Hello everyone, well basically I’ve just started getting into the electronics and I’m finding that my passion is growing, with a thirst to learn more and more each day. But I’m still fairly new to it so my knowledge isn’t very good but I’m keen to learn. So any help or advice you guys can give will be much appreciated.

My project: I wanted to build a very small circuit that could stop dc motors from cycling after the connection has been disconnected, so I did some research for the best method to do this, I decided to put a circuit together using two Mosfets one N-Channel and one P-Channel to make a short that will stop the motor dead.

The battery I’m using is and 8.4v 3300mah, below is a basic diagram similar to what I put together that I found on Google.

Now this worked brilliantly but I noticed the short is rather aggressive, is there a way to reduce the amount it shorts easily? I tried using a 10watt 0.56ohm resistor between the source of the P-Channel which worked well and dramatically reduced the aggression of the short, but the resistor got very hot after a while, so I’m guessing this may not be the best option. Does anyone have any suggestions on how I could reduce the break without the circuit or components getting to hot, also bare in mind that I would like this circuit to be as small as possible without using a microprocessor? Any suggestions much appreciated.

 

Was there mention of using a delay between run and brake signals?

 

No it runs instantly, soon as the N-channel stops the P-channel shorts the motor by shorting out the negative terminal on the motor by sending its power to the N-channels drain.

 

Now this worked brilliantly but I noticed the short is rather aggressive, is there a way to reduce the amount it shorts easily? I tried using a 10watt 0.56ohm resistor between the source of the P-Channel which worked well and dramatically reduced the aggression of the short, but the resistor got very hot after a while,

Using a large wattage resistor is a common way of introducing controlled braking, if the motor is large and stops an overhauling load from high speed, there is a great deal of energy to dissipate in a short time.
Max.

 

Thanks for the response is there a better (Smaller) method of reducing the current to the motor from the source?

 

You can pwm the P- channel mosfet using a 555. Adjusting the Duty CYCLE to reach the disered stop.

Cheers

Alberto

 

Awesome thanks i will have a look into it

 

The braking energy still has to be dissipated somewhere. A resistor would probably be my choice too.

With PWM, 2 FETs and a freewheel diode (or with a halfbridge) you can do regenerative braking, just keep in mind the braking energy will then be fed back into the PSU which can cause its own problems.

You should explain what the motor is driving, and how often it needs to brake etc so we can better understand the amount of energy over time that you need to deal with.

 

Thanks, the resistor worked really well heats not too much of an issue but i would like to downsize the resistor a bit is there a better layout i should do? as the 10k resistor is rather large.

 

Ofc the resistor would have to be the proper wattage for the job. On CNC machines and such, the braking resistor is often a electric kettle heater spiral/stove eye as thats cheaper than a huge resistor.

 

Since you show a battery... Depending on the application you might be able to use regenerative braking. That would return the energy to the battery.