Simple question on rectifier diodes

  • Thread starter C. E. Steuart Dewar
  • Start date
C

C. E. Steuart Dewar

Guest
I am building a small alarm box - there are four alarms which trip form
c contacts and I use them to pass 12vdc which powers lights on the alarm
box. The contacts are tripped by a 2.5ghz receiver (transmitter which
actually detects the alarms is about a mile away), but the contacts are
only briefly pulsed.

I latch the pulse with a relay (connect N/O back to +12 where the signal
comes in) and that all works nicely.

I then want to have a siren go off if any one of those alarms goes off.
I thought I could pass the +12 from any of the four relays on through 4
rectifier diodes then connecting them all to the siren, assuming that
this would prevent the current from one alarm passing back through the
diode to another alarm light. It almost seems to work except that my
latched relay shuts off - as if there is something unexpected happening
current-wise with the rectifier diode.

I could also solve this by just tying together 3 relays (2 relays with +
12 and COM for the 4 alarms, and then taking the N/O outputs into +12
and COM of a third relay), but I would like to understand why using
rectifier diodes does not work in this case as I had thought a rectifier
diode just provided zero resistance in one direction and basically
infinite resistance in the opposite direction but it seems in this case
to sink the +12 momentarily, shutting off the relay, or??

cesd
Gorilla Haven
Morganton, GA
 
C. E. Steuart Dewar wrote:

I am building a small alarm box - there are four alarms which trip form
c contacts and I use them to pass 12vdc which powers lights on the alarm
box. The contacts are tripped by a 2.5ghz receiver (transmitter which
actually detects the alarms is about a mile away), but the contacts are
only briefly pulsed.

I latch the pulse with a relay (connect N/O back to +12 where the signal
comes in) and that all works nicely.

I then want to have a siren go off if any one of those alarms goes off.
I thought I could pass the +12 from any of the four relays on through 4
rectifier diodes then connecting them all to the siren, assuming that
this would prevent the current from one alarm passing back through the
diode to another alarm light. It almost seems to work except that my
latched relay shuts off - as if there is something unexpected happening
current-wise with the rectifier diode.

I could also solve this by just tying together 3 relays (2 relays with +
12 and COM for the 4 alarms, and then taking the N/O outputs into +12
and COM of a third relay), but I would like to understand why using
rectifier diodes does not work in this case as I had thought a rectifier
diode just provided zero resistance in one direction and basically
infinite resistance in the opposite direction but it seems in this case
to sink the +12 momentarily, shutting off the relay, or??

cesd
Gorilla Haven
Morganton, GA
Click to expand...
The _diode_ can't sink your +12V, but maybe the siren is when it starts
up? It would have to be a pretty hefty siren, or a pretty wimpy 12V
supply, because relays generally take their time about releasing.

Here's a test: Latch one or more of your relays, then connect the siren
to your +12V directly (like with a pushbutton switch). If your relays
unlatch then your power supply is too wimpy.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
"C. E. Steuart Dewar" <cesd@gorilla-haven.org> wrote in message
news:MPG.1af83df133067c4a989806@news.tds.net...
I am building a small alarm box - there are four alarms which trip form
c contacts and I use them to pass 12vdc which powers lights on the alarm
box. The contacts are tripped by a 2.5ghz receiver (transmitter which
actually detects the alarms is about a mile away), but the contacts are
only briefly pulsed.

I latch the pulse with a relay (connect N/O back to +12 where the signal
comes in) and that all works nicely.

I then want to have a siren go off if any one of those alarms goes off.
I thought I could pass the +12 from any of the four relays on through 4
rectifier diodes then connecting them all to the siren, assuming that
this would prevent the current from one alarm passing back through the
diode to another alarm light. It almost seems to work except that my
latched relay shuts off - as if there is something unexpected happening
current-wise with the rectifier diode.

I could also solve this by just tying together 3 relays (2 relays with +
12 and COM for the 4 alarms, and then taking the N/O outputs into +12
and COM of a third relay), but I would like to understand why using
rectifier diodes does not work in this case as I had thought a rectifier
diode just provided zero resistance in one direction and basically
infinite resistance in the opposite direction but it seems in this case
to sink the +12 momentarily, shutting off the relay, or??

cesd
Gorilla Haven
Morganton, GA
Click to expand...
Is this an electronic siren with a loudspeaker, or a mechanical siren with
an electric moter (Do they still make these)? If the latter, it probably
takes heavy startup current and momentarily pulls the supply voltage down.
What might work is to power the relays through a fifth diode, and hang a BIG
electrolytic on it to keep the voltage up.

Tam
 
Hi

It could be that the siren is generating noise or flyback pules which are
reverse biasing the diodes and allowing signal to flow the opposite
direction.

Make sure the reverse breakdown voltage of the diodes is high or you can
also put a diode from the +12V to the siren with the cathode at +12. This
will clamp any spikes to 12.6V and stop any high voltage spikes turning on
the diodes in reverse.

Roland D


"C. E. Steuart Dewar" <cesd@gorilla-haven.org> wrote in message
news:MPG.1af83df133067c4a989806@news.tds.net...
I am building a small alarm box - there are four alarms which trip form
c contacts and I use them to pass 12vdc which powers lights on the alarm
box. The contacts are tripped by a 2.5ghz receiver (transmitter which
actually detects the alarms is about a mile away), but the contacts are
only briefly pulsed.

I latch the pulse with a relay (connect N/O back to +12 where the signal
comes in) and that all works nicely.

I then want to have a siren go off if any one of those alarms goes off.
I thought I could pass the +12 from any of the four relays on through 4
rectifier diodes then connecting them all to the siren, assuming that
this would prevent the current from one alarm passing back through the
diode to another alarm light. It almost seems to work except that my
latched relay shuts off - as if there is something unexpected happening
current-wise with the rectifier diode.

I could also solve this by just tying together 3 relays (2 relays with +
12 and COM for the 4 alarms, and then taking the N/O outputs into +12
and COM of a third relay), but I would like to understand why using
rectifier diodes does not work in this case as I had thought a rectifier
diode just provided zero resistance in one direction and basically
infinite resistance in the opposite direction but it seems in this case
to sink the +12 momentarily, shutting off the relay, or??

cesd
Gorilla Haven
Morganton, GA
Click to expand...
 
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