Ways to trigger a number of power transistors in sequence

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Could some some electronics guru provide hints/suggestions. I have to trigger 16  
power transistors in sequence, in round
robin fashion. I am currently running  
SPICE simulations on a scaled down version
consisting of 4 transistors. A pulse
propagates down a serial in parallel  
out shift register. Each output is  
tied, via a resistor to the base od
each transistor. So far, simulation  
results are encouraging.  

What are other ways of achieving  
the same goal ? For example, if a  
i-4 multiplexer is used, how would
one generate the enable sequence
00 -> 01 -> 10 -> 11 ? All suggestions
are welcome. Thanks in advance.

Re: Ways to trigger a number of power transistors in sequence
On 07/07/2017 07:10 AM, snipped-for-privacy@gmail.com wrote:
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Why not two cascaded decade counters with decoded outputs like the  
74HC4017, driven from a clock? The master reset is active high and  
asynchronous so just send outputs 0-15 to your transistor driver circuit  
and tie output 16 from the second chip in the cascade to the master  
reset of the first.


Re: Ways to trigger a number of power transistors in sequence
snipped-for-privacy@gmail.com wrote:
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A 4 bit counter and 16 channel mux like the 4067 is IMO the straight forward  
solution. Instead of a 4067 two 4051 will also do.  
If the power transistors are no MOSFETs or darlingtons you will need a
buffer for the base current for each power transistor.


--  
Dipl.-Inform(FH) Peter Heitzer, snipped-for-privacy@rz.uni-regensburg.de

Re: Ways to trigger a number of power transistors in sequence
On Fri, 07 Jul 2017 04:10:00 -0700, dakupoto wrote:

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(you did not say frequency or if you have an external oscillator)

Use a CD4060 and a CD4514.  The 4060 will give you the oscillator
But if this is for a long term production product I'd find a different
way.  A micro controller with 16 GPIO pins would do everything you need.
Or a little Microchip ATtiny25 with a 4 to 16 decoder.

--  
Chisolm
Republic of Texas


Re: Ways to trigger a number of power transistors in sequence
On 07/07/2017 12:26 PM, Joe Chisolm wrote:
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If the OP's collector current requirements are modest enough (they also  
didn't define "power") they should remember this IC where you get 8 NPN  
Darlingtons in a single package, suitably clamped for driving mechanical  
relays etc:

<http://www.ti.com/product/ULN2803A>


Re: Ways to trigger a number of power transistors in sequence
snipped-for-privacy@gmail.com wrote on 7/7/2017 7:10 AM:
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What you are asking is a bit like asking "how long is a piece of string?"  
Generating a sequence of pulses is a vague way to put it.  What happens if  
there is some overlap in the pulses?  What happens if there is no overlap in  
the pulses?  Can you tolerate glitches in outputs that are not pulsing at  
that point?  What is the time scale of the pulses?

Using a binary counter to drive a decoder can result in narrow spike pulses  
when the counter changes more than one output at a time.  For example when  
the counter changes from 3 to 0 the decoder can see 1 or 2 as a very brief  
intermediate state and generate a spike on that output.  There are several  
ways around that issue.  One is to use the shift register you started with.  
You still have the issue of not controlling the overlap of the pulses but  
you won't get spikes on the other outputs.

Another approach is to use a Gray code for the counter where only a single  
output bit changes on any count.  One form of Gray code is called a Johnson  
ring counter that is a shift register loop with an inverter in the loop.  
For example, with three bits you can generate six states; 000, 001, 011,  
111, 110, 100 and back to 000.  Decoding a state is a matter of detecting  
the edge of the transition between 0 and 1 using two input AND gates and  
inverters.  There are many example circuits about.

Still, none of this controls the timing of the adjacent pulse edges.  If you  
need to worry about the presence or absence of overlap of the pulse edges  
then you need to either use different circuits, or perhaps apply analog  
techniques to shape the pulse edges.

As others have indicated, a simple MCU can help with the issues of  
generating basic pulse sequences with or without overlap but may need to be  
augmented with analog circuits if you need fine pulse edge adjustments.

So how long a piece of string do you need?

--  

Rick C

Re: Ways to trigger a number of power transistors in sequence
On 07/07/2017 12:56 PM, rickman wrote:

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I don't think any modern synchronous binary counter will do the former.  
There can be other more subtle timing problems but I don't think  
synchronously-clocked transmission gate "D" flip-flops will have major  
trouble with transient indeterminate states like a ripple counter would.

There will surely be some residual overlap and indeterminacy even with a  
synchronous counter as nothing truly happens at exactly the same instant  
in time when there are different delays involved, but it will be from  
the propagation delays and rise/fall times which are in the 10s or 100s  
of nanoseconds max for even 30 y/o CMOS and gee whiz how fast does this  
thing need to be?

Re: Ways to trigger a number of power transistors in sequence
bitrex wrote on 7/7/2017 1:20 PM:
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Glitches is glitches...  You'll have to tell me what acceptable values are.  
How long is *your* piece of string?

--  

Rick C

Re: Ways to trigger a number of power transistors in sequence
A ring counter ?

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