So yeah i have homemade batteries on all my cordless tools. Each battery pack (5S) has a balancer BMS module and a big MOSFET that opens when the battery goes below 18v. All of the tools (rated at 18v) are standard DC brushed motors with pwm control, which i think implies that they must have a freewheel diode so therefore i don't need to add one.... i guess.
does anyone have experience with this?

 

What would the benefit be from having a free-wheeling diode connected across the motor in a battery powered device???
And why would a diode be added at extra cost for no benefit???

 

What would the benefit be from having a free-wheeling diode connected across the motor in a battery powered device???
And why would a diode be added at extra cost for no benefit???

well if it's drivin by pwm, pwm + inductive load = freewheel diode, right? i don't know for sure. Maybe if your MOSFET gate signal is slewed up n down the right way, you can reduce the inductive kick n avoid the diode. I'm not familiar enough with these circuits to know

 

Unless you also designed and built the tool's PWM control module, my suggestion is to not mess with it. It is a fair guess that the ones designing the speed control knew what they were doing.

 

There is a benefit. Switch would wear down much less and last longer. And diode does not cost an arm and a leg.

 

I have worn out a few corded and cordless tools, and never once has the switch been the part that failed.

 

I have worn out a few corded and cordless tools, and never once has the switch been the part that failed.

so have i, which is exactly why i suspect there must be some kind of mechanism to damp the arcing. i need to know because i'm making my own battery packs. Maybe the only way to know is to open it up and check. I've designed enough PWM controllers to know that there are a few ways to do it.

 

There is a benefit. Switch would wear down much less and last longer. And diode does not cost an arm and a leg.

it's not a simple question actually. Maybe the internal capacitance of the battery will damp the arc. Or maybe the pwm circuit has a parallel cap across the MOSFET. Or maybe the gate signal going to the MOSFET is carefully ramped up and down to minimize the inductive spike. Or, of course, it might have a parallel diode. I've seen the schematics of a lot of appliances, and have been repeatedly amazed at the ingenious tricks and complicated circuit dynamics engineers have used to lower the cost and raise the reliability of electric devices. A lot of times it's just a matter of experiment and experience. Either way, it's not the switch, it's the MOSFET inside the switch assembly that ya gotta worry about.
But maybe that's what you meant idk.

 

so have i, which is exactly why i suspect there must be some kind of mechanism to damp the arcing.

the cordless tools are fairly new. speed control is the norm and that is more than a diode.

 

the cordless tools are fairly new. speed control is the norm and that is more than a diode.

yeah they control the speed with pwm, which means there must be some kind of damping to protect the switching mosfet. The question is what kind. But now i'm thinking that no matter what kind of circuitry the tool has, it's just better to put a (redundant) diode across the battery leads just in case. I don't wanna open up the tools and check. screw it.

 

Stop and think about what the load presented to the switch at the input to a PWM circuit actually looks like. Certainly it is not like a plain inductance. AND, if the goal really is to protect the contacts against the inductive spike voltage, will that voltage actually be present in a PWM circuit??? AND, for that matter, analyze how inductive a brushed motor looks to a PWM driver circuit.

 

MisterBill is right... only really simple tools - with inductive load (motor) but without speed control could potentially benefit from adding a diode.

 

Consider also that in a "really Simple" battery power tool there is no device too be protected from any inductive voltage spike when the switch opens. Any voltage developed across the motor inductance will be applied to an open circuit in series with the battery.

 

Stop and think about what the load presented to the switch at the input to a PWM circuit actually looks like. Certainly it is not like a plain inductance. AND, if the goal really is to protect the contacts against the inductive spike voltage, will that voltage actually be present in a PWM circuit??? AND, for that matter, analyze how inductive a brushed motor looks to a PWM driver circuit.

Those are good points but I don't know and i'd have to scope every cordless tool to find out. The MOSFET might be vulnerable to spikes, it depends on a lot of things, and given the cost of an extra diode, it don't matter. I realized that, since i'm using the same battery for different tools, it's a reasonable precaution to take.

 

for DC tools (and that is what battery powered tools are) load is inductive (motor). if the power switch is the only control, it is subject to oxidation/wear due to arcing when current is interrupted. the arc is significantly smaller when suitable suppression is used. so cheap tools may simply rely on large enough contacts for the power switch.

tools with speed control have circuits that already take care of that.

so what is the exact tool that you are dealing with? what kind of control it has? what is the voltage/current?