Cement Mixer switches

[Radian]

Am I the only one who finds the no-volt cut-out switches on cement mixers pack up as a matter of routine? Both mixers I've bought went the same way eventually. Granted they were cheapo models, but our builder had the same thing happen to him here and his was a Belle. Rather than use a G-clamp or buy a new switch (which I've done a couple of times) I think I've finally cracked the problem for good...

[Roundtuit]

Wheelbarrow and shovel?

[Temp]

Are these the switches that stop the mixer turning on after after a power cut? 

 

What was your solution?

[Radian]

8 hours ago, Temp said:

Are these the switches that stop the mixer turning on after after a power cut? 

 

Yes. They are essential really, not so much for personal safety as when used in static power tools like table saws, but to protect the mixer gearbox should the drum contain a full load when the power was lost.

 

And for the fix, now I have some spare time to document it for anyone sufficiently interested...

 

To fix it I first looked at the way it was made and figured out why it might fail. The on button has no mechanical latch so would spring back were it not for a solenoid energised along with the motor. This effectively keeps the on button held in just as if you were stood there holding it yourself.

 

The off switch has no latch either and just momentarily interrupts the power to the solenoid which releases the on switch - as if you finally took your finger off it.

 

So much for how they're meant to work. The clue as to why they fail is in the buzz you sometimes hear when you use them. This buzz is from the 50Hz 240/120VAC mains supply (60Hz in some countries).

 

The solenoid is energised by this alternating current therefore so is the magnetic attraction developed by the coil. But the magnetisation of the materials used in the solenoid can't keep up with the rate of change of the magnetic field so smooth it out into a constant force that keeps the switch held in.

 

My guess is that over time the constant vibration and temperature swings endured on building sites knock the magnetisation out of the materials so reducing the force available to keep the switch held in the on position.

 

My solution is to convert the AC supplying the solenoid coil into DC having the equivalent power. This loses the reliance on the material properties that degrade over time and finally put this issue to bed for anyone that wants to implement it.

[Radian]

In case anyone handy with a soldering iron (and good life insurance) wants to replicate this, I offer my circuit diagram with all the usual caveats:

 

 

On the left is a simulation of the bare solenoid connected to 240V RMS to estimate the power requirement.

I used a multimeter with LCR ranges to characterise its inductance and DC resistance. Both these may change on your coil - especially if 120V is used.

 

On the right is the simple half-wave rectifier with a current limiting resistor to maintain the same power in the coil.

C1 has to be high-voltage e.g. 400V rated, D1 can be any 400V 1A diode (e.g. 1N4004) and R1 must be rated for something like 10W or more. It has to dissipate 4 or 5 Watts.

Again, these values may need to be tailored to other switches but I would guess any 240V mixer switch that measured around 5000 Ohms on a multimeter would work OK with these values.

Edited December 31, 2021 by Radian