I have a 12V battery connected to an electromagnet, and I need to be able to change this input voltage from 12V to 9V using a transistor. I was thinking of something that changes the voltage path to a more resistive one? I am just starting out with electrical engineering, so if someone could help clarify this problem it would be great!
What current does the solenoid draw? If the battery is lead acid it is useable over a 10-14V range typically, so if you definitely need 12 to 9V you would have to use a switch mode power supply. If the solenoid will work satisfactorily at lower voltages, then none of the above will be an issue, and the simplest circuit is variable resistor, but since you have not specified a solenoid current, I have no way of knowing what power rating would be needed, it may well be impractical. Next option is an emitter follower. The output voltage can be adjusted with a variable resistor. You have to be aware of power dissipation in the pass transistor (again, you need to specify the solenoid current), and put a diode across the solenoid to clip the high reverse voltage flyback if the power is turned off, damaging your control circuit. Now there are a number of problems with this simple circuit, which mean that additional parts are needed to overcome them. With a fixed load resistance, and no need for high precision (temperature stability etc) you would probably get away with it if the current is low enough. Article here https://learnabout-electronics.org/PSU/psu22.php covers some of the basics. Quite a bit to consider for such a simple request! There are any number of integrated circuit solutions available as well, including monolithic if the current/power is low enough. Cheers, Richard
Thanks for your detailed response Richard, but I've found an alternative way to approach this after consulting a few others. Thanks for bringing up the idea with diodes for backcurrent, very useful!
By the way, if the solenoid turn off time is critical, you need to allow the back emf to swing to a much higher voltage to reduce it, so an RC snubber or zener diode rather than a typically used silicon diode is necessary, cheers, Richard
Hi Mourizio di Paolo Emilio, First of all thank you very much for your feed back ,In my circuit d1 acts as a polarity protection mainly for the ic and the in put current is also limited to 6amps by d1. d3 protects Q1 and d4 protects the relay driver transistor from back emf.the max rly can go ony a max 120ma to 150ma not more than that. so you can use the same 9v for some other ckt also. To avoid chattering of the relay a 100mfd 40v cap should be used parallel to the rly coil since high magnetic field if produced across the rly coil that is why i mentioned 40v rating for the cap min value of the cap should be 47 mfd 40v.if you are using a rly driver transistor use a 100mfd 25v cap from the base of the transistor to ground thisalso helps to avoid chattering of the relay. i have allready uploaded theschematic earlier any way i will do it again. Regards V.Sambath kumar.
Hi
let me remind you one more thing for d1 it is better to use a schotty diode because the forward drop will not be more than 0.4 volts .you can use MBR1545 .It is a 15a 45v silicon rectifier.in the schematic the voltage rating of 47 mfd cap is showen as 25v it should be 40v.
The discussion addresses controlling the voltage supplied to an electromagnet from a 12V battery, aiming to switch it down to 9V using transistors. Key considerations include the solenoid current draw, as this affects the choice of components and power dissipation in the transistor. A simple variable resistor or an emitter follower circuit with a variable resistor can adjust output voltage, but power dissipation and heat must be managed. Protection against back EMF generated by the solenoid is critical; diodes across the coil are recommended, with suggestions to use RC snubbers or zener diodes for faster turn-off times. A logic-based switching method using multiple gates to select between 12V and 9V was proposed. Practical circuit designs include relay driver transistors with polarity protection diodes (preferably Schottky diodes like the MBR1545 for low forward voltage drop) and capacitors (47–100 µF, 25–40V rating) to prevent relay chatter. The relay coil current should be limited to about 120–150 mA. Schematics were referenced but not included in the text. Overall, the solution involves careful transistor selection, voltage regulation methods, and protective components to safely switch the electromagnet voltage between 12V and 9V. Summary generated by the language model.
