rc snubbers, spark absorver circuit needed.

Dear All,

I need to design a circuit or find some sort of component, that i can plug into my batery, before supplying power to the device.

The device or component needs to be able to abosorve the spark produced on a switch.

The circuit is a simple direct supply to a dc motor controlled by switch.

Type of batery LIPO 11.1V 2000mAh 25C
mechanical switch spst
Motor may burn 20A peak.

I was looking into RC snubber, and other type of suppresion are spark absorver circuit, but they all need to be in the switch or with the motor, and I need something external to the circuit, than we can plugin or plug out.

Sorry if i dont explain my self properly.

Best regards.
Frank.

It sounds like you are trying to reduce the potential to arc over the switch terminals when you open the switch supplying power to the motor, is that correct?

If so, a simple reverse-biased diode across the motor terminals might be sufficient to handle the back-emf from the motor (depending on the size of the motor/inductance of windings). The RC snubber is more effective at filtering off transient spikes from things like switch-mode power supplies, where there is some normal operating frequency and the spurious emissions have a defined frequency content.

The snubber MUST be between the switch and the motor, anywhere else and its a waste of time.
Frank

Hi all!,

Thanks for your replies.

I know I can not use snubbers the way I said I wanted to.

But what do you think I could use to protect the contact on the switch from the sparks due to the inductive kicks of the motor.

The device needs to be like a coupler or some sort of device which i can plug between the batery and the motor controlled by the switch.

Any ideas??.

Thanks again.

Best regards.;-)
Frank.

gutisie said:

Hi all!,

Thanks for your replies.

I know I can not use snubbers the way I said I wanted to.

But what do you think I could use to protect the contact on the switch from the sparks due to the inductive kicks of the motor.

The device needs to be like a coupler or some sort of device which i can plug between the batery and the motor controlled by the switch.

Any ideas??.

Thanks again.

Best regards.;-)
Frank.

Click to expand...

Two words: Snubber Diode

If the low-side of the motor is grounded, you could hang a diode in shunt to ground from the input wire, prior to the swtich.


https://www.edaboard.com/threads/169546/, see post #2

hi!!,

thanks for your reply.

Where could i get those?, and what characteristics?
Thanks again.
Best regards,
Frank.

gutisie said:

hi!!,

thanks for your reply.

Where could i get those?, and what characteristics?
Thanks again.
Best regards,
Frank.

Click to expand...

You would just use a regular diode (not zener)... a silicon or schottky barrier diode should do the trick. Digikey - Rectifer diodes

Digikey, Newark, Mouser, local electronic parts store should carry them. Since the motor can draw 20A, you probably want a beefy diode, such that it won't smoke when the back-emf from the motor surges through it. You might lean more toward the schottky diode, since it has a lower votlage drop, it'll generate less heat for the same current through it. You'll need to make sure the reverse-breakdown voltage is higher than your battery supply, so the diode doesn't go into avalanche break down when you supply voltage to the motor. For a 12V system, go with 15V or better.

As for power handling/sizing, that's more difficult to do from a rule of thumb. Since this motor will be pulling 20A, the diode could see the same current, but only for a few moments... then the motor will start to spin down, and the current through the diode will drop. Maybe start with a 3A (average curent) rectifier diode, axial leaded. Something like this, perhaps: Digikey - diode.

If you want to really figure it out, fire up LTSPICE and calculate the power dissipation, then use that to determine how big of a diode you really need.

Hi all!!,

I have tried the snubber diode, but the problem is that it needs to be between the switch and the motor, other wise it does not work.

I have done a little drawing so I can show you what I have in mind.

Thanks for your help guys.

Best regards.
Frank.
**broken link removed**

The problem is that the motor basiclly truns into a voltage source that can produce tens to hundreds of volts when current is disrupted (V = -L*di/dt). When you open the switch, you get (Vbattery + Vmotor_emf) volts across the switch.

I'd say leave the switch on the motor closed, and add another switch + snubber in the "protection circuit" box you indicated. Can't you open up the motor case and get access to the wires to add a protection diode somewhere inside?

hi thanks enjunear,

yes i can, in fact at the moment i can put a transistor (mosfet) to protect the switch, but the thing is to know if it is posible do it as i suggested.

Nevertheless i will think about having the switch closed at all times......

Thanks.

Any more ideas?...

gutisie said:

hi thanks enjunear,

yes i can, in fact at the moment i can put a transistor (mosfet) to protect the switch, but the thing is to know if it is posible do it as i suggested.

Nevertheless i will think about having the switch closed at all times......
Thanks.

Any more ideas?...

Click to expand...

If you have a nice, big FET with a low Rds(on) (< 0.01 ohm), then I'd just use that as your current-control device (your "switch"), and leave the physical switch built into the motor in the ON position. Place your snubber diode in shunt to ground between the MOSFET and motor/switch. Then the motor turn off transients won't cause a voltage spike to occur across the transistor (can cause it to breakdown and conduct the turn-off transient's energy back to the battery).

Be careful to size your transistor correctly. At 20 ADC, you could create a lot of heat in a FET with a relatively high on-resistance (if Rds-on = 0.1 ohms @ 20A, then you would be dissipating 2 watts of heat on the FET). The Vishay SUD45P03-10 is a nice low on-resistance part, but doesn't quite meet your current needs. Something in that part family should do the trick.

Thanks a lot enjunear.

Mosfets are another issue. I dont know how to calculate nor the resistances or the required heat sink if any.

At the moment I normaly use IRL2203N or another one with similar spec. I put a 10k 1/4w between gate and sorce.
Then i put a 100 ohms 1/4w resistor in series with the gate. And it works!!!

How can i calculate this resistors to optimize the current going to the motor, or the efficiency of the fet.
How do i calculate the heat sink if required?

Thanks a lot guys.

Best regards.