Electric lawnmower...purpose of diode...

[eccerr0r]

I was looking at an old battery powered electric lawnmower I picked up as the previous owner didn't want to buy a new board to replace a ruined old board. Tried to study how it was designed - it appears it has a huge 80A contactor...or rather, a 80A relay. It appears to be SPDT to control the motor. A 30A relay protects the circuitry.

The weird thing I don't quite get is how this thing is connected. The common contact is connected to the motor. The NO contact is connected to the battery, which is expected, so when the relay is activated, the common is connected to the NO contact and runs the motor.

The weird thing is that the NC contact of that relay also has a connection - it's connected to (apparently) a diode's anode! The cathode of the diode appears to be connected to ground. This is a BIG diode... a stud diode.

While this appears to also provide faster slowing of the blade when the motor is shut off, couldn't a resistor or simply connecting the NC contact to ground do, instead of an "expensive" diode? Would the diode really protect the relay contacts that much better than just using a resistor?

[stj]

Re: Electric lawnmower...purpose of diode...

it kills the reverse transient pulse and provides static breaking.
most electric stuff with blades has to stop within a very short period to meet safety standards

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

Yeah but why a diode and not a resistor?

[stj]

Re: Electric lawnmower...purpose of diode...

a resistor wont kill the pulse.
i'm surprised they didnt just short it with a relay - thats a common method

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

A low ohmage resistor would eat the pulse (it's a brushed DC motor, there isn't much of a pulse, voltage there would be mostly dependent of the RPM at the time of cut off)...why not just use a 0 ohm resistor (i.e. short)!

Instead there's this fancy shmancy diode...

[CapLeaker]

Re: Electric lawnmower...purpose of diode...

Nope, it won’t.

[stj]

Re: Electric lawnmower...purpose of diode...

a resistor would have to be solid metal oxide- and very expensive.
a cheap wirewound resistor would be a broadcast antenna!!!

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

Actually, still... there is no real (large) pulse. Yes it's inductive but most of the energy that's stored is not in that magnetic field, but rather rotational energy of the blade. Besides, if there really is that much energy, why are car starter motors not treated the same way?

[stj]

Re: Electric lawnmower...purpose of diode...

cars have a diode-pack in the alternator.
also, newer cars are stuffed with MOV's and reverse-protection diodes in every electronic module

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

diode pack in the alternator is not for protection.
Shutting off a starter motor does not generate a reverse pulse.
Not all cars have MOVs.

So... keep guessing?

[sam_sam_sam]

Re: Electric lawnmower...purpose of diode...

I would think that if you used a resistor for this it would have to be huge to do the same thing that the diode does but that is just a guess

But yea it is mandated by government that the blade has to stop with in a certain time to supposedly protect the user from being injured so who really knows

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

Diode needs to be equally huge, both need to dissipate the same amount of energy to dissipate energy slowing down the blade!!!
Come on guys, think! So far it looks like all theories that have been presented have counterexamples.

So far the only possible reason I can think of is that the package of the diode (which is a stud diode so it has a nut to hold it onto a bracket) is so convenient that they can consistently mount it and not have it vibrate off from all the shaking as the mower turns and rolls over uneven terrain, versus some PCB mount power resistor. That's all I can think of, else a resistor seems like the better option.

Any counterexamples for this reasoning, other than the diode is not cheap which was the original concern?

---

BTW:

Nope, it won’t.

Yes a short or low ohm resistor would eat the pulse - if there was much of one to begin with. This is more mechanical energy than inductor di/dt energy here. Note that you are not restricted to the same solutions for this situation as for flyback transformers and relay windings due to the nature of this circuitry.

---
I just thought of another possible advantage of a diode:
You can spin the blade with your hand if there's a diode there up to a certain speed. If you have a short or resistor, at 0 RPM it will always give resistance to spinning. I'm not sure of the practical reasoning for this but if someone could give a good explanation for having this functionality (perhaps to reduce returns "hey my motor is jammed I want a refund!") then this would be a good reason for a diode!

[redwire]

Re: Electric lawnmower...purpose of diode...

For safety reasons- circular saws, cutoff saws, lawnmowers etc. have to brake when switched off, rather than slowly spin down.
They are brushed universal motors in most power tools.

With a PM motor in a lawnmower, you get generator-action it acts like a dynamo with the momentum of the spinning blade(s).
Which means braking makes full voltage and current for a short time - too much for a cheap switch or N.C. relay contact.
Add a resistor? Let's say for 50A. A typical wirewound is maybe 10-30x power for short overloads. A 10W 0.02ohm part seeing 50W pulse is OK but maybe not reliable enough?
I think using a diode, it's a cheap resistor in the sense that it can take very high surge currents, has some heatsinking, they like to fail short (safe).
If you started/stopped the lawnmover 10 times it would cook the braking resistor.

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

if you start/stopped the lawnmower 10 times it would cook the diode too! I still see no benefit with a diode than a resistor, if it fails short, it'd be no different than a resistor.

Also how often can one really make a surge, once per second at most? If you don't let it spin up all the way, you'll get less energy to dissipate. And again the amount of energy that needs to be dissipated is the same whether it's a diode or resistor.

Not only this, the resistor here can be ±100% and still be useful in braking.

BTW if someone is curious the diode looks like a MUR5010 but I didn't look at the part number. I'm sure a slow recovery unit would work just fine here too so it's probably not a MUR5010, but you can get almost two 50W 0.02Ω±1% resistors for the price of one MUR5010 ...

[petehall347]

Re: Electric lawnmower...purpose of diode...

is the proposed resistor in parallel when the motor is running ? if so it likely won't even run because power will shut off . if its only parallel when switched off normally its a safety issue in case of loss off power say through a bad motor connection etc ..a diode can sit there right across the motor and be fine . yes i know the resistor can be switched in electronically but would cost more and be not as reliable .
anyway i am probably talking crap again .
on another note i don't suppose you know who makes the motors for stihl battery equipment . ? bloody expensive with their name on the sticker .

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

The proposed resistor is only in parallel with the motor when the relay is off, in the exact same position as the diode - just swap it straight out. The contactor/relay prevents the diode from eating power.

They indeed are for stopping the motor rotation as quickly as possible when the "operator detected" switch is lost...

[redwire]

Re: Electric lawnmower...purpose of diode...

That second 30A relay does not make sense, I wonder if we have the whole picture.
We don't know the peak current for braking. It depends on the motor winding resistance, magnets etc. and likely ramps down to zero in a second or two. What does the motor put out/consume for peak power?

A relay has a weaker current rating for the N.C. contact, compared to the N.O. contact. This is because contact pressure is only from the return spring. So an 80A relay might only be rated 40-60A for the N.C. contact. Check the datasheet to see what its limit is.

I know a braking resistor is technically right here but there's a risk of it fusing open if it's small and cheap. DO-5 diode is small 15-17mm, extra labour to wire in. MUR5005 can take 50A for 1sec. and blows the doors off a resistor for surge current and not failing open. I think it's the ratio of average to peak current a part can take.
Or the diode is perhaps a zener or it's doing something else.

[eccerr0r]

Re: Electric lawnmower...purpose of diode...

whoops. I made a mistake in the OP - it's not a 30A relay, it's a 30A circuit breaker.

My guess is that the motor is ⅓HP, and the steady state current is about 10A. However the total rotating mass and RPM is needed to know how much total energy the device needs to dissipate...

The DO-5 is a fairly large diode IMHO and can actually deal with 50A continuous with proper heatsinking. The actual diode is not on a heatsink but on a small metal bracket.

I wouldn't think that one could use a 1% resistor but just use a wad of nichrome to dissipate the energy?