SSR Relay stuck in active position
SSR Relay stuck in active position
(OP)
Before I begin, I am completely out of my element in this project, so my apologies if I don't use correct terminology.
I am trying to control a microliter dispensing valve using a relay control circuit. This circuit needs to be fast, as I need the droplet sizes to be very small. My digital IO controller outputs a 5V TTL signal, which used to drive a 5V reed relay (with a 51V zener diode parallel to the relay for protection). The common pin of the relay is connected to a +13.8VDC 3A power supply, and the normally open pin is connected to the positive end of the dispensing valve. The negative end of the valve connects to the negative end of the power supply. The valve is supposed to be driven by 12V, with a threshold current of 0.209 mA.
With the reed relay, I was able to get a decent dot size of around 5 mm, but this valve is supposed to be able to make dot sizes smaller than that. Therefore, I switched over to a solid state relay (input is 5V; output is 120VAC 1.6A) in an attempt to possibly make the circuit respond faster.
However, following the same wiring scheme as with the reed relay, my results were not what I expected. The relay closes with no problem. However, once the relay is in the closed position, it appears to be stuck in the closed position even when the 5V input is cut off. If I remove the valve and just put a voltmeter to see what's going on, the circuit acts as expected. What's going on?
Did I choose the wrong kind of SSR relay, or is there something wrong with my wiring or choice of power source?
I am trying to control a microliter dispensing valve using a relay control circuit. This circuit needs to be fast, as I need the droplet sizes to be very small. My digital IO controller outputs a 5V TTL signal, which used to drive a 5V reed relay (with a 51V zener diode parallel to the relay for protection). The common pin of the relay is connected to a +13.8VDC 3A power supply, and the normally open pin is connected to the positive end of the dispensing valve. The negative end of the valve connects to the negative end of the power supply. The valve is supposed to be driven by 12V, with a threshold current of 0.209 mA.
With the reed relay, I was able to get a decent dot size of around 5 mm, but this valve is supposed to be able to make dot sizes smaller than that. Therefore, I switched over to a solid state relay (input is 5V; output is 120VAC 1.6A) in an attempt to possibly make the circuit respond faster.
However, following the same wiring scheme as with the reed relay, my results were not what I expected. The relay closes with no problem. However, once the relay is in the closed position, it appears to be stuck in the closed position even when the 5V input is cut off. If I remove the valve and just put a voltmeter to see what's going on, the circuit acts as expected. What's going on?
Did I choose the wrong kind of SSR relay, or is there something wrong with my wiring or choice of power source?





RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
But given my current setup, is there anything I can do to limit the current then once the output signal is shut off? Or should I look for DC type solid state relays instead?
RE: SSR Relay stuck in active position
If you are working with 60 cycle AC, the quickest you may be able to switch a SSR is 1/120 sec. If 50 cycle then 1/100 sec.
Since you've already tried swithing the solenoid with DC (using a reed relay), instead I suggest you use your DC solenoid and switch it on and off with a MOSFET or transistor. You can find logic level input MOSFETS which will work directly from your 5V TTL signal.
Switching DC on/off at the ground side is simple with a N-channel MOSFET or NPN transistor. With a N-channel MOSFET drain or NPN transistor collector, you connect one end of your solenoid to your positive DC supply, and the other end to the MOSFET source or transistor emitter which switches to ground. Control a transistor with a resistor in series with the base (about 1K ohms). For a MOSFET, the TTL control signal goes to the gate. A resistor in series with the gate is usually not required with a MOSFET but you can have a resistor - in otherwords a N-MOSFET or NPN transistor are interchanable in such a switch circuit even though the name of the pins are different.
If you must switch on the high (positive) side then you can use a P-channel mosfet or PNP transistor. Logic level translation to drive this arrangement takes a little more consideration but you should be able to find simple circuits with a web search. P-MOSFETS or PNP transistors are not as fast in switching, but the difference for such a circuit is in the microseconds.
A diode should be place across the solenoid coil to provide a path for the flyback from the coil as the device switches to the off state. Cathode of the diode to the positive connected solenoid side. This is the same principal as the 51V zener you used previously.
