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'[EE] RC Filter ? - Square Wave To flat DC'
2011\05\30@182858 by Jason White

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Hello All. I'm currently working on a project that requires me to use
2 1:16 multiplexers to interface with various peripherals due to
limited I/O. I'm curious how much (RC?) filtering would be required to
make a short fast pulse (eg. 1-10khz 1-5% duty cycle) a suitable
signal to drive a relay.

Thanks

relay: odd Chinese 20A H-Bridge relay on hooked up via opto-coupler,
No other specs
multiplexers: CD74HC4067M
MCU - 16f690


-- Jason White - Python/C++ Programmer and Electronics Hobbyis

2011\05\30@194732 by Dwayne Reid

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At 04:28 PM 5/30/2011, Jason White wrote:
>Hello All. I'm currently working on a project that requires me to use
>2 1:16 multiplexers to interface with various peripherals due to
>limited I/O. I'm curious how much (RC?) filtering would be required to
>make a short fast pulse (eg. 1-10khz 1-5% duty cycle) a suitable
>signal to drive a relay.
>
>Thanks
>
>relay: odd Chinese 20A H-Bridge relay on hooked up via opto-coupler,
>No other specs
>multiplexers: CD74HC4067M
>MCU - 16f690

Use a signal diode feeding a parallel RC network (one end of RC grounded, the other end to the cathode of the signal diode).  The junction of the diode / R / C then feeds a CMOS or MOSFET input of some sort, depending upon what you want to do.

If this is just driving the relay coil, a small MOSFET (2n7000 or similar) is fine.  Gate to the D / R / C network, source to gnd, drain to the bottom side of the relay coil, top side of the relay coil goes to your relay supply (+12V?).  Don't forget the diode across the relay coil.

Choose R to be somewhere between 10k - 100k, then pick C such that the voltage across C remains well above the turn-on voltage of the mosfet while you are generating pulses but decays quick enough so that the relay turns OFF quickly.  R * C should be anywhere from 3x to 5x longer than the period between your pulses.

As you can tell, most of this can be pretty loosey-goosey.  Use a scope to monitor the voltage at the gate of the mosfet if you are concerned..

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\05\30@200816 by Robert Rolf

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Jason White wrote:

{Quote hidden}

Use a diode plus capacitor to create a peak detector. Use a FET as switch to the opto. Resistor to ground determines time before FET turns off. High impedance of gate lets you use small cap values.
It WILL have a slow turn off which may cook the opto.
In which case use a 74HC14 schmitt trigger to square it up (no FET needed). (available as a single channel SMT part SN74LVC1G17DCKR & similar)

Or use something like a 74HC123 retriggerable monostable with a timeout longer than your refresh rate. (also available as a single channel SMT part).

Robert

2011\05\30@211237 by Brent Brown

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> Jason White wrote:
> Hello All. I'm currently working on a project that requires me to use
> 2 1:16 multiplexers to interface with various peripherals due to
> limited I/O. I'm curious how much (RC?) filtering would be required to
> make a short fast pulse (eg. 1-10khz 1-5% duty cycle) a suitable
> signal to drive a relay.
> Thanks
>
> relay: odd Chinese 20A H-Bridge relay on hooked up via opto-coupler, No
> other specs multiplexers: CD74HC4067M MCU - 16f690

Just a suggestion, you might get a lower part count/space/cost solution if you instead look at cascaded serial shift registers (74HC165, 74HC595 etc) for output expansion. Then you won't need R,C & possibly no FET either. I'm sure there are examples at piclist.com but I can't find any right now.
-- Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  .....brent.brownKILLspamspam@spam@clear.net.nz

2011\05\30@215017 by Harold Hallikainen

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>> Jason White wrote:
>> Hello All. I'm currently working on a project that requires me to use
>> 2 1:16 multiplexers to interface with various peripherals due to
>> limited I/O. I'm curious how much (RC?) filtering would be required to
>> make a short fast pulse (eg. 1-10khz 1-5% duty cycle) a suitable
>> signal to drive a relay.
>> Thanks
>>
>> relay: odd Chinese 20A H-Bridge relay on hooked up via opto-coupler, No
>> other specs multiplexers: CD74HC4067M MCU - 16f690
>
> Just a suggestion, you might get a lower part count/space/cost solution if
> you
> instead look at cascaded serial shift registers (74HC165, 74HC595 etc) for
> output
> expansion. Then you won't need R,C & possibly no FET either. I'm sure
> there are
> examples at piclist.com but I can't find any right now.

I agree. There are parts that Allegro used to make (6832 and similar) that
have a serial in, parallel out shift register plus a parallel latch plus
high current drivers. You shift the data out, latch it, and the output
fets are updated. TI makes similar 8 bit parts, but I can't find the part
number right now.

Harold


-- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available

2011\05\30@215843 by Brent Brown

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{Quote hidden}

Yes, I use TI's TPIC6A595. Serial operation as per standard 74xx595, eight low side MOSFET outputs, 350mA each with short circuit protection and voltage clamp.... so you can get away with no diodes across your relays.

-- Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brownspamKILLspamclear.net.nz

2011\05\31@070954 by Jason White

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Thanks for the responses, Ill look into the shift registers.

On Mon, May 30, 2011 at 9:58 PM, Brent Brown <.....brent.brownKILLspamspam.....clear.net.nz> wrote:
{Quote hidden}

>

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