Chris said:
Hi All
I've been trying to construct a pressure regulating device, using a 12v
latching solenoid and conventional valve, to protect a filter in a home
irrigation system. I am using a pressure switch which goes contacts
open at high pressure/contacts close at falling pressure. These two
states in turn trigger 555 timers which in turn drive small 12v SPDT
relays.
The relays are configured so that in the relaxed state the circuit
connects to the 0v line and when exited by the pulse from the 555,
switches to supply 12v for each relay in turn. The purpose of this
switching configuration is so that the 12v latching solenoid, which
operates the valve, can be switched to close the valve when pressure
reaches a high threshold, but then by reversing the supply voltage, is
switched to open the valve at a low threshold.
This all works perfectly - until the latching solenoid is connected,
whereupon the action of the solenoid causes the relays to bounce
erratically.
I have tried various combinations of (non polarised) capacitors and R/C
combinations across the terminals of the solenoid, and/or across the
relay. I have also tried using seperate 12v power supplies to power
the two sides of the device (timing - relay activation side and power
supply for solenoid side)but the action of the solenoid still causes
the relays to bounce erretically.
I feel I am running out of options to solve this dilema, but am now
considering the use of switching transistors or solid state relays.
Only I'm not sure how to switch such devices on and off to give the
necessary pulse to switch the solenoid on then off etc.
Any help would be greatly appreciated
Hi, Chris. First off, I'm not sure you've got the right valve for the
job. The protection circuit you're using won't prevent filter damage
in the event of a power outage. I'd really recommend getting another
solenoid valve with a 12VDC coil rated for continuous duty. That would
make your control system much easier (view in fixed font or M$
Notepad):
` +12V COM
` | |
` | Normally Closed |
` | Pressure Switch |
` | PS1 |
` | ___ T |
` o--------UUU-----o----------o|o---------o
` | RY1 | -|- |
` | | |
` | | |
` o--------|<------' |
` | D |
` | |
` | RY1 |
` | || ___ |
` o--------||----------o------UUU---------o
` | || | SV1 |
` | | |
` | '------|<----------o
` | D2 |
` | |
`
`
` | |
` | Normally Closed |
` | Pressure Switch |
` | PS1 |
` | T ___ |
` o--------o|o---------o------UUU---------o
` | -|- | SV1 |
` | | |
` | '------|<----------o
` | D1 |
(created by AACircuit v1.28.5 beta 02/06/05
www.tech-chat.de)
The big advantage of this setup is that if the power goes off, the
valve closes, preventing overpressure and damage. Period. The top
circuit would be used if the pressure switch contacts aren't beefy
enough to drive the solenoid, and the bottom one is if they are.
Choose a diode big enough so that it can handle the current. If your
solenoid uses less than 4 amps or so, you could consider something like
a 1N5402. Bigger diodes are available. The power off business is a
big problem, and I think you should seriously consider replacing your
control valve.
But I get a feeling that you're casting about for a way to use the
existing valve, and swapping it out basically isn't going to happen.
Since that's the case, let's take a look at your existing control
circuit:
` +12V COM
` | |
` o----------. .----------o
` | | | |
` | | | |
` | | |/ |
` | NO --- --- NC |
` | ------- |
` | C|/ |
` | | |
` | C| |
` | C| |
` | C| |
` | | |
` | C| / |
` | ------- |
` | NO --- --- NC |
` | | /| |
` | | | |
` | | | |
` o----------' '----------o
` | |
` | |
(created by AACircuit v1.28.5 beta 02/06/05
www.tech-chat.de)
This is the driver circuit you've described for the latching solenoid.
Possibly you've got a magnetic or mechanical latch internal to the
valve. This is an intelligent way to drive a latching coil with other
relays -- you're OK there.
Unfortunately, things start falling apart elsewhere. You're saying
you've got one normally closed contact as the output of the pressure
sensor, and that's somehow triggering two 555s. Since the 555 set up
as a one shot triggers on a logic transition from "1" to "0", I'm not
sure how you get separate negative-going triggers for the two separate
555s with one normally closed pressure switch contact. Possibly your
pressure switch has a form C contact (normally open and normally closed
contacts)?
But you're also saying you're having problems with the ICs due to
electrical noise from the valve solenoid. This can be a bear to
troubleshoot, especially with massive inductive loads and/or poorly
regulated power supplies. Snubbers don't work too well with massive
inductive loads, either. Diodes across the load are preferrable, but
since you're driving the solenoid coil from either direction, that
won't do. You haven't described your system well enough to offer more
than general advice about 555s, which might or might not be of help.
