Need Help Understanding a Circuit

OldIBMer's picture

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Joined: Jan 23, 2020

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Need Help Understanding a Circuit
January 24, 2020 - 12:08am

I have 3 garden tractors which have the circuit below controlling their PTO clutches.  With 12V applied to the clutches thru the PTO switches, ground is applied via an interlock circuit (black box).  That interlock circuitry is understood to be sensitive to having a positive voltage applied.  Can someone tell me how the diodes are protecting the interlock circuit.  And if they are not doing that, what their function might be?  Looks like the diodes could be removed and the PTO's would function just fine.  I appreciate any insight that might be given on this;  I have just not been able to figure it out.

Gerald

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Reverse current protection diode
January 24, 2020 - 4:41pm

Hey I am not sure if your circuit is correct because I can't figure out how Front/Rear PTO Lite would work in this arrangement.

But the diode there should be a reverse current protection diode that is used to prevent the current from the PTO (inductor) to discharge into the 12V (probably controlled). As we know inductors store charge when they powered and discharge when power is removed, so this diode prevents the inductor from discharging back into the 12V input pin.

The circuit will work even without the diode, but it increases the probability of the controller (from which 12V signal is provided) to fail 

OldIBMer's picture

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Reverse current protection diode
January 25, 2020 - 7:58am

Thank you for your response.  I double checked that my crude drawing does agree with the tractor schematic.   

I was thinking that the PTO lites will work since when +12V is applied to the diode cathodes, the diode will not conduct allowing the PTO interlock ground to complete the lite circuit.  (Indeed, the circuit does work;  I'm just puzzled by the way the diodes were implemented.)

The 12V comes directly from a normal car battery and has a rectifier to control the charging voltage, but I'm not aware of any need to provide special measures to protect that circuitry; it seems pretty bulletproof.  I had not considered the inductor discharge from the electromagnetic clutches.  Seems like that would be one more reason to place the diodes so that they would protect the interlock black box.  Anyway, the designers would have known that, so any ideas about why the circuit was made the way it is?  I feel like I'm missing something here.

Gerald