Hey guys,

I'm working on a new project that has me scratching my head.

The circuit controls two 12v winches, using PWM to get them to sync. In fact the two winches are pulling on a tilting solar panel array to track the sun. At horizontal they move the same amount, but as it tilts more one way then one winch needs to turn further.

But here's my concern. I've got about 30 ft of 6 ga wire from the battery to the control circuit, and another 10 ft to the winches. These winches draw about 30 amps. I may only be pulsing in the 50 hz range. But when that FET turns off, I'm going to still have 30 amps coming down that wire causing a monster surge on both the +12v and ground wires.

My concern is that my 3.3 regulator will struggle to protect my microcontroller. It will also require higher voltage tolerant FETs, or possibly even activate the FETs due to the surge.

For protecting the regulator, I'm thinking of a 22 ohm resistor followed by a 15v zener diode and capacitor. Let's say my 3v logic draws 50mA with some LEDs, that's a whole 1v drop across the resistor (burning only 50mW power, so a 0805 or 1206 resistor will work there. Go the other direction, let's say I have a 30v surge. That will drop about 15v across the 22ohm resistor, wow, 10 watts! And nearly an amp, but this is a short pulse. I'll make sure the resistor is rated for the current.

But what if the surge is 50v, or 100v? Anyone have any guidance? I can't get the battery closer, I have 3 towers sharing one battery. And the winches will cycle very little, maybe every 15 minutes for a couple of seconds. And I can slowly move the pivot back at night to return to morning position.

 

Could you attack the problem another way, a mechanical solution? In rigging (lifting steel,etc) they use a (don't remember the real name for it) "lift beam". In essence a longer beam with a central lift point. This allows one crane or winch to lift longer pieces form one point. Just the first picture I found showing what I mean.

 

For protecting the regulator, I'm thinking of a 22 ohm resistor followed by a 15v zener diode and capacitor. Let's say my 3v logic draws 50mA with some LEDs, that's a whole 1v drop across the resistor (burning only 50mW power, so a 0805 or 1206 resistor will work there. Go the other direction, let's say I have a 30v surge. That will drop about 15v across the 22ohm resistor, wow, 10 watts! And nearly an amp, but this is a short pulse. I'll make sure the resistor is rated for the current.

That high current would be of a very short duration so you don't have to worry about the resistor current or power rating.

 

Shortbus,

I've already built the structures and the panels are on them. So I'm committed to the design. If all fails, I can change things. But until I have a failure this is my path.

Crutschow, you are correct but you would also be surprised. If I went crazy & picked a tiny 0201 chip, it might explode when 1 amp goes through it. In fact I'm not sure the current would burn up the trace going to the resistor! But I just checked, a nice 0805 is rated at 2 amps so I have room to play there.

As I've looked into this, I estimate 32 uH inductance in the wire from the battery to the circuit/FET. Check my math. E(capacitor) = 1/2 x C x V^2. And E(inductor) = 1/2 x L x I^2.

At 32 uH and 30 amps, E(inductor) = 0.0144. If I assume E(capacitor) = E(inductor), reverse the capacitor formula and I assume 300uF, that gives me V = 9.8 volts. I assume that is the 'surge', so really I will get a 12+9.8 = 21.8 volt spike.

Is this correct? In fact, if I bump up to 800uF, I only have a 6v spike (18v peak) and would easily be able to use 25v rated capacitors. Now I'm not considering the ESR of the caps. Am I off base, or is my math correct?
Thanks

 

It's always a good idea to put a fast diode (schottky) clamping the PWM output to the positive supply rail.
This diode helps prevent the FET from getting killed by ESD, and by inductive overshoots caused by parasitic wiring inductance.

Then the drain is clamped by the body diode for negative voltages, and by the schottky for positive voltages.

Most FET driver circuits leave the drain without any positive over voltage protection, this invites death from random static electricity, when for example you disconnect the load and drag the wires over the carpet.

 

It's always a good idea to put a fast diode (schottky) clamping the PWM output to the positive supply rail.

Schottky diodes are fine for that but any diode will work well.
All diodes turn on quite rapidly, which is the critical parameter for protection from transients.
Fast diodes, such as Schottky's turn off faster, but that's not usually important in transient protection.
Here is a discussion and the results of some tests on that.

 

For the protection diode, I have 4 diodes. Each terminal on the winch has a diode to ground (cathode to winch) and another diode to +12v (cathode to +12v rail). Repeat 2 diodes on 2nd terminal of winch.

The idea is that when the winch turns off and the motor inductor keeps current flowing, if the voltage goes over +12v it will drop current to +12v. And if the voltage gets below 0v, it drops current into the ground rail.

But let me repeat my previous question, any comments about my calculation of the capacitance/surge concern?
Thanks

 

Let me ask, what 3V regulator are you using?

The "surge" may not even exist. The "kickback" is usually in the same direction as the applied voltage. You do want to protect the drivers.

If you can get the driver closer to the winch all the better.-

Can you "measure" your concern with a scope?

You probably should have gate protection and Vsd protection. So, yea, a schematic would help a little. What FET and what components you have surrounding can help too and so would what 3 V regulator are you using.