But if you throw in another relay and diode, a couple of largish caps,
and a couple of bleeder resistors, you can have a complete control
system straight from the 1950s, when men were men, women were glad of
it, and relay logic ruled the earth (view in fixed font or M$ Notepad):
` +12V COM
` | Normally Closed |
` | Pressure Switch |
` | PS1 |
` | ___ T |
` o---------UUU-----o-------------o|o-----------o
` | RY1 | R1 -|- |
` | | ___ |
` o---------|<------' .-|___|-. |
` | D | | |
` | | C1 | CRY1 |
` | ___ | +|| | NC||/ |
` o---------UUU-----o-o---||--o---|| |
` | RY2 | || /|| |
` | | |------------o
` o---------|<------' ||C |
` | D .---|| |
` | C2 | NO|| |
` | ___ +|| | |
` o---------UUU-----o-----||--o |
` | RY3 | || | |
` | | R2 | |
` | | ___ | |
` o---------|<------o---|___|-' |
` | D |
` o-------------------------------------. |
` | CRY2 CRY3 | |
` | | |
` | NC||/ ||/NC | |
` o------------------|| ||----' |
` | /|| ___ /|| |
` | |--o-UUU-o--| |
` | ||C | SV1 | C|| |
` | .---|| | | ||------------o
` | | NO|| | | ||NO |
` | | | | |
` | '-------|-----|----------------o
` | 1N5402 (2 ea.) V V |
` | - -
` | | |
` o----------------------o ----'
` |
(created by AACircuit v1.28.5 beta 02/06/05
www.tech-chat.de)
Depending on the status of the pressure switch PS1 on power-up, RY1
will energize or not. Depending on it's status, the coil of RY2 or RY3
will energize for a brief period of time, depending on the coil
resistance and the value of cap you've chosen. This will provide a
momentary pulse to open or close the valve as in your circuit. When
PS1 changes state, RY1 will toggle, causing the other relay to turn on,
repeating the process. This gives you a complete open loop control
system that resets itself on power-on.
Component selection is dependent on the coil chosen for the relays.
Assuming you've got a 400mW coil (360 ohms at 12VDC), you need 1N4002
diodes or equivalent for diodes D. The value of caps C1 and C2 should
be chosen to get an appropriate time delay on the relay pulse. For a
400mW coil, you could use a 1000uF, 25V cap. That will give you a
pulse time of a couple of hundred milliseconds. If you want or need a
longer pulse, you can bump the cap up to 2200uF at 25V. R1 and R2 are
set to bleed off charge from the cap when the relay contact opens, and
work as a voltage divider when the contact is closed. A value of about
10X the coil resistance is a good starting value. That will give you
about 3.3K, 1/4W resistors. The thing is, it will take about 10
seconds for these resistors to bleed off voltage on the cap to 10% of
the 12V, which ensures relay turn-on the next time the contact closes.
But if you bump the cap up to 2200uF, recharge time will also more than
double. I'm assuming here that your system is slow enough so that at
least a 10 second recharge time on the pulse timers is OK. If you've
got a fast system, you may have to rethink this. By the way, note the
two beefy diodes at the bottom of the ladder. It costs an extra diode,
but you can snub a bidirectional inductive load with diodes. It's
actually done with H-bridge drivers and motor drivers all the time.
Note also that, if your solenoid is more than 3 or 4 amps, you'll have
to choose bigger diodes. And if your solenoid is less than half an
amp, go with 1N4002s.
There is also a bit of a glitch in this system. If power-up takes
place with PS1 closed, RY2 will energize, and CRY2 will make for a
couple of milliseconds, until RY3 kicks in. This is a one shot event
on power-up, though, and shouldn't be a problem. It usually takes a
lot longer for a big solenoid to energize than a small control relay,
so it probably won't affect the valve at all. So it shouldn't be a
problem for you.
Again, this won't really help you in the event of a power outage. But
there are a number of things you can do about this. The simplest would
be putting a small lead-acid battery in parallel with this setup. Your
system uses less than a watt when the solenoid isn't being changed, and
since the pulses to the valve solenoid are brief, the battery should be
able to handle the load for a long time. If that's not good enough,
you can add another relay or two to disable the circuit on power-off,
and shut down the system on battery power. Please post again if this
is of interest.
Hope this has been of help. ;-)
Good luck
Chris