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tomcostanza



Joined: 19 Oct 2008
Posts: 45

PostPosted: Sat Jan 12, 2019 7:19 am    Post subject: EE needed Reply with quote

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.


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PostPosted: Sat Jan 12, 2019 8:46 am    Post subject: EE needed Reply with quote

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 . . .


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user9253



Joined: 28 Mar 2008
Posts: 1396
Location: Riley TWP Michigan

PostPosted: Sat Jan 12, 2019 8:50 am    Post subject: Re: EE needed Reply with quote

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.


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PostPosted: Sat Jan 12, 2019 8:50 am    Post subject: EE needed Reply with quote

The adjustable pulse width generator might be
the first thing to try.

See https://tinyurl.com/y92d76o4


Bob . . .


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user9253



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PostPosted: Sat Jan 12, 2019 9:02 am    Post subject: Re: EE needed Reply with quote

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.


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tomcostanza



Joined: 19 Oct 2008
Posts: 45

PostPosted: Sat Jan 12, 2019 10:01 am    Post subject: Re: EE needed Reply with quote

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.


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tomcostanza



Joined: 19 Oct 2008
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PostPosted: Sat Jan 12, 2019 10:09 am    Post subject: Re: EE needed Reply with quote

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.


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PostPosted: Sat Jan 12, 2019 10:14 am    Post subject: EE needed Reply with quote

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: 45

PostPosted: Sat Jan 12, 2019 11:16 am    Post subject: Re: EE needed Reply with quote

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.


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PostPosted: Sat Jan 12, 2019 1:54 pm    Post subject: EE needed Reply with 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 . . .


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user9253



Joined: 28 Mar 2008
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Location: Riley TWP Michigan

PostPosted: Sat Jan 12, 2019 4:57 pm    Post subject: Re: EE needed Reply with quote

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.
Smile


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PostPosted: Sat Jan 12, 2019 5:05 pm    Post subject: EE needed Reply with 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?
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 . . .


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PostPosted: Sat Jan 12, 2019 8:50 pm    Post subject: EE needed Reply with quote

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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