SSR Relay stuck in active position 4

[jozaeta]

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?

 

[BrianR]

The scheme jozaeta proposes is the standard configuration contained in many MIL-SPEC relays, a diode and a zener. Commonally a 42V zener for 28VDC coil relays.

 

[fsmyth]

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>

 

[jozaeta]

Thank you for the response fsmyth. My valve is not that exact model. The 500 Ohm resistor is the internal resistance of the valve, not a separate resistor. In my valve's case, the resistance is 24 Ohm.

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.

 

[itsmoked]

2-5ms is meteoric! Good luck....

 

[jozaeta]

Yes, I know it's next to impossible. In reality I just want to get my circuit to function as fast as I can make it. 50ms would be a feat for me.

 

[itsmoked]

Yeah 2-5ms is like ... custom relays and energy balancing stuff.

Like dot matrix print heads with inertial rebound, etc.

 

[OperaHouse]

I'd glean the LM1949 literature for some further understanding. When I drove a glue solenoid at 2ms on and 2ms off to make dots it took driving the 6V valve with 60 volts. Steady state voltage only gives you the maximum power you can dump into the solenoid. If you want to drive it fast, it takes a large voltage to overcome the inductance. Any spike suppression slows down the turn off. For this reason use as high a voltage driver and mov/zener as can be tolerated. You can get (20) MJ16012 450V transistors on eBay for $10 shipped with a gain of about 20. At that price you can destroy a few! It took me 2 months to tune my system for maximum speed.

 

[fsmyth]

Well, actually, the examples I listed were for exactly that.
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.