# Using a 5Vdc output to control another circuits current output

#### Kempi

Joined Sep 18, 2023
2
Hi,
Can someone help, I have a National Instrument module controlled through LabVIEW outputting a Max 5Vdc. I want to use this power to control another circuit's current output from 0.016A to 0.1 A. This is to operate an electro magnetic brake. Any help is greatly appreciated.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
446
Maybe I’m not understanding your question exactly, but if you apply 5V across a 50 ohm resistor the current will be 100mA - or 0.1A as your describe it. If the resistance of your electromagnetic brake is less than 50R you just need to add a resistor in series. If the resistance is greater then 50R you will need to buffer the voltage with an Op Amp into the base of a transistor with a 50R resistor from emitter to ground and connect the electromagnet between the collector and a high enough voltage rail to deliver the current through its resistance. By feeding back the emitter voltage to the input of the op amp you control the current in the resistor which is mostly flowing through the collector, hence the electromagnetic brake

If this is in line with your requirement but not clear from my description I could sketch if for you.

#### Kempi

Joined Sep 18, 2023
2
Thanks for your reply, I appreciate your help, and I think your understanding of my problem is correct. I have attached the output details from my labview module and the input requirements for the magnetic brake. If you would sketch the circuit it would help. Again thank you, is the car in the pic a BMW 328? super looking.
Best regards,
Ian

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#### Jerry-Hat-Trick

Joined Aug 31, 2022
446
I'm relying on your specified requirement to drive a maximum current of 100mA. The brake is rated at 1A which is a whole different problem. And I'm assuming the resistance of the brake is 20 ohm as stated so it will need a voltage of 2.0V across it to drive 100mA. Your 0 to 5V input can deliver 1mA so I've used a voltage divider to drop that to 0 to 2V for the positive input of the op amp.

The output of the op amp into the base of the transistor will rise to turn the transistor on until the voltage across the brake which is fed back to the negative input of the op amp is equal to the voltage into the positive side. Breaking the circuit at the X marked "A" and inserting a multimeter set in the 200mA range should allow you to calibrate by adjusting the value of R1 or R2 if necessary. For low input voltages to work you will need a rail to rail single supply op amp - microchip do a range of these. Use pretty much any NPN power transistor - so long as the gain is 20 or more the op amp should be able to supply sufficient base current. You probably won't need a heatsink.

All this hinges on you being happy driving the brake with just 10% of its rated current

Yes, good spot, it is a BMW 328. I'm building a replica on a Triumph Herald chassis using a glsss fibre bodyshell made from a buck which was built by someone who restored a real one. The picture is of a real one!