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Delaying the saturation of a transistor

M

Mr. J D

Jan 1, 1970
0
Ok I am looking on how to delay the turn on of a transistor, or any
other component. The precision of the timing circuit doesnt matter, it
just needs to slow down the saturation of the transistor for safety
purposes; to allow a digital component (a Tinylogic XOR Gate to be
precise) to activate before the transistor does.
If I were to use a resistor in series with a capacitor attached to
ground, and the base of the transistor attached to the resistor, It
will not only delay the turn on, but it the saturation of the
transistor will constantly be going up and down. This is what I want to
avoid, the transistor saturation going up and down. I just want it to
delay and then hold at a specific saturation.
 
E

Eeyore

Jan 1, 1970
0
Mr. J D said:
Ok I am looking on how to delay the turn on of a transistor, or any
other component. The precision of the timing circuit doesnt matter, it
just needs to slow down the saturation of the transistor for safety
purposes; to allow a digital component (a Tinylogic XOR Gate to be
precise) to activate before the transistor does.
If I were to use a resistor in series with a capacitor attached to
ground, and the base of the transistor attached to the resistor, It
will not only delay the turn on, but it the saturation of the
transistor will constantly be going up and down. This is what I want to
avoid, the transistor saturation going up and down. I just want it to
delay and then hold at a specific saturation.

I don't think you means saturation. Why do you ?

Graham
 
S

Stanislaw Flatto

Jan 1, 1970
0
Mr. J D said:
Ok I am looking on how to delay the turn on of a transistor, or any
other component. The precision of the timing circuit doesnt matter, it
just needs to slow down the saturation of the transistor for safety
purposes; to allow a digital component (a Tinylogic XOR Gate to be
precise) to activate before the transistor does.
If I were to use a resistor in series with a capacitor attached to
ground, and the base of the transistor attached to the resistor, It
will not only delay the turn on, but it the saturation of the
transistor will constantly be going up and down. This is what I want to
avoid, the transistor saturation going up and down. I just want it to
delay and then hold at a specific saturation.
But what is the minimal delay required?
If it is in miliseconds or parts of it then a 5V relay shorting the base
to ground and activating on the power rail would create such delay.
Reed relays may be too fast and mechanical relays may draw too much
current, so give us some info as to what are the limits in which to move.

Have fun

Stanislaw
Slack user from Ulladulla.
 
J

John Woodgate

Jan 1, 1970
0
dated Sat said:
I don't think you means saturation. Why do you ?
Mr. J D may not be a native English speaker. He means 'collector
current'.
 
E

Eeyore

Jan 1, 1970
0
John said:
Mr. J D may not be a native English speaker. He means 'collector
current'.

I suspect you're on the right track but if so why doesn't he want the collector
current to go up and down ?

Maybe he wants a delayed swich on with a 'snap' action ? In which case a
comparator or dedicated reset type circuit would be appropriate.

Perhaps he can advise.

Graham
 
T

Tam/WB2TT

Jan 1, 1970
0
Mr. J D said:
Ok I am looking on how to delay the turn on of a transistor, or any
other component. The precision of the timing circuit doesnt matter, it
just needs to slow down the saturation of the transistor for safety
purposes; to allow a digital component (a Tinylogic XOR Gate to be
precise) to activate before the transistor does.
If I were to use a resistor in series with a capacitor attached to
ground, and the base of the transistor attached to the resistor, It
will not only delay the turn on, but it the saturation of the
transistor will constantly be going up and down. This is what I want to
avoid, the transistor saturation going up and down. I just want it to
delay and then hold at a specific saturation.
You can prevent the transistor from going into deep saturation by connecting
a diode with low voltage drop from base to collector; cathode to the
collector. You can trim that even more by adding a low value resistor
between directly in series with the base of the transistor. The ON voltage
of the transistor will be slightly raised.

Tam
 
E

Eeyore

Jan 1, 1970
0
Tam/WB2TT said:
You can prevent the transistor from going into deep saturation by connecting
a diode with low voltage drop from base to collector; cathode to the
collector. You can trim that even more by adding a low value resistor
between directly in series with the base of the transistor. The ON voltage
of the transistor will be slightly raised.

Baker Clamp !

