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Best method to sync large amount of current from 555 in astable mode?

M

Michael Noone

Jan 1, 1970
0
Hi - I need to drive a fairly power hungry circuit (I think a couple
hundred ma, but I'm not positive just yet on that) from the output of a 555
in astable mode. My first thought was a darlington transistor or a mosfet.
Does that seem sensible? I don't know what speed it will be operating at
just yet (haven't been told), but I get the feeling it should be fairly
fast, probabaly in the MHz range, and I'm not sure if high power
darlingtons or fets can operate that fast. So, I was wondering if somebody
here might be able to lend me a little guidance? A part number would be
great, as I have to admit I'm slightly new to electronics (I'm a second
year EE at UIUC)

Thanks!

-Michael Noone
 
J

John Fields

Jan 1, 1970
0
Hi - I need to drive a fairly power hungry circuit (I think a couple
hundred ma, but I'm not positive just yet on that) from the output of a 555
in astable mode. My first thought was a darlington transistor or a mosfet.
Does that seem sensible? I don't know what speed it will be operating at
just yet (haven't been told), but I get the feeling it should be fairly
fast, probabaly in the MHz range, and I'm not sure if high power
darlingtons or fets can operate that fast. So, I was wondering if somebody
here might be able to lend me a little guidance? A part number would be
great, as I have to admit I'm slightly new to electronics (I'm a second
year EE at UIUC)

---
It would probably be better if you got all your ducks in a row before
you start looking for specific hardware but, in general, a 555 is not
a good oscillator for what you want since it's rated at 500kHz fmax as
an astable. Also, "in the MHz range" is pretty broad. Can you get a
little closer?
 
M

Michael Noone

Jan 1, 1970
0
---
It would probably be better if you got all your ducks in a row before
you start looking for specific hardware but, in general, a 555 is not
a good oscillator for what you want since it's rated at 500kHz fmax as
an astable. Also, "in the MHz range" is pretty broad. Can you get a
little closer?

Damn - I did not know about that limitation of the 555. I may be
entirely incorrect about that speed, as I said it was just a guess. I
was given very vague specs on Friday for it. I'll try to find out more
specific information tomorrow. For higher speeds, what would I use in
place of an 555? The first thing that comes to mind is a
microcontroller, say an AVR or a PIC. Sorry for being so vague, I
promise I'll get better information as soon as I can. Thanks,

-Michael
 
J

Joerg

Jan 1, 1970
0
Hi Michael,

Usually I take 1/6th of a Hex Schmitt trigger and use it as an astable.
The data sheet says how that is done. For PWM control a FET, transistor
or whatever can be hung from the input to ground to steer the pulse
width. A CD40106 may be best here because you can run it at 10V or so
which provide enough drive for a FET.

Of course this is only useful if it is a loop structure. A Schmitt is
not suitable for setting exact pulse widths. If you have a micro
controller on board anyway you can always create a loop to set the pulse
width.

Regards, Joerg
 
J

Joerg

Jan 1, 1970
0
Hi Michael,

Forgot to mention: What I meant were clean CMOS versions of Schmitt inverters such as the 74HC14 or CD40106.

Regards, Joerg
 
T

Tim Wescott

Jan 1, 1970
0
Michael said:
Damn - I did not know about that limitation of the 555. I may be
entirely incorrect about that speed, as I said it was just a guess. I
was given very vague specs on Friday for it. I'll try to find out more
specific information tomorrow. For higher speeds, what would I use in
place of an 555? The first thing that comes to mind is a
microcontroller, say an AVR or a PIC. Sorry for being so vague, I
promise I'll get better information as soon as I can. Thanks,

-Michael

What are you trying to do?

You may be able to use one of the gajillion different PWM regulators out
there -- at considerably less than an amp you may be able to use an
on-board pass transistor. If you're not actually doing PWM regulation
then look at the data sheets and view the chips as a collection of
interesting blocks to see if they'll do what you want.

This suggestion, of course, depends _heavily_ on what you're trying to
do, so if you let us know we will have a better chance of helping you.
 
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