Author Topic: Current limiting to an inductive load - do I include its resistance in calcs?  (Read 1327 times)

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Offline dsegelTopic starter

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I'm using a transistor to drive a coil with 18V DC. The coil has a resistance if 45 ohms. I want to limit current to about 350ma. Do I need to add a 50ohm resistor in series, or does the coil resistance already limit the current?
 

Online magic

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Of course it does. 18V/45Ω=400mA so you only need to add a little more resistance in series.
 

Offline MrAl

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I'm using a transistor to drive a coil with 18V DC. The coil has a resistance if 45 ohms. I want to limit current to about 350ma. Do I need to add a 50ohm resistor in series, or does the coil resistance already limit the current?

Hi,

Be prepared to catch the back EMF from the coil when you turn it off.  The voltage can shoot up very very high and cause a bad shock.
Typically a diode is used or zener or something else to absorb the energy.
 
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Offline dsegelTopic starter

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I was planning on using something like a TVS diode because the coil is driven in both directions (i.e. the current direction could be either way). How do I choose a voltage rating? Is 30V breakdown enough for an 18V coil? I measured the voltage spike with my scope to be around +/- 150-200V.

And I can just put the diode across the coil terminals, right?
 

Offline MrAl

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I was planning on using something like a TVS diode because the coil is driven in both directions (i.e. the current direction could be either way). How do I choose a voltage rating? Is 30V breakdown enough for an 18V coil? I measured the voltage spike with my scope to be around +/- 150-200V.

And I can just put the diode across the coil terminals, right?

Hi,

Well it partly depends on how often you have to turn on the coil and how long it stays off for, and the total energy that has to be dumped.  The energy will partly dictate the diode power rating because it has to dissipate energy.
Do you know the inductance of the coil?
And how often do you turn it on and turn it off, like the frequency?
 

Offline Rerouter

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The diode voltage is based upon what will damage the transistor, equally the higher the voltage of the TVS diode, the faster the energy in the spike is dumped, With the trade off of the diode being heated much more,

The max current will always be the same as the max current through the inductor, (ignoring shoot through) its just how quickly you want to dissipate that spike.
 

Offline dsegelTopic starter

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The coil will only be going on for 200ms  and off for at least 900ms each time.

Don't know the inductance; I'll see if I can find that or figure it out.
 


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