I designed a basic on/off control circuit using 555 timer for a 3v hobby dc motor as shown in below. enter image description here

Without the motor connected, the output off the 555 timer is as shown below. (the mosfet drain is similar just inverted) enter image description here

But when I connect a dc brushed motor, I suppose the noise seen when connected to dc motor is because of arcing at the brushes. (Please clarify if I am wrong) enter image description here In the above image, channel 1 is connected to 555 output pin and channel 2 is connected to the MOSFET's drain pin.

As seen in the oscilloscope, I see that this noise has propagated from the MOSFET (STP16NF06) drain to gate. Due to with 555 timer switches off & triggers erroneously and so the PWM waveform switching is in microseconds rather than seconds.

I further investigated by testing the following circuit: enter image description here

When I switch on the gate, the motor runs. But the oscilloscope showed the following noisy waveforms with Channel 1 as Gate voltage and Channel 2 as Drain voltage enter image description here

Please help me remove this noise. I tried adding adding ceramic 1uf capacitors one at each motor terminal, but didn't help.



Upon reviewing datasheets, the FET has adequate internal diode and looking at the scope spikes, I suggest examining if the ground signal has similar spikes , it is measurement error from excessive long inductive ground clip path.(> 1cm)

If ground is made clean with clipless probing to ground with 2 pins <1cm apart, then repeat the same type measurement across 5 to 0V . If noisy then add a low ESR bulk tantalum cap and if not quiet <5% ripple and more, lower ESR caps until low noise and then repeat the same test method (clipless tip&ring) on above results and if not resolved, show all captures of gnd, Vcc, Vgs and Vds.

=== 1st cut

The 555 timer has a push pull output which is much lower impedance then the gate. Although there is a Miller capacitance between drain and gate there can be some transient Vgs.

But the pulse with suggests that the reverse recovery time and ESR of the clamp diode is too high causing twice the supply voltage to appear on the motor during turn-off as we expect with a low side switch.

The diode is supposed to act as a similar ESR high speed current rectifier to BEMF or high side switch to V+.

Thus re-examine the choice of diode for similar current ratings with FET and thus greater body capacitance to FET yet short Trr recovery time. This is normal to absorb the motor stored energy as it gets switched off.

Also , include links to datasheets of all parts in question, to find motor DCR , diode ESR, FET RdsOn and supply ESR of bulk capacitor.


Try placing a TVS diode between terminals of DC motor, with suitable breakdown value (slightly over supply voltage). It will likely eliminate brush generated spikes.


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