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EE needed

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tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Sat Jan 12, 2019 7:19 am Post subject: EE needed

This is a non-aviation question so please forgive my blatant request for info.

I have a solenoid rated as follows: 12VDC  29mA  4.0W (printed on the coil). I don't have any more information about it.

I need to open it for a VERY short time (around 30ms), then let its spring close it.  If I just apply 12VDC for 30ms, it barely has time to open and I get inconsistent results.  So:

1) Being an inductor, it's opposing the change in current in the 1st place.

2) It barely overcomes that, and then the current goes away.



I'm thinking I can apply much more than 12V with a proportional increase in current, since it's only for 30ms, but don't know how to figure out how much for how long.  I would very much appreciate someone giving me a non-differential equation answer if there is one.



Give a man a fish and you feed him for a day.  Teach a man to fish and you feed him for a lifetime.



Thanks so much.

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jan 12, 2019 8:46 am Post subject: EE needed

At 09:19 AM 1/12/2019, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "tomcostanza" <Tom(at)CostanzaAndAssociates.com>



 This is a non-aviation question so please forgive my blatant request for info.

 I have a solenoid rated as follows: 12VDC  29mA  4.0W (printed on the coil). I don't have any more information about it. 	




   29mA? That's only 0.35 watts. Could it be 290mA?

   What's the physical sized of this device. 4W dissipated

   into a small mass gets pretty hot in a hurry.



  	  Quote: 	
		  I need to open it for a VERY short time (around 30ms), then let its spring close it.  If I just apply 12VDC for 30ms, it barely has time to open and I get inconsistent results.  So:

 1) Being an inductor, it's opposing the change in current in the 1st place.

 2) It barely overcomes that, and then the current goes away. 	


   Yup. How critical is the timing for being 'active'?

   What are the tolerances on that 0.030 seconds?

   You could build an adjustable pulse generator that

   would be tailored to produce the pulse length needed

   to achieve the 0.030 second activation requirement.



  	  Quote: 	
		  I'm thinking I can apply much more than 12V with a proportional increase in current, since it's only for 30ms, but don't know how to figure out how much for how long.  I would very much appreciate someone giving me a non-differential equation answer if there is one. 	


   What voltages are available in your system.

   Inductive time constant is t=L/R. One way

   to get smaller T is increased R. We used

   to speed up response time on stepper motors

   with big resistors in winding leads . . . wastes

   a lot of POWER but for our needs, response time

   was critical, warming up the room not so much.



 

  	  Quote: 	
		  Give a man a fish and you feed him for a day.  Teach a man to fish and you feed him for a lifetime. 	


   There are also some constant current generator

   circuits that might be more attractive . . .

   but they still toss off a majority of the

   power.



   The adjustable pulse width generator might be

   the first thing to try.



 

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sat Jan 12, 2019 8:50 am Post subject: Re: EE needed

When you say open and close, I assume that you are talking about a valve or 

something, not the electrical circuit.

You wrote "but don't know how to figure out how much for how long".  It seems 

that you are able to vary the time.  So instead of increasing the voltage, why 

not just increase the time?

 Are the current or watts correct?  Looks like something is off by a factor of 10.

I calculate 69 volts.

E x E = 33.3 x 12 x 12

E = 69 volts

Hopefully someone will correct me if wrong.

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jan 12, 2019 8:50 am Post subject: EE needed

The adjustable pulse width generator might be

   the first thing to try.



   See   https://tinyurl.com/y92d76o4



 

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sat Jan 12, 2019 9:02 am Post subject: Re: EE needed

I should have posted 69 volts maximum for 30 milliseconds.  The actual voltage

 to get the desired results can be determined by trial and error.

_________________
Joe Gores

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Sat Jan 12, 2019 10:01 am Post subject: Re: EE needed

Bob,



 	  Quote: 	
		  29mA? That's only 0.35 watts. Could it be 290mA? 

What's the physical sized of this device. 4W dissipated 

into a small mass gets pretty hot in a hurry. 	




I noticed that as well.  But that IS what's printed on the coil.  FYI, it's about 1"x1"x0.75"



 	  Quote: 	
		  How critical is the timing for being 'active'? 

What are the tolerances on that 0.030 seconds? 	




The timing is the only thing that IS critical.  It's a project of my own design, so I can vary just about anything.  

 	  Quote: 	
		  What voltages are available in your system. 

Inductive time constant is t=L/R. One way 

to get smaller T is increased R. 	




As I said, I can vary just about anything.  But wouldn't adding a resistor in series just further reduce current and exacerbate the problem?

 	  Quote: 	
		  The adjustable pulse width generator might be 

the first thing to try. 	




Again, I can vary the drive to the solenoid any way I want, and can set the pulse width to whatever I want.  But what voltage/current would I use for the pulse?



TI makes a chip (DRV110) that does just what I want, but I suspect it's designed for longer engage/hold times.  My short t is what's screwing it up.

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Sat Jan 12, 2019 10:09 am Post subject: Re: EE needed

user9253,



I didn't understand your calculation.



I calculate 69 volts. 

E x E = 33.3 x 12 x 12 

E = 69 volts 



Where does 33.3 come from?



If I use the 4W number:



V = P/I = 4/0.029 = 137V



But for how long?  Obviously 137V through a 12V coil would let the magic smoke out in very short order.

