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circuit efficiency and component question

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New Member

The circuit below was designed to allow 2 amps to go to a load from a 12v source, and then depending on the logic signal 1/0 to limit the current flowing by routing it through the other transistor. I was wondering if there is anything obviously wrong with this design or if there is a really easy way to do this in another manner that i can't see (possibly an IC or something). Is there some sort of H-bridge chip that i could use in this application? If not, what transistors can be turned on with a logic signal that can handle that current and voltage?
**broken link removed**


Active Member
The circuit can work, but i don't understand for what You need?
Why need two transistor, when just one output existiert?
Describe what You need exactly.


Perhaps I can rephrase it:

A 12V current limited power supply, where current limiting (to 2A) can be turned on/off via a logic signal.


Well-Known Member
You really don't want to try to build a current limiter by relying on the beta of a transistor. Have a look at the schematic below. I haven't tried it, but I think it will work. It uses a BF MOSFET to short out the current sense resistor when you don't want it. You'll need to carefully study the **broken link removed** to select your component values.


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Well-Known Member
Here's another way, which I also have not tried. It works by reverse biasing the base-emitter junction of the current-sense transistor (internal to the chip). Actually, I think this will be a foldback limiter when current limiting is "off", with a current limiting "knee" of about 6 amps (if you have set it up for 2 amps with the limiting enabled). If you want the limit to be even higher, change the 4.7k resistor to 1k.


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New Member
Hello all, this is my first post here so please forgive my noobie qualities. First off I am working with Bonanz on several aspects of a project and the circuit at the top of this post is one of them. I asked him to post that circuit here because he told me this forum was where we would get the most knowledgeable replies.

I have read the above posts and I am a little confused. Basically what I was trying to do with the circuit was mimic a mechanical switch with transistors.

The two resistors R1 and R2 represent two electromagnets in parallel. They will be operating under two conditions, the first condition they will be drawing 2 amps max and providing a set force( top part of the circuit), and the second condition is a default condition where they will have less current and provide less force ( bottom part of the circuit). I wanted to be able to switch between the two conditions using a high or low output from some digital logic.

Ideally this works in multisimm, turn on the circuit, draw 2 amps and providing the force we need. Then at some point send out a low signal from the digital logic to the bottom of the circuit and "turn" that part of the circuit, which would give us a smaller force from the electromagnets.

The main problem I am having at this point is, finding transistors that will handle two amps +, and I am wondering if this is even an efficient way to mimic a mechanical switch. I am no expert, which is apparent in my lack of skill to find a transistor to do this, much less one that hasn't been discontinued. So any ideas would be great and thanks for the help.


Well-Known Member
Why didn't you just post the problem in the first place? I sure went down the garden path.
Do you need to be able to turn all the current off with a logic level also?
If not, it only requires one switch. Can the switch(es) be in the ground leg of the electromagnets?
When you limit the current to 2 amps, you are going to have to dissipate 8 watts someplace unless you use a switching regulator. Does this make any difference to you?
Answer these questions and I will help if I can.


New Member
Sorry it took so long for me to reply.

Yes, we need to be able to turn all the current off with logic level.

No, we cant use the ground leg of the electromagnet.

We are not to worried with having to dissapte the 8 watts.


Well-Known Member
Sorry I'm such a cranky old fart.
The schematic below just uses a resistor to limit the current. You can work out the logic for full drive, reduced drive, and off.
Here's the datasheet for the MOSFETs. Mouser has them for $2.30 each. They are overkill, but you shouldn't have to heatsink them, as the one drawing 3 amps should dissipate less than 150mw. If you pick different ones, remember that the power dissipation will be I^2*Rds. So, lets say you pick one with Rds=0.2 ohms. The transistor will dissipate 1.8 watts, and will have a voltage drop of (0.2 ohms * 3 amps)=0.6 volts.
I'm sorry if I spoiled the learning experience of letting you do your own design. That would be better for you. You could use PNP switches, but, as I said earlier, you can't rely on beta to limit current. It changes from device to device, and it changes with temperature, and remember that the part that is wasting 4 volts is dissipating 8 watts. If that were one of your PNPs, it would get hot and be very unpredictable.
If you want to use saturating PNPs with resistor current limiting (as below), or a stable PNP current source, I could help you with that too. Just remember that a PNP drawing 3 amps is going to require a significant amount of base current, which is going to have to be supplied by an NPN, which needs to saturate. You might wind up needing more base drive for your NPNs than your logic gates can supply. The MOSFETs below won't switch very fast, but they require no steady-state gate current, which makes them very attractive for slow applications like this.

I added flyback protection diodes to the electromagnets. If they are physically very close to each other, you may only need one diode.


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