RE: SSR Relay stuck in active position
I just did some research on mosfets, and it does appear that I can use them to switch my circuit. I'm a bit confused on N-type and P-type mosfets. Since my driver outputs 5V to activate the valve, wouldn't I actually want an N-type mosfet? And are most mosfet gates tripped with voltages ranging 2-7 volts? I tried looking on an electronic component site for a mosfet I can use, and they don't list the gate voltage, only voltage and current ratings.
If that is correct, then would this picture be the way I should wire my circuit?
RE: SSR Relay stuck in active position
Yes, just use an N-Type mosfet as you have it.
You should use a mosfet that is called a "logic" mosfet. They are specifically designed to be turned more fully on by 5V then typical fets that need about 7 to 10V.
Go to International Rectifier. They are the pros in that field and they have a good site.
http://www.irf.com/product-info/hexfet/
Pick one with about 55V or more.
http://ec
Go to the BOTTOM where there is a RDS of 4.5V. Pick any of these.
http:
This might work, you never said what the current of the solenoid was.
RE: SSR Relay stuck in active position
The threshold current for my valve to be about .209 A. The continuous voltage (5VDC) and the coil resistance 24 Ohms were used to come up with that number. Here is a PDF of the drawing for the valve: h
RE: SSR Relay stuck in active position
The coil when un-energized will cause a large inductive voltage kick that will be a polarity that is opposite of the driven polarity. You want none of this to get to the fet. But due to the Dirac function (Straight up) if you tried to just block it, the voltage would sky rocket (hundreds of volts) probably breaking something.
So the solution is to put a standard diode in parallel with the coil and reversed to the normal flow direction. It will then in effect short the previously mentioned voltage spike thru the inductor/coil itself. The resistance that is inherent in the copper winding of the coil will dissipate this shorting current without damage.
You can probably use a regular "rectifier" type diode. But I would use a faster one. Something like:
http://www.diodes.com/datasheets/ds26002.pdf
The fet I mentioned b4, is probably over kill but if you are just making one or two units it will take some abuse and be easy to handle. So I'd still use it.
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
Are you saying the circuit I drew will not provide enough current to the valve? Because I just did some calculations and, depending if I did them right, it does seem that I'd only be providing a 13mA current, when the valve needs 209 mA (why am I not getting .209mA as you stated? Isn't V=5V, R=24ohm, therefore i=V/R in amps?). If that is the case, would I just have to use a smaller resistor instead of 1k? But don't I need that 1k resistor?
Again, thank you for the help in this very confusing matter.
RE: SSR Relay stuck in active position
As for the current. He's probably confused by my accidental inclusion of a decimal point. .209 = 209mA I switched in the middle of the stream because typically currents under an Amp, EE's switch to mA.
I was confused by your 209mA.
I get that with V/R=I 5/24 = 208mA
Your circuit I'm looking at above is 13.8V.
13.8/24 = 0.575mA
or 13.8/(24+1k) = 13.5mA
If you need 208mA but want to use 13.8V then you need a different resistor.
V = I X R; V/I = R; 13.8/208mA = 66.5Ohms
66.5 Ohms - 24 Ohms = 42.5 Ohms
42.5 is what you need.
Don't forget that: Power = I x I x R.
Power = 208mA x 208mA x 42.5
Power = 1.8 Watts.
You should stay below about 1/2 the resistors rating for reliability... You need a 3 or 4 watt resistor. (Big)
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
scary. Not that FET's are inappropriate, but they DO need
to be handled a bit differently than BJT's. If you do use
an FET, make sure you add a gate resistor to one of the
rails, or add some sort of gate driver. There are numerous
examples of such at: http://people.txucom.net/fsmyth/pages/mosfet2.html
Root around. There are plenty more examples.
I won't post the simpler relay drivers (probably gazillions
available to a search engine, but here are some that solved
a particular need (all level-shifters with TTL inputs:
http://people.txucom.net/fsmyth/driver-2.gif
and one with current control:
http://people.txucom.net/fsmyth/ccdrive1.gif
All are probably overkill for driving a small relay, but
should give you an idea of the components and some general
values. A single 2N2222, 2N3904 or similar small-signal
general-purpose transistor would probably suffice for your
purpose.
