Hi folks! This my first post. I am a retired chemical engineer who loves to DIY. Do not know too much about electricity although I did take Linear Circuit Theory as a technical elective and I can usually figure things out thinking about electricity as fluid flow. Quick project background: I have a Time Delay Module (TDM) on my garden tractor which instantaneously kills ignition and, after a second or so delay, kills the electric PTO clutch. If my butt lifts off the seat switch, current to the TDM is killed. Mine started to malfunction and I went to my tractor forum for help. Discovered another person (I'll call him Bill) had the same problem. Instead of spending $120 on a new TDM, he decided to build his own, which he has done successfully. I tried to do the same. Mine worked a few times and then the transistor fried. Why?
Here is the TDM schematic from the tractor Service Manual:
https://lh5.googleusercontent.com/-4CaKNQyx6ag/UF3lWcsp1xI/AAAAAAAAAy0/Ki1o79iPsPg/s562/%21cid_887DC29CB5F8481E86285FE760C88CDC%40WilliamVAIO.png
Bill exposed his TDM circuit and discovered the Service Manual had R1 in the wrong position. He also identified the components. Here is his sketch:
https://lh5.googleusercontent.com/-...q7Mi2jh4m78/s783/time+delay+interlock+(2).png
Here is a picture of Bill's TDM:
https://lh4.googleusercontent.com/-...AAzk/pmR4FbtQEj4/s631/IMG_20120922_170143.jpg
I exposed the circuit on my TDM. It did not have D3. My transistor is a MPSA06 instead of a 2N3904. My R1 is 6.8K, not 7.5k ohms. I had already bought parts so I built mine with R1 =8.6k ohms, T1 = 2N3904, D2 = 1N4004, C1 = 10 microF electrolytic. I did not install D3 as shown in the Service Manual because my TDM did not have it