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Old 06-21-2012, 11:37 AM   #1
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Default Solenoid diode?

Done a ton of research.....found comments that all is fine without a diode on the solenoid. Just curious, why there is no problem with this. My '96 TXT Series has not had a diode since I have owned it.

All this research has been towards an effort to verify that the solenoid is going bad. Disassembled it and cleaned the contacts but it still clicks with no movement on about 10% of the time for initial pedal action.........
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Old 06-21-2012, 11:45 AM   #2
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Default Re: Solenoid diode?

so is it the solenoid or the pedal micro switch?? the next time it does it, have your DVM with you, and see if your getting pack volts at the two small post on the solenoid
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Old 06-21-2012, 01:01 PM   #3
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Default Re: Solenoid diode?

The pedal micro works fine.....solenoid clicks every time....
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Old 06-21-2012, 02:57 PM   #4
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Default Re: Solenoid diode?

Remove the resistor for testing and see if the solenoid fails to supply power when the cart won't go.
The job of the diode is to (protect) extend the life of the solenoid contacts. Sure they run fine without them but, for how long?
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Old 06-21-2012, 04:50 PM   #5
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Default Re: Solenoid diode?

Will do. After reading so many past replies, I was not sure if that was the route to go.

So, about the diode, explain a bit as to how it protects the solenoid contacts. Read something about it not allowing reverse power flow under certain conditions......maybe when changing directions before stopped??????......
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Old 06-21-2012, 05:17 PM   #6
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Default Re: Solenoid diode?

A voltage surge occurs any time an electrically generated field collapses, in your case when the solenoid is deactivated. A voltage, opposite in polarity to the original applied voltage is generated by the collapsing magnetic field.

A good example of this is in an automobile ignition system. When the breaker points open, the current flowing to the ignition coil is shut off and the magnetic field built up in the coil collapses. The resulting inductive kick voltage is high enough to jump the gap at the spark plug.

The same effect also happens whenever a solenoid is shut off. The voltage surge can reach several hundred volts which will arc across the switch contacts and quickly destroy them. Switch life can be reduced to one-tenth of normal.

The solution is to add a diode in parallel across the solenoid terminals. A diode acts as a one-way valve for electricity. In normal operation, the electric current can't flow through the diode, so it flows through the solenoid coil. When the driver releases the throttle, the current is shut off to the solenoid and the inductive kick flows backwards through the diode rather than through the switch contacts, bleeding off the high voltage spike and saving the solenoid contacts.

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Old 06-21-2012, 05:36 PM   #7
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Default Re: Solenoid diode?

Thanks....the relationship to the cars points helped since I have worked on classic cars for years.....
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Old 06-22-2012, 05:42 PM   #8
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Default Re: Solenoid diode?

I believe the purpose of the diode and the resistor are getting a little skewed in this thread.

A series cart has, or ought to have , a diode across the solenoid's coil (Small terminals) and a resistor across the solenoid's contacts (Big terminals)

Diode: As mentioned, the coil generates a voltage spike in the opposite polarity of the activation voltage. The diode is now forward biased and shunts the spike back into the coil, depleting it. If the diode was not there, the set of switch contact that open to deactivate the solenoid, would have been subjected to a high voltage spike and an arc might occur between them since they are still opening when the coil's magnetic field collapses. The voltage spike moves through the wire at near the speed of light, while the switch contacts are mechanical and move at a relative snail's pace, so they will be very close together and have a low arc-over voltage. The switch that typically opens to deactivate the solenoid is the throttle microswitch, which is also the most used microswitch in the system.

Resistor: Aptly called the Precharge Resistor. The controllers use Pulse Width Modulation (PWM) to control current flow to motor. There are a dozen or so MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) in parallel that are either turned fully on or fully off about 18,000 time a second and the ratio of on time to off time determines the average number of Amps the motor is allowed to draw. The output waveshape to the motor is akin to a squarewave and squarewaves generate tons of electrical noise so there are filter capacitors galore across the MOSFETs to suppress it. Otherwise, your cart would be a broadband jamming system that would blank out all radios, cell phones and other devices that communicated with RF. Anyway, when discharged, a capacitor's initial current draw approaches infinity, but when charged or partially charged, it is far, far, far less. The resistor keeps the filter caps charged so the current through the solenoid contacts when the first close is in the hundreds of Amps instead on in the thousands of Amps.

FWIW: Off topic a little. I don't have the foggiest idea why there is a resistor and diode pair across the solenoid contacts on a DCS controller.
The resistor is needed, but if there is a reason for the diode across the contacts, I haven't figured it out yet.

If there is a diode or similar arc protection built into the controller circuit that supplies B- to the solenoid coil on DCS and PDS controllers, a diode is not needed across the coil in a DCS or PDS cart, but it certainly won't hurt to have one.
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Old 06-22-2012, 06:25 PM   #9
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Default Re: Solenoid diode?

Well, I have not had a diode on it since I bought the cart last Jul. Use it daily....sometimes a short time, sometimes all day.

Now, the issue I am having......the dang cart is running good since I posted the problem so when it takes a crap again, I will do some more testing.
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Old 06-23-2012, 05:36 AM   #10
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Default Re: Solenoid diode?

Quote:
Originally Posted by scottyb View Post
Remove the resistor for testing and see if the solenoid fails to supply power when the cart won't go.
The job of the diode is to (protect) extend the life of the solenoid contacts. Sure they run fine without them but, for how long?
Not wanting to start a fight, but does the diode really protect the contacts or just the controller? There are two separate circuits.

In industrial applications, a capacitor is sometimes placed in parallel to the contactors. That's also what old school cars did.
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