12v Magnetic coil switching

Thread Starter

ashokchandra

Joined May 13, 2018
37
Hi All,
We are using 12V magnetic coil in our project and switching on/off by linear led constant current regulator(bcr420uw6 ic). controlling bcr420u from micro -controller and current consumption of coil around 70mA
when i turn on magnetic coil the bcr420u ic heating up more after some time the going bad.
can any one suggest me how to solve the issue without heat or suggest me any alternate solution using different circuits.
below attached our application schematics.

Thanks,
Ashok
 

Attachments

ronsimpson

Joined Oct 7, 2019
438
Why are you using the bcr420uw6?
With 12V on the coil, there should be 70mA. (not using the IC)
What is the purpose for the IC? You don't need to regulate the current.
Also when you turn off the coil, the coil will "flyback" and kill the IC.
1576087335845.png
 

Thread Starter

ashokchandra

Joined May 13, 2018
37
Why are you using the bcr420uw6?
With 12V on the coil, there should be 70mA. (not using the IC)
What is the purpose for the IC? You don't need to regulate the current.
Also when you turn off the coil, the coil will "flyback" and kill the IC.
View attachment 194322
Hi,
Thanks for your time and suggestions.
in our case inductor value 10mH And current of 70mA.
supply voltage 12V.
can i use your suggested circuit ?
will it cause any heating issue ?Please confirm.


Thanks,
Ashok
 

MrAl

Joined Jun 17, 2014
7,095
Hi ,

magnetic coil resistance 0.2 ohms.


Thanks,
Ashok
Only 0.2 Ohms? You sure? That sounds too low. What is the magnet coil used for?
If it really is that low you need a good high power transistor.
But just to note, your drawing shows 70ma at 12v which would be 12/0.070=171 Ohms.
 

Thread Starter

ashokchandra

Joined May 13, 2018
37
Only 0.2 Ohms? You sure? That sounds too low. What is the magnet coil used for?
If it really is that low you need a good high power transistor.
But just to note, your drawing shows 70ma at 12v which would be 12/0.070=171 Ohms.
Hi,
yeah sure.we are generating magnetic radiation of 40uf.
suggest me any circuits which handles 12@70mA of current.
 

MrAl

Joined Jun 17, 2014
7,095
Hi,
yeah sure.we are generating magnetic radiation of 40uf.
suggest me any circuits which handles 12@70mA of current.
Well now you are verifying the 0.2 Ohms on the one hand but on the other hand you are still stating 70ma. The two are not the same. Or do you have two circuits now each with a different resistance?

Also, what is magnetic radiation of 40uf, are you perhaps using a capacitor antenna?
 

BobTPH

Joined Jun 5, 2013
2,250
Well now you are verifying the 0.2 Ohms on the one hand but on the other hand you are still stating 70ma.
If you had read the OP, you would know that the coil is being driven by a constant current source at 70 mA with a source voltage of 12V.

That results in 9.6V dropped across the pass transistor, and thus, more than half a watt dissipated.

Bob
 

MrAl

Joined Jun 17, 2014
7,095
If you had read the OP, you would know that the coil is being driven by a constant current source at 70 mA with a source voltage of 12V.

That results in 9.6V dropped across the pass transistor, and thus, more than half a watt dissipated.

Bob
Yeah but then later he said "0.2 Ohms".
That's why i asked which it was really or if there might be two different circuits being discussed.
 

Alec_t

Joined Sep 17, 2013
10,896
If the coil resistance is only 0.2Ω, the coil drops 1.4V @70mA. So the voltage drop across your linear regulator IC is 12V-1.4V = 10.6V. The power dissipated in the regulator is 10.6V x 0.07A = 0.74W, which will make the IC very hot unless it is on a suitable heatsink. What heatsink are you using?
You mention 'magnetic radiation'. At what frequency are you trying to switch the coil on/off?
Make sure you have a freewheel diode across the coil, or you will fry the IC (if you haven't done so already).
 
Last edited:

Danko

Joined Nov 22, 2017
869
can any one suggest me how to solve the issue without heat or suggest me any alternate solution using different circuits.
Try this solution:
1576235731874.png

EDIT: Seems you should not to go this sophisticated way.
Simple put 120Ω, 1W resistor in series with coil in your circuit post #1
and your problem will solved.
 

Attachments

Last edited:

BobTPH

Joined Jun 5, 2013
2,250
Use a lower supply voltage. All of the heat comes from using 12V where only 1.4V is needed.

Or use a switching regulator instead of a linear one.

Bob
 
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