<als>
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
As far as diode vrs zener, it doesn't really sound like switching speed is all that critial. Therefore, it would be much cheaper to use a sloppy old 1V4001 type diode to disapate the energy from the coil. Remember, size your diode approriately based on the amount of current the is flowing through the coil when energized (this will be the maximum amount of current that can be instaneously supplied by the inductior when the voltage drops out).
RE: SSR Relay stuck in active position
The back emf caused by interrupting current thru an inductor, that would seriously abuse the fet, is now going to be handled by the zener. The zener voltage should be about half the rating of the fet for its safety. According to Ohouse I guess that stored energy can be more quickly consumed by the zener and the power supply than by recirculating it thru only the coil.
Something we've all left out is that as your circuit sits you could turn on the solenoid and leave it on indefinitely as long as you have that resistor wattage correct.
However if you have a duty cycle over a period of a second you can mess with the resistor's resistance value and power rating.
For instance if you are allowed 208mA continuously then if you are never ON more than say, half the time, 50% duty cycle, then you could probably decrease the resistor value for quicker opening. Or conversely, use a lower power resistor because the average power thru it will be less.
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
Is this your valve? Is the 500 ohm resistor part of the
relay? If so, then this will go a long way into explaining
the relatively slow actuation. It also is probably why they
show a 36v supply. It may be that it is only representation
of the coil's resistance, and not a discrete resistor.
The circuit shown is pretty standard, but I question the
usefulness of a 110v zener. If you are intending to use
one of the more commonly available TTL-compatible MOSFET's,
the maximum voltages are more in the 50v neighborhood.
If you do use a MOSFET, you can remove the diodes across the
relay, which in extreme cases can slow the release, and do
very little to protect the driver, as shown.
If you do, in some cases you can rely on the body diode in
the MOSFET to protect it, but it would not be a bad idea to
add an ultra-fast diode from drain to source.
Stick with the transistor shown, if possible. It has plenty
of speed, a 300v maximum, and should be easy to find.
If you find that you do not have enough base drive available
it can be replaced with a Darlington equivalent.
In either case, the higher supply voltage is required to
enable faster turn-on.
<als>
RE: SSR Relay stuck in active position
I'm a bit wary of initially wiring my circuit without the protection of diodes, so I will try it with them connected first. If the actuation is still not as fast as I want, I will try locating a higher voltage power source.
I think I finally have a grasp of what is going on, and therefore have confidence to wire my circuit (once my components arrive). Thank you all very much for your help! I definitely would not have been able to figure this out by myself.
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
Like dot matrix print heads with inertial rebound, etc.
RE: SSR Relay stuck in active position
RE: SSR Relay stuck in active position
I had at one time experimented with using the solenoids from
an old high-speed printer (the assembly was about fist-sized) to drive a firing pin for remote-operated
shotgun. Progressed into using the voice-coil head actuator
from an old hard disk drive. But that's another story.....
joz, I finally got around to looking at your link. Should
have done that long ago, but since you descibed it as a
drawing, it skipped it. My bad. It distinctly lists it as
having a maximum operating frequency of 600 Hz. Based on
that, and the fact that max. continuous is about a watt,
the D.C. resistance is 24 ohms, and the impedance listed as
4100 ohms, with a working voltage of 4.3v, I would tend to
treat it more as a large reed relay or one winding of
a tiny stepper. The inertial mass of the armature in the
solenoid will be your limiting factor; all the drawings
listed as examples, and the functional ones you posted,
are quite a bit faster than the solenoid will ever be.
Also, over-driving this is likely to be a bit twitchy;
the high impedance suggests that there will be a significant
reflex voltage. Protect your drivers. Monitor valve heat.
Use current control, if possible. It may in fact be
necessary to use a half-bridge driver to control on AND
off timing. Didn't cite any of those, but I got plenty
of examples. Some even work properly. :)
<als>
As a P.S., you might look into electrostatic control of
your droplets. Not a topic I feel qualified to comment on,
but is apparently the answer to really fast and/or precise
liquids delivery.