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Marv asks:

1. Which works better (diodes vs MOV) or are they about equal?

2. How do I know which one is right for the task at hand?



1. The Diodes work better for eliminating noise. However, the MOV's are

safer, and handle higher current. The flyback spike voltage on the diode is

around -0.7Volts. The MOV's spike voltage would be around -14 Volts, so you

get more noise with the MOV. However, the MOV is safer because it will

generally fail open and a diode fails shorted. If the diode fails, you

won't be able to actuate the solenoid.



2. The coil of the solenoid tries to maintain a constant current going

through it. When you de-energize the solenoid, the magnetic field around it

collapses and generates a huge negative voltage. This voltage can be

eliminated with the flyback diode or clipped with an MOV. Since the

solenoid coil tries to maintain a constant current, the current going

through the diode will be the same amount of current used to energize the

solenoid. The voltage rating is the same as the voltage applied 12 Volts

for a 12V system. For safety reasons you should at least double the

voltages you expect. For small relays I recommend a 1N4001 rectifier diode.

This diode is rated at 1Amp 50 Volts.

Here are some Motorola part numbers

1N4001 50V 1A

1N5400 50V 3A

MR750 50V 6A



The MOV (Metal Oxide Varistor (look up Varistor when searching for it in a

catalog)) is a non polarized device, and conducts if you exceed its voltage

rating. You want to pick an MOV with a knee voltage slightly higher than

what you expect to apply to the solenoid. I would pick 14 Volts for a 12V

system. The amazing thing about MOV's is that they are very small for their

current handling capabilities.

Here are some Harris part numbers. Note that these are peak and not

continuous duty currents. However, all you need for flyback is peak current.

Part Volt Amps Size

V18ZA05 14V 50A 5mm dia

V18ZA1 14V 250A 7mm dia

V18ZA2 14V 500A 10mm dia

V18ZA3 14V 1000A 14mm dia

V18ZA40 14V 2000A 20mm dia



In my opinion there is no need to use anything higher than the first one,

the V18ZA05. For the best of both worlds, you could put the MOV in parallel

with the solenoid, and then put the diode in series. This would eliminate

the -14V spike noise the MOV allowed and you would still be safe because if

the diode failed short, all that would happen would be the noise would be

allowed to get back on line. You would not loose the control of your

solenoid. I think I will use this method on any larger solenoids that draw

more than 2 amps. I am not this far along in my construction so I don't

know if there are such large solenoids in my L360 Kit.



All these parts are available at Newark Electronics 1-800-463-9275.

Radio Shack also has these components. Radio Shack only resells these

components, they do not manufacture them, so there is no quality issue

here.



Regards,

Ed



[I decided to forward this to The List as it contains lots of really

interesting information that I thought others of you out there will find

valuable... I hope you don't mind, Ed.  



There have been reports recently of flap and gear solenoids sticking (I

assume with welded contacts) and it occurs to me that the motors which they

are controlling are also inductive loads of sorts, are they not?  If that's

the case, then similar protection for the relay's contacts ought to be able

to be afforded by components similar to these MOV's/diodes, as the relay

contacts are nothing more than the switches which are controlling these

motors, right?  (The reason for the flyback control is to protect the

switch, not the relay coil as stated long ago at the beginning of this

thread...)  So, the question is what can we do to protect those relay

contacts to prevent arcing when we fire-up/shut-down those motors?  Or is

this thinking totally out of whack?? <M>]