LED indicator on Solenoid

[vinic]

I'd like to wire LED indicators to a series of solenoids. They are on a 24vdc circuit. The attached schematic shows what the original layout was on the left, and what I've tried already, on the right.

Before I fry another power supply, I thought I'd ask some folks that may actually know how to do this. Any help would be greatly appreciated!

 

[VEBill]

I was just about to suggest double-checking that the "-24" is actually just the return for the +24 (that's been my assumption).

It's very unlikely to be 48 volts, but if the solenoids are burning hot then make sure that the voltage is actually 24 volts across the coil.

 

[vinic]

I'm only reading 24 volts across the coil. It does not get hot right away, it's been on for few minutes now and is just now getting fairly warm. Can hold my hand to it for 30 seconds before it gets uncomfortable. My guess is that it would continue to build heat?? but have not gone there. Not really into burning up components, regardless at how it may seem!

From your reaction, this is not typically even without some form of solenoid driver in place??

 

[VEBill]

24 volts and 1.3 amps is about 30 watts. So it should be getting hot exactly as this amount of power indicates. Which sounds like what you're describing.

More advanced driver circuits can apply full power to snap the solenoid, and then reduce the current to a much lower holding current. It all depends on the application.

 

[vinic]

Out of curiosity I hooked up the old system to this relay coil, and it acts the same way; gets hot after being energized for a minute or so. I suppose this is the way it's always been and I just did not notice it. Most of these valves are located in an area where I don't get near them very often.

I'd still like to try to build a driver for this so just now ordered a handful of DRV102 units to test. The flag feature might come in handy; the word I get from some others using the same setup is that there's a high chance of failure.

thanx again for your feedback... it's been very helpful.

 

[IRstuff]

A high chance of failure implies that something is being operated outside of its safe operating regime. This is a potentially hazardous condition and should not be allowed to continue.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[LionelHutz]

Being able to hold your hand to a coil for 30 seconds before having to remove your hand isn't hot by any means.

It'd be interesting to see what that driver will do for you.

 

[vinic]

Using a solenoid driver on this seems to do the trick. Have been able to drop the current from 1.145 amps down to 218 mAmps and still keep the solenoid engaged. The relay coil maintains it's original temp after quite an extended hold time.

Here's a little vid demonstration:

http://www.youtube.com/watch?v=XF-xmFFJ6vg

 

[OperaHouse]

Just because you can doesn't mean you should. I wouldn't operate it less than 400ma for reliable operation. I have designed solenoid drivers and mechanical devices have their good and bad days.

For those who want to go cheap, A resistor of equal resistance to the solenoid can be placed in series with the power. A very large value cap of 4700uf or more is ten placed across the resistor, positive to the supply positive. The capacitor supplies the initial high closing current. As the capacitor charges the current drops to half. Off periods must be long enough for the capacitor to fully discharge.

 

[vinic]

Thanx for the 400ma tip, OperaHouse. My intent was to find a happy medium with this, and not take it to and keep it on the edge. Having a figure in mind helps a lot.

I like the idea of just a cap and resistor. So the current ends up cut in half with your setup? I don't think down time is an issue with my system. It typically takes 10 to 15 minutes at minimum to change the temperature of a tank (volumes are 500 gallon minimum). I assume that's plenty of time to discharge the cap?

Question: what type of capacitor would you recommend?? I found a 25V Radial Electrolytic Capacitor for $0.90 in small quantities.

 

[OperaHouse]

At that price, I would use two. Lot cheaper than a controller. That was only a guess at a capacitance to start at. I should warn you not to put the capacitor near the resistor because of heating. Elevated temperatures of a valve that may be on most of the time could cause the capacitor to fail in just a couple of years.

 

[vinic]

Positioning the cap away from the resistor is duly noted. It looks to me like the easiest way to mount them (for my setup) would be on the opposite side of the pcb.

Would you mind looking over the diagram attached to see if I have the arrangement of these components correct?

The main question is the placement of the cap and R3 in relation to the flywheel diode.

The relay coil is showing 18 ohms, so that's the rating of R3 (1/2 watt). Cap is the 4700 microF.

Unless I hear differently, I'm about to pull the trigger on this order and throw an entire 15 bucks at it!

 

[VEBill]

Capacitor isn't in the right place. If I understand the suggestion of Mr. House, it was intended to be in parallel with the resistor (to provide an initial "short circuit" around the resistor at start up).

Double check your "1/2 watt". Maybe remove the fraction bar.

 

[vinic]

That configuration makes sense to me now. (see attached update)

Not sure how I came up with 1/2 watt, other than two too many decibels. Power actually calculates out to 32 watts (strange how it's the same as the relay coil . Does the resistor need to be rated that high? That'll kill my $15 budget. Eight of these in the control box will build a bit of heat, though it would be rare, if ever, that they'd all be on at the same time.

 

[VEBill]

"32 watts"

Double check your power calculation.

Hints: 18 ohms in series with 18 ohms is 36 ohms. Steady state current is not as high as your latest calculation. Also, if the initial pulse is very short and not endlessly repeated, then the additional power of the initial pulse can typically be ignored.

PS: The capacitor trick is clever, but an active solution might be cleverer.

 

[vinic]

Yep. Feeling pretty dumb about now. Your help and patience is greatly appreciated! So your saying the steady state power for the resistor would only be about 12 watts?

By "active solution" are you referring to a device like the DVR102, because I do like the way that works. It's fairly pricey, but if I can feel comfortable that it would be reliable and long lasting, then it's well worth the $100 (total) investment.

I do have yet another question on the DRV102 setup:

They recommend a Schottky diode for D1, but I'm lost as to how to select the properly rated diode for this. Here is the description from TI:

"A flyback diode is (D1) needed with inductive loads to conduct the load current during the off cycle. The external diode should be selected for low forward voltage. The internal clamp diode provides protection but should not be used to conduct load currents. An additional diode (D2), located in series with Out pin, is required for inductive loads."

Is 'low forward voltage' simply the 24vdc? Is the RMS voltage of any concern here?


Another thing that is not clear to me, and may or may not be important, is the Input pin (1). Here is their description:

"The input is compatible with standard TTL levels. The device output becomes enabled when the input voltage is driven above the typical switching threshold, 1.7V. Below this level, the output is disabled. With no connection to the pin, the input level rises to 3.4V. Input current is 20µA when driven high and 80µA with the input low. The input may be driven to the power supply (VS)
without damage."



The first time I read this, I thought I'd need to apply a voltage greater than 1.7 to this pin for there to be any output. Simple enough as I had a pack of voltage regulators sitting around from a previous project. It worked to have that connected, but it also worked with no voltage applied. Not sure how, or what this pin is used for now!? Again, it's not terribly important, other than I'm just trying to learn as much about this as I can. Simply switching the 24vdc on and off seems to be a viable, if not easiest solution for my app.

Thanx again for the assistance ( and education!)

 

[VEBill]

"...you're saying ...about 12 watts?"

About 8 watts. So reach for a 10 or 15 watt resistor. I mentioned '12' mostly because it was funny (?) about the fraction bar removal from '1/2'.