Solenoid - Short to Battery and Normal Operation?

[pawan kumar]

Hey Friends,

I am not clear with this, correct me if i am wrong:

A solenoid is just a copper wire with a lot of turns. So, after a few milli seconds, the solenoid almost acts like a wire.

If this assumption is true, how do we detect/distinguish between a Short to Battery and a Normal operation.

Your input would help me understand.

Thanks in advance,
Pawan

 

[BradtheRad]

Solenoids and electromagnets draw current, converting it into a physical force. In proper operation they are simply a load on the power source. They are reliable, pretty much. However suppose you increase the voltage, which is not proper operation... then they start to act as a short circuit. They draw overmuch current, which can result in ruined components. If you wish to detect this, then you must install a fuse, or a monitoring circuit, etc.

 

[pawan kumar]

Hi BradtheRad,

Thanks for the reply.
I wish to know how Solenoid controller IC's detect this. Not necessarily due to a defective solenoid, Accidentally, if a wire comes in parallel to the load(solenoid), the IC senses this and turns the circuit off. I would like to know how this happens.
Let me elaborate my question :
I am required to detect the presence of a solenoid between the driver and the battery. For this, my approach was to measure the voltage at the Drain pin of the IC, (of course when turned off). If that is equal to the Battery voltage, the solenoid is not removed. I now have a conflict in determining a short to Battery, which is also having a similar result. Hope I convey my thought.
Pawan

 

[dick_freebird]

Solenoids rated for continuous duty have enough internal
resistance to make the direct-battery connection keep a
safe current level. Have to look at the particulars.

 

[c_mitra]

A solenoid is just a copper wire with a lot of turns. So, after a few milli seconds, the solenoid almost acts like a wire.

Most, well almost all, solenoids used in instruments are made with a fine copper wire that have considerable resistance. Therefore solenoids have considerable DC resistance and of course some AC inductance.

Hence solenoids used in small instruments are DC rated for a certain voltage. If the label says 6V, 600sOhm that means the solenoid will hold on at 6V and 10mA current. It *does not* appear as a short to the controller circuit.

For big solenoids, made with fat wires (some with copper hollow tubes with water circulation) the inductance is large and the resistance is low. Such solenoids, used for producing magnetic field production, are driven by currents and the driver is tuned as a constant current driver. The driver may send 100A at 6V but it will not appear as a short to battery.

 

[pawan kumar]

Hi Mitra,

Thanks for the reply. guess I am getting clearer....

can you please tell what the voltage at A would be when the attached circuit? The answer would make my idea clear.

Thank you,
Pawan

 

[c_mitra]

The transistor is off and there is no current and the voltage at A will be 12V.

Now, see that if there is no current flowing, there will be no voltage drop and every point will *appear* as short.

 

[pawan kumar]

Hi Mitra,
There's a voltage source with a current limit resistor to the left. This, in case SHORT, allows 6mA to flow through.. but, in case of A, I am unable to predict the o/p.

Please give a second look at it.
Thanks,
Pawan

 

[BradtheRad]

A low-ohm resistor is used to measure current, by taking a volt reading across the resistor.

If you wish to detect the solenoid's presence, you might install a low-ohm resistor inline with the solenoid, and pass a small current through them.

A drop in supply voltage indicates a load has been added, which burdens the supply. Supply current can be detected by measuring voltage across a low-ohm resistor.

A solenoid is an electro-mechanical device, which can be damaged by a nick in the windings. It can cause them to short, or go open circuit, or change resistance. Therefore your safeguards need to have several ways to monitor any and every fault that could occur in the solenoid, if that is what you wish to do.

 

[pawan kumar]

Exactly... That's where I get the doubt. The low ohm resistor is going to behave in the same way when there's a short, as well as in the normal solenoid case. I would like to know if there's any difference in voltage behavior in the low ohm resistor
Thanks,
Pawan

 

[c_mitra]

I cannot figure out whether a connection is present at a crossing. It is a good habit to put a solid circle if there is a connection. (see post #6)