# Indicator LED for electromagnet

Discussion in 'The Projects Forum' started by magnet18, Feb 25, 2011.

1. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
Hi everyone,
I have a simple circuit with a 9V connecting to a switch (NC), to an electromagnet, and back. What I need is a way for an LED to light up when the switch is pressed.
I tried putting it in parallel, both with and without a resistor, but it wasn't receiving enough current to light up, the magnet took it all.
I'm worried that if I place the LED in series it will either get blown or it will limit the current to the magnet to the point where it can't pick anything up.
Oh, to throw another wrench in the mix it has to go in a cardboard tube, 2 inches cross and far enough up there that I can't get my hand or any tools up there, so if at all possible I'd like to do this without any transistors. I might have to resort to using one though.
Any ideas?

2. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Can you post your current schematic?

hgmjr

3. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
So much for trying to explain it without one.
Here ya go

And it doesn't work.
The LED and the battery are both good, the magnet works, but all together the LED doesn't do squat.

• ###### LED magnet indicator.GIF
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Apr 5, 2008
17,121
3,001
Hello,

You will need a current limiting resistor.
Without it the led will burn very quick.
You can calulate it using the following formula:
R = (Vsupply - Vled) / Iled.

Also the back EMF can still destroy the led.

Bertus

5. ### hgmjr Moderator

Jan 28, 2005
9,030
215
You say that you tried paralleling an LED and a series current limiting resistor and the LED did not light?

Maybe the 9V battery cannot adequately drive the solenoid and thus the battery is being overloaded.

hgmjr

6. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
Yes, I know about the current limiting resistor, I also tried it like this

And when the switch was open, the LED was lit fine, when the switch was closed, the LED went out.

• ###### LED magnet indicator2.GIF
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Apr 5, 2008
17,121
3,001
Hello,

When the led goes off whith the second circuit, the internal resistance of the battery is to high and the voltage will drop very much.

What kind of battery and electromagnet are you using?

Bertus

8. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
Battery- standard 9V
Electromagnet, I really don't know...
Got it a few years ago and didn't even have a part # then.

Is it possible that 2 9V's in parallel or series could solve this?

Dec 26, 2010
2,147
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There are a couple of things you might do to get around this problem.

1) Re-wind the electromagnet with many more turns of thinner wire, so that the coil resistance is higher - the battery voltage will hold up better. (This may be impracticable if the magnet is a commercially made item.)

2) Use a battery with more current capability (be careful - don't use a really low internal resistance battery with a very low resistance coil - fire / injury hazard).

You might actually find that your present magnet works better with just a few AA cells in series than it does with the 9V battery. I would think that a holder for up to four AAs would fit inside your tube

It would be a good idea to check the coil resistance first, as even AA cells won't last long with less than several ohms resistance per cell.

As mentioned elsewhere, an anti back emf diode across the coil (1N4001 or whatever) is required, otherwise the switch-off surge can fry the LED.

10. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
In your circuit the LED should be on regardless of the position of the switch. If it goes out when you close the switch then the coil is sucking the battery down. If it's off all the time then the battery is dead. The only other explanation is it's not wired as you indicate.

Dec 26, 2010
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Those 9V batteries are pretty feeble, especially the zinc-carbon layer type. Good alkaline types have more oomph, but really they can't manage much current for long.

Edit: I wonder if the magnet could be short-circuited, either completely or partially. Does it still work?

12. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
It still works, and it worked the last time before I used it before now, a few years back.
We were using a 12V lead-acid then.
I think I'm just going to put 2 9V's in series, with it wired as it is now.
The LED's on when the magnets off, and vica versa, good enough for me.
It only needs to be on for 45 seconds or so, in 5-10 second bursts, so it should be OK. I'll stick a diode in the line just in case.

Dec 26, 2010
2,147
301
Probably two 9V batteries in parallel would work better,and for longer: can you not see why?

The diode goes in parallel with the coil BTW, reverse biased of course.

14. ### magnet18 Thread Starter Senior Member

Dec 22, 2010
1,232
125
WHOOPS, I meant parallel, my bad.

15. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
From an available power standpoint the difference between a little 9V battery and a lead-acid is equivalent to comparing a BB gun to a 105 Howitzer. BTW, Ohms Law applies to electromagnets too. That's how you should be calculating your current requirements .