Electronic switch for electromagnet

Thread Starter

EduardoBat

Joined Jul 16, 2019
29
Hi, everybody! Everything's ok?
So, with a 12 V battery, I need to suply at least 11 V to an electromagnet, like image 1. My problem is: I need an on-off switch to control the state of this electromagnet. This circuit is set on a drone, so it needs to be controlled via the APM controller. The APM can send a 5 V signal to the switch, making the circuit look like image 2. The eletromagnet consumes 250 mA. First thing i've tried, was using 3 parallel PNP transistors with maximum Ic current equal to 100 mA. When I set the circuit on, it burned (maybe the base resistance were too small?). The circuit is in image 3. My questions are:
1) What is the best switch I can use? (SCR, diode conection, zenner, etc..)
2) Should I use a NPN?
3) If I keep using PNP, how can I define it resistances?
If anyone have another hint, please tell me.

Thanks already, Eduardo.

image 1.jpgimage 1
Image 2.jpgimage 2 image 3.jpgimage 3
 

djsfantasi

Joined Apr 11, 2010
6,510
Why not a logic level n-channel MOSFET?
Here’s one to look at, but it’s overkill.
You need one rated for 30V and 1A (or higher). The only additional components are a 10KΩ resistor, from gate to ground, and a power diode reverse-biased across the solenoid contacts.
 

Vinnie90

Joined Jul 7, 2016
86
In general it's not a good idea to place BTJs in parallel, because when they overheat their beta increases and so does the current flowing through them until they die (it's kind of a positive feedback).

I'd also use a N-MOS for this specific purpose. You can put MOS in parallel because their Rds increases when they overheat and so they reduce the current flowing through and cool down (negative feedback).

Don't forget to add a flyback diode in parallel to the electromagnet so that when you turn your swith off the current can still circulate
 

djsfantasi

Joined Apr 11, 2010
6,510
In general it's not a good idea to place BTJs in parallel, because when they overheat their beta increases and so does the current flowing through them until they die (it's kind of a positive feedback).

I'd also use a N-MOS for this specific purpose. You can put MOS in parallel because their Rds increases when they overheat and so they reduce the current flowing through and cool down (negative feedback).

Don't forget to add a flyback diode in parallel to the electromagnet so that when you turn your swith off the current can still circulate
What I said...
 

Thread Starter

EduardoBat

Joined Jul 16, 2019
29
Why not a logic level n-channel MOSFET?
Here’s one to look at, but it’s overkill.
You need one rated for 30V and 1A (or higher). The only additional components are a 10KΩ resistor, from gate to ground, and a power diode reverse-biased across the solenoid contacts.
It seems great, thank you. I' ll try, but I didn't understand something: the resistance positon is "A" or "B" in the image?This way_.jpg
 

Thread Starter

EduardoBat

Joined Jul 16, 2019
29
In general it's not a good idea to place BTJs in parallel, because when they overheat their beta increases and so does the current flowing through them until they die (it's kind of a positive feedback).

I'd also use a N-MOS for this specific purpose. You can put MOS in parallel because their Rds increases when they overheat and so they reduce the current flowing through and cool down (negative feedback).

Don't forget to add a flyback diode in parallel to the electromagnet so that when you turn your swith off the current can still circulate
If I find a 250 mA plus mosfet I wont need to put then in parallel. My fear now is about the current loss because of the mosfet. Thanks.
 

Thread Starter

EduardoBat

Joined Jul 16, 2019
29
As you can see by the prior post, the correct answer is both. I was referring to position (b), but a smaller resistor in position (a) is also recommended.
I got it, thank you both. One more question, this setup is only for the N polarity? I read that the N polarity need a negative voltage to switch. My APM use only positive voltage as signal. Another thing is: it would be better for me to use something like a p-polarity depletion mode mosfet, because I need my eletromagnet to turn on when my drone turn on. How would be the setup and the sizing for this?
 

djsfantasi

Joined Apr 11, 2010
6,510
No, an n-channel does not need a “negative polarity” if by that you mean a negative voltage.

N-channel MOSFETs are typically used to switch a load to ground. MOSFETs are switched by voltage (whereas BJT transistors are switched by current. A logic level MOSFET needs a much lower voltage to turn on and thus are compatible with logic or microprocessor inputs. A high logic level voltage applied to the gate, switches the load to ground.

Assuming it is wired correctly.

P-channel MOSFETs typically switch a voltage to the load. It can be much higher than the logic voltage. Their operation is different than an n-channel MOSFET. When the gate is grounded, the load is connected to the supply voltage
 

Thread Starter

EduardoBat

Joined Jul 16, 2019
29
No, an n-channel does not need a “negative polarity” if by that you mean a negative voltage.

N-channel MOSFETs are typically used to switch a load to ground. MOSFETs are switched by voltage (whereas BJT transistors are switched by current. A logic level MOSFET needs a much lower voltage to turn on and thus are compatible with logic or microprocessor inputs. A high logic level voltage applied to the gate, switches the load to ground.

Assuming it is wired correctly.

P-channel MOSFETs typically switch a voltage to the load. It can be much higher than the logic voltage. Their operation is different than an n-channel MOSFET. When the gate is grounded, the load is connected to the supply voltage
No, an n-channel does not need a “negative polarity” if by that you mean a negative voltage.

N-channel MOSFETs are typically used to switch a load to ground. MOSFETs are switched by voltage (whereas BJT transistors are switched by current. A logic level MOSFET needs a much lower voltage to turn on and thus are compatible with logic or microprocessor inputs. A high logic level voltage applied to the gate, switches the load to ground.

Assuming it is wired correctly.

P-channel MOSFETs typically switch a voltage to the load. It can be much higher than the logic voltage. Their operation is different than an n-channel MOSFET. When the gate is grounded, the load is connected to the supply voltage
Sorry, I didn't understand. So, if I use a N polarity mosfet, when will it conduct the load current and when will it block it?
Which, for my application, will be the difference between using a depletion and a enhancement mosfet?
 

djsfantasi

Joined Apr 11, 2010
6,510
So, using an N-channel MOSFET, when the signal from the APM is high (a positive voltage), the load will be connected to ground and current will flow.

If this is opposite to what you want, you may be able to change the signal by software in the APM. If not, you can connect two MOSFETs in series with the first FET acting as an inverter. That MOSFET could be a low current MOSFET, such as an 2N7000.

Or you can use a P-channel MOSFET on the high side of the load, which switches on when the gate is grounded and off when it’s high. There are a couple of design consideration when using a P-channel. If there is a large voltage difference between the signal and load voltages, you may need a driver circuit which would invert the signal and then you’d have the same problem.

I’d go with the software or additional MOSFET as an inverter.
 
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