555 Output Driver Revisited

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
I have shown this schematic once before, but it has been almost 10 years ago. This will create a true Rail to rail output for a standard 555, It allowed me to practice my schematic drawing skills, and I was bored.

Why Rail to Rail?

and what is the big deal about it? When driving a MosFet, you really need a rail to rail signal. A CMOS 555 comes with this feature, but a standard 555 has a very soft upper voltage due to the Darlington high side drivers, It might work as is. it is the might part of the previous statement this circuit addresses. a stock 555 will drive up to 200ma. I can foresee circuits where you need more current drive, as in driving a speaker. This circuit, becomes a non inverting buffer, as the transistors both invert the signal. I consider this a negative, but ce'st la vie. If you want even more drive and an inverted signal you can always add another two transistors.

...............................555 Output Drivers.png
 

DickCappels

Joined Aug 21, 2008
6,012
I like this circuit. It is important to note that if the NE555 is removed both of output transistors will be on with predictable consequences.
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
A valid point. The LEDs might have some say in that though, until one or both fail due to PIV, which they are not equipped to handle.They are there to drop 3.5 Volts, and dramatically reduce switch over surge on Q1 and Q2..Time to update the drawing...

..............................................555 Output Drivers.png

Sorry, but that circuit is an inverting buffer. ;)
When the 555 output is high, the NPN is on, pulling the output low, and vice-versa.
I suspect we are arguing semantics. U1 is an invertor, and Q1, and Q2 is also an invertor. The 2 together create a non inverting gate with up to a 1A drive (assuming the transistors can handle it). The LEDiodes are the weak link, I tried to limit current down to 20ma. in them. A 2N2222 and a 2N2906 (my go to transistors) have a minimum β of 50 and a max current of 600ma. If you need more drive you can always go with 2 darlingtons (but then, there goes the Rail to Rail). There probably are better transistors with higher gain and current specs that can be used instead.
 

Audioguru

Joined Dec 20, 2007
11,251
Beta is used when a transistor is linear amplifier with plenty of collector to emitter voltage (10V for a 2N2906) and is not saturated. Rail-to-rail is when the transistors are saturated, not as linear amplifiers. When saturated the normal for a little transistor is a base current that is 1/10th the collector current.
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
The 1/10 rule is only a guideline for an unknown transistor. Transistor families come with a minimum and maximum beta. The only p/n that comes close to a beta of 10 is 2N3055, which has a nominal beta of 15. and is a high wattage type in a TO3 case. We can control a circuits limitations by selecting Transistor families w/ known specs. Both transistors are getting approx 20ma (Q2 slightly less due to the 555 Darlington drop, but can easily be compensated for by adjusting R2 dow in value (around 10Ω or so at 4.5 Volts and 360Ω at 12VDC).It is true at 4.5 is a bit starved, using the extreme low power supply voltage brings this out. At 5.5V the indeterminate conditions go away, yo can also replace DS2 with small signal silicon diodes and the circuit will straighten up.
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
At 4.5VDC the 555 will output.3.2V replaceDS2 with a silicon device. Calculating R1:
R1=4.5v(PS)-.6(DS2)-.6(Q2 BE)/.02 (target current in Amps)≅100Ω

At this point the transistor is saturated. The Leds were only a kludge, and not that core to the design.
 

crutschow

Joined Mar 14, 2008
23,808
I suspect we are arguing semantics. U1 is an invertor,
Okay, I didn't read your post carefully. :oops:
I didn't realize you were using the 555 as an inverter and not a timer.
But why have the 555 if it's not being used as a timer? :confused:
The 1/10 rule is only a guideline for an unknown transistor. Transistor families come with a minimum and maximum beta.
True about the beta value.
But that beta is defined with a collector-emitter voltage of several volts, not when it is saturated.
You don't use that value of beta to saturate the transistor.
If you look at the saturation voltage spec for most standard BJT's, it's done at a forced beta of 10.
For most transistors you could probably used a forced beta of 20 for good saturation, but I wouldn't go higher than that.
 
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Audioguru

Joined Dec 20, 2007
11,251
The curves in a datasheet are for a "typical device" that you cannot buy. You get whatever they have which can have minimum beta.
The datasheets for most little transistors guarantee a maximum saturation voltage loss when the base current is 1/10th the collector current.
Don't you want all of your circuits to work properly even the many that have transistors with minimum beta?
 
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