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# To generate 12V and 1.5A across a coil

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##### Newbie level 4
Hello.

I am designing a pulse width controller to control a solenoid coil.

At 100% dutycycle, i need 12V across the coil and 1.5A flowing through the coil. The inductance of the coil is not known. How do i solve this issue?

Thank you!!!

I think you have to manage the value of inductance.
Remember some point on inductance
1) In steady state coil acts like a short ckt.

2) when current is going to change across it a voltage occur across it.
V=L* derivative of current I wrt. Time
(sorry I can't express here the formula with symbol)
i.e. V=(inductance)*(rate of change current flow through the coil)

I think now it will be a bit easier to solve your problem.
Good luck.

You said 100% duty cycle which is straight DC.

You state a target current of 1.5 A maximum.

It will reach maximum after about five time constants no matter what the inductance (Time constant = L / R).

After that time current flow is limited exclusively by ohmic resistance.

You needn't take into account opposing EMF because EMF has then dropped to zero.

Therefore use Ohm's law to determine what your ohmic resistance should be.

12 divided by 1.5 = 8 ohms resistance.

Either build it into the coil by winding sufficient feet of the proper gauge wire.

Or perhaps you intend to add components inline which will add their own resistance?

What gauge wire is rated to safely carry 1.5A in dense windings?
I'm looking at engineer's tables. 18 gauge looks right.
To get 8 ohms you need 1000 feet. 5 lbs worth. That will make a large solenoid. This is all just theory of course.

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kunilkuda

### kunilkuda

Points: 2
The duty cycle is going to be varying.... from 0 to 100%. I am supposed to maintain a predetermined pressure in a common rail system in BOSCH using a solenoid coil which they are providing. The problem is, they don't have any specifications of the coil.

I am using a microcontroller to generate the PWM based on a feedback signal. This solenoid valve is going to completely open at 12V and 1.5A. The voltage across the solenoid valve is going to be a square wave. The only data about the coil we have is, the end-end resistance of the coil is 4Ω measured with a multimeter.

That's odd. Solenoid is current controlled device, not voltage. So you should be able to power it even with 6V, as long as you can generate 1.5amps across it. Unless you're talking about very special solenoid.

Points: 2
If the resistance is 4 ohms and the current is 1.5A then you only need 6V + your switching transistor Vsat, say 6.5V total. If you generate this by using a 12V 50/50% (squarewave) at a high frequency>> R/L then the mean current should be spot on, reduce the "on" time to reduce the current.
Frank

Your supplier will probably say the 4 ohm solenoid reaches its maximum performance at 1.5A, which occurs at 6V supply (as kunilkuda stated). Power dissipation 9W.

Then he'll add that it can endure up to 12V indefinitely in a short circuit scenario. 4 ohms will carry 3A. Power dissipation 36W. Still feasible though the coil will get hot.

Regardless, during the testing stage you'll want to monitor how hot the coil gets. Can you keep your hand on it? If not it's too hot. At that point the supplier may say 'We're working on making a coil that delivers better performance and draws less power."

To get the equivalent of 6V DC, you can pulse 12V through the coil at 50 % duty cycle.
The coil will develop 6V average internal EMF. Oppositional 6V during the power cycle. Propulsive 6V during idle cycle.

About how you drive the coil...

You're probably aware that it's crucial your PWM must not switch to high impedance in idle cycle.
Breaking a circuit with a coil will cause it to generate a hi voltage spike. Components can break down.

You'll need to short circuit the coil during idle cycle. You can use a flywheel diode (or is it a freewheeling diode?) to do this.
Thus current will never completely shut off while you apply PWM.

The diode should be rated to handle twice whatever current can possibly go through it.

Hi, your problem has been solved for several hundred times. You can buy for ~50-100\$ a proportional controller from any hydraulic valve manufacturer. I wouldn't waste my time to make it myself as there are some small tricks which will allow you to operate your valve or not. For a few pieces it is not worth the effort to spend a few months development. Even when you buy a finished product, you will need to adjust about 4-5 parameters (the gain, offset, demagnatizing current amplitude and frequency...). If you need any help, send an e-mail to my private account.

---------- Post added at 13:27 ---------- Previous post was at 13:21 ----------

4 Ohm coil resistance is correct for a 12V, 1.5 A proportional valve. The manufacturer has to take into account some tolerances such as heating (up to 50 % resistance increase), resistance tolerance (+/- 10%), voltage supply tolerance (+10 / -15%), voltage loss on the current controller and ambient temperature range.

At 100% dutycycle, i need 12V across the coil and 1.5A flowing through the coil. The inductance of the coil is not known. How do i solve this issue?

If I understood you well, when you connect your coil to 12V DC, its DC current equals to 1.5A.
This lets us calculate the internal resistance (R_coil) of the coil:
R_coil = V_dc/I_dc = 12 / 1.5 = 8 Ohms (it is already found earlier)
But you said that it is 4 Ohms if measured at a low current (by an Ohmmeter).
I wonder if this resistance of 4 Ohms is the meter reading for both polarities. I try to find out if there is an eternal diode in parallel (perhaps some other things :wink: )
Please confirm. (By the way, could you measure the DC current if the supply is only 6 or 5V?)

Now for the coil inductance:
We can use the uC PWM to measure it but first do you have an idea of the circuit that will drive the coil, or not yet?

To be continued...

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Hi, your problem has been solved for several hundred times.
Hopefully, the original poster has approached a solution within the seven weeks until he asked the question.

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