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

ceengland7(at)gmail.com
Guest

Posted: Sat Jan 12, 2019 10:14 am Post subject: EE needed

On 1/12/2019 12:01 PM, tomcostanza wrote:

 	  Quote: 	

 Bob,

> 29mA? That's only 0.35 watts. Could it be 290mA?

> What's the physical sized of this device. 4W dissipated

> into a small mass gets pretty hot in a hurry.

 I noticed that as well.  But that IS what's printed on the coil.  FYI, it's about 1"x1"x0.75"

> How critical is the timing for being 'active'?

> What are the tolerances on that 0.030 seconds?

 The timing is the only thing that IS critical.  It's a project of my own design, so I can vary just about anything.

> What voltages are available in your system.

> Inductive time constant is t=L/R. One way

> to get smaller T is increased R.

 As I said, I can vary just about anything.  But wouldn't adding a resistor in series just further reduce current and exacerbate the problem?

> The adjustable pulse width generator might be

> the first thing to try.

 Again, I can vary the drive to the solenoid any way I want, and can set the pulse width to whatever I want.  But what voltage/current would I use for the pulse?

 TI makes a chip (DRV110) that does just what I want, but I suspect it's designed for longer engage/hold times.  My short t is what's screwing it up.

 --------

 Clear Skies,

 Tom Costanza

Details would help a lot. Solenoid controlled valve, or relay? If relay, 

what voltage, current, and AC or DC?

If it's a relay, replacing it with something solid state would simplify 

your problem by quite a bit.

Charlie

---

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tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Sat Jan 12, 2019 11:16 am Post subject: Re: EE needed

Some more data points:



I have several of these devices from 3 "manufacturers"



 	  Code: 	
		  Vendor     Written on device            Measured

-------------------------------------------------------------

1              12V 2.5W                       22 ohms   440mA

2              12V 5W                          35 ohms   320mA

3              12V 29mA  4.0W             35 ohms   320mA

	


So it seems Bob's question about "is it really 29ma" was exactly right.  The coil is sealed, so I can't even guess about the wire size.



I guess I don't have enough info to solve this mathematically, so I'll have to do it empirically.



Thanks to all for their input.

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jan 12, 2019 1:54 pm Post subject: EE needed

 	  Quote: 	
		  

 I guess I don't have enough info to solve this mathematically, so I'll have to do it empirically.

 	


   Too many unknown variables. From the time it's energized

   to the instant that full travel is realized has a lot of

   variables only slightly related to the t=L/r rise.

   Putting an adjustable pulse generator in the loop

   will let you 'dial in' a pulse width that achieves

   the desired effect. 



 

 

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sat Jan 12, 2019 4:57 pm Post subject: Re: EE needed

I assumed, maybe incorrectly, that 1 full second is the maximum amount of 

time to apply high voltage without damaging the coil.  One second divided by 30

 milliseconds equals 33.3

Watts = E squared / R

Power at high voltage for 33 milliseconds = power at 12 volts for 1 second

E squared / (33.3 x R) = 12 volts squared / R

The Rs cancel.

Thus E squared / 33 = 12 squared

E x E = 33.3 x 12 x 12

E = 69

Maybe my reasoning is flawed, but 69 volts will cause less smoke than 137 V.

_________________
Joe Gores

alec(at)alecmyers.com
Guest

Posted: Sat Jan 12, 2019 5:05 pm Post subject: EE needed

Do you think trying to activate a coil for so short a period with a timed pulse is reliably repeatable over variations in temperature etc?

What if the supply voltage has variations?

Or the spring in the relay changes elasticity as it ages?

Hate to be a pooper but maybe an approach with solid state switching would be more reliable?



Sent from my iPhone



On Jan 12, 2019, at 16:53, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

 	  Quote: 	
		     	  Quote: 	
		  

 I guess I don't have enough info to solve this mathematically, so I'll have to do it empirically.

 	


   Too many unknown variables. From the time it's energized

   to the instant that full travel is realized has a lot of

   variables only slightly related to the t=L/r rise.

   Putting an adjustable pulse generator in the loop

   will let you 'dial in' a pulse width that achieves

   the desired effect. 



 

 

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jan 12, 2019 8:50 pm Post subject: EE needed

At 07:05 PM 1/12/2019, you wrote:

  	  Quote: 	
		  Do you think trying to activate a coil for so short a period with a timed pulse is reliably repeatable over variations in temperature etc?

 	


   Your automobile's fuel injector

   valves do it . . .



  	  Quote: 	
		  What if the supply voltage has variations?



 Or the spring in the relay changes elasticity as it ages?



 Hate to be a pooper but maybe an approach with solid state switching would be more reliable? 	


   Those are some of the variables I

   cited. Fuel injected engines have

   the advantage of being able to monitor

   mixture in real time and adjust

   pulse width and timing to maintain

   design goals irrespective of a

   constellation of intervening

   variables.



   Your concerns for maintaining

   consistent solenoid behavior

   'open loop' are valid.



   But we don't know tolerance to which

   the behavior is expected. It may be

   that once the t=L/R factor is

   accounted for, the rest of them

   are operationally insignificant.



   It may be that the 'desired behavior'

   can be measured and used to close the

   loop on a variable pulse width generator.



 

   Bob . . .

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