Low pass filter for transient suppression

Thread Starter

odm4286

Joined Sep 20, 2009
238
Hello everyone, is a low pass filter a viable way to deal with transient voltages? I'm working on a DC motor driver which uses a FT232R for USB to RS232 communications. Right now the FT232R stops working whenever I run my motor, from my scope I can see transient spikes that go as high as 7V. I'm wondering if I can use a low pass filter to correct this. I would use a TVS diode but its tough to find a through hole component with the correct clamping voltage. I've also tried a zener diode, it helps but it just isn't fast enough to catch everything.

Lastly, I've verified this isn't a software bug. If I send my packet without the motor attached everything operates as expected. I get a nice square wave out of my PIC output for a given duration and the micro sends a ready packet back after a given duration. Thanks in advance everyone.
 

bertus

Joined Apr 5, 2008
19,934
Hello,

I assume the motor is a DC motor.
Does it have a backEMF diode accross it.
Could it be that the motor is making spikes on the powersupply rail?
Then try to split the motor power and MCU power.

Bertus
 

Thread Starter

odm4286

Joined Sep 20, 2009
238
Hello,

I assume the motor is a DC motor.
Does it have a backEMF diode accross it.
Could it be that the motor is making spikes on the powersupply rail?
Then try to split the motor power and MCU power.

Bertus
It's all powered from a single 12V supply. A 7805 supplies a regulated 5v to my ICs. The 12V is used for the motor. Yes, I have a fly back diode connected to prevent inductive kick.

Where do you see the 7V spikes?
The 5 volt rail, it's better now with the zener and bypass caps I've added but I still get spikes. I'll check their duration tonight after work. They are VERY fast.
 

bertus

Joined Apr 5, 2008
19,934
Hello,

Does the 7805 have the recommended capacitors on the in and outputs?

3-pin_low_power_regulators_with_capacitors.png

Bertus
 

DickCappels

Joined Aug 21, 2008
5,752
I once had a microcontroller that was among other things driving a Polariod film back, which had a wire brush DC motor. It turned out that noise was being picked up the wiring going to the driver and coming back up the I/O line and "messing with" the I/O port logic. It wasn't until I added RF chokes and bypassing to everything around the controller including I/O and power that the film back stopped dumping the entire package of film on the lab floor. Just as with ESD and lightning noise can find very sneaky paths to exactly where you don't want it.

If you find you need to filter the USB D+ and S- signals you can use small ferrite beads and low value capacitors to ground -I know this works for USB 2.0 but I don't know about the higher speeds.
 

Thread Starter

odm4286

Joined Sep 20, 2009
238
I once had a microcontroller that was among other things driving a Polariod film back, which had a wire brush DC motor. It turned out that noise was being picked up the wiring going to the driver and coming back up the I/O line and "messing with" the I/O port logic. It wasn't until I added RF chokes and bypassing to everything around the controller including I/O and power that the film back stopped dumping the entire package of film on the lab floor. Just as with ESD and lightning noise can find very sneaky paths to exactly where you don't want it.

If you find you need to filter the USB D+ and S- signals you can use small ferrite beads and low value capacitors to ground -I know this works for USB 2.0 but I don't know about the higher speeds.
That's crazy, at least I'm not the only one with this problem. My D+ and D- lines should be ok as I don't transmit while the motor is running. Vcc on the other hand, needs some clamping to prevent the spikes.
 

ebp

Joined Feb 8, 2018
2,332
Diode-type transient suppressors are zeners that have be designed to withstand high transient currents. Zeners are extremely fast.

Do not allow the 5 V regulator and the motor circuit to share any current paths if possible. Connect the wiring for the motor straight back to the source and do the same for the 5 V regulator. Keep power and ground conductors twisted or tightly bundled into pairs as much as possible.

Add a "bulk" capacitor in parallel with the input cap for the regulator. Ceramic capacitors perform well at high frequency and it is possible to get resonance between wiring inductance and a ceramic cap that can make transients worse instead of better. A bulk capacitor, such as a 10 µF aluminum electrolytic, will shift the resonant frequency and act in "lossy" fashion that helps to limit the resonant peak. An alternative is a resistor of a few ohms in series with another ceramic cap (i.e. that network in parallel with the existing ceramic cap), but that is usually more of a nuisance than the electrolytic except in surface mount. A similar RC network can be useful across a brush type DC motor.

Capacitors, both bulk and ceramic near the motor for its supply can also be helpful in keeping transient currents local instead of allowing them onto the connecting wires.

0.22 µF are fine as alternative sot 0.1 µF, and in fact likely to be better. It is important that the leads be kept as short as possible and the input cap be placed as close as possible to the regulator, all in the interest of keep inductance out out paths where it isn't wanted.
 
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