with any logic output (except maybe ECL ;-) ) and that will make
the 556 (at least the sectiou whose reset pin you're manipulating) stop.

You do have to account for the stretched first half-cycle after you
release reset, of course.

Good Luck!
Rich

 

[Donald]

Thanks for answering, i meant when I turn the transistor off. I had a
suggestion from some other people to use the Rs, reset pin, instead of
the ground, because of internal groun connections, etc.
However I am having a hard time realizing how to use a transistor to
control the Rs pin. I could use an inverter connected to a PNP, but i
am looking for something more simpler.

With all the free pic web sites out there, why not post a schematic.

Then everyone will know exactly what you are trying to convey.

A pic is worth a thousand 'i meant'S.

 

[colin]

some models internally connect to gnd (node 0)

You could try making your own model, of course you would have to be carefull
if you used comparators or flip flops that they dont also use GND
internally.

maybe just copy the 555 transistor level model from the data sheet ?

Colin =^.^=