Graham
 
F

Fred Bloggs

Jan 1, 1970
0
Ok I am looking on how to delay the turn on of a transistor, or any
other component. The precision of the timing circuit doesnt matter, it
just needs to slow down the saturation of the transistor for safety
purposes; to allow a digital component (a Tinylogic XOR Gate to be
precise) to activate before the transistor does.
If I were to use a resistor in series with a capacitor attached to
ground, and the base of the transistor attached to the resistor, It
will not only delay the turn on, but it the saturation of the
transistor will constantly be going up and down. This is what I want to
avoid, the transistor saturation going up and down. I just want it to
delay and then hold at a specific saturation.

At least you recognized that wrapping that delay into the single
transistor will also slow the rise/fall times. The R+C method shunts the
high frequency transistor drive and slows it down. The only way to
prevent this is to Schmitt trigger the transistor input, with either a
tiny logic Schmitt using an RC on its input, or another transistor. A
tiny logic Schmitt is probably the cheapest, most compact, and most
consistent performer of your choices. Of course, this answer is given in
isolation and without knowledge of your specific circuit. If "safety" is
critical, the *best* way is to use the XOR output as an input to
whatever eventually drives the transistor. This may get complicated with
an XOR and I won't waste time on conjecture without additional information.
 
M

Mr. J D

Jan 1, 1970
0
Fred said:
At least you recognized that wrapping that delay into the single
transistor will also slow the rise/fall times. The R+C method shunts the
high frequency transistor drive and slows it down. The only way to
prevent this is to Schmitt trigger the transistor input, with either a
tiny logic Schmitt using an RC on its input, or another transistor. A
tiny logic Schmitt is probably the cheapest, most compact, and most
consistent performer of your choices. Of course, this answer is given in
isolation and without knowledge of your specific circuit. If "safety" is
critical, the *best* way is to use the XOR output as an input to
whatever eventually drives the transistor. This may get complicated with
an XOR and I won't waste time on conjecture without additional information.

Thanks for all the replies. I am a native english speaker. To clear up
what I meant: I only need a few milliseconds of delay time, and then
stay at the peak. Like a capacitor but with only one rise, and no fall.
Someone mentioned using a diode to do this, can you explain more in
detail?
 
J

John Woodgate

Jan 1, 1970
0
dated said:
Thanks for all the replies. I am a native english speaker.

Apologies. The use of the word 'saturation' instead of 'collector
current' has echoes of French terminology.
 
J

Jim Thompson

Jan 1, 1970
0
Thanks for all the replies. I am a native english speaker. To clear up
what I meant: I only need a few milliseconds of delay time, and then
stay at the peak. Like a capacitor but with only one rise, and no fall.
Someone mentioned using a diode to do this, can you explain more in
detail?

You could do it logically. See...

http://analog-innovations.com/SED/CrudeDelay.pdf

For your case just use the "TAU" output and drop the XOR portion.

...Jim Thompson
 
E

Eeyore

Jan 1, 1970
0
Mr. J D said:
Jim Thompson wrote:

Thanks for the schematic. I don't want to do it logically, as it
require an excess of parts, I have very limited space. I was almost out
of space when I decided to add an XOR gate.

Can't you use a gate with schmitt inputs ?

Graham
 
F

Fred Bloggs

Jan 1, 1970
0
Mr. J D said:
Thanks for all the replies. I am a native english speaker. To clear up
what I meant: I only need a few milliseconds of delay time, and then
stay at the peak. Like a capacitor but with only one rise, and no fall.
Someone mentioned using a diode to do this, can you explain more in
detail?

The diode suggestion has to do with limiting the excess minority carrier
charge in the transistor base by shunting base drive current around the
base and directly into the collector at small BC forward bias voltage,
which is why a Schottky diode is best and sometimes this combination is
called a Schottky transistor. It is a speedy switch because the storage
time, or roughly delay in time after removal of base drive until
collector current begins to fall , is a function of the excess minority
carrier base charge. This has little to do with your problem. You're not
going to beat a tiny logic Schmitt trigger used to drive the transistor:
View in a fixed-width font such as Courier.

..
..
..
.. dual .---> OUT
.. schmitt |
.. .--|<|--. |
.. | | |\ |\ |/
.. IN >--+--[R]--+----| o--| o-[Rb]-|
.. | |/ |/ |>
.. === | | |
.. | C | | |
.. gnd1---------+-----+----' ---
.. gnd2
..
..
.. RxC~ 1.5ms
..
..
..
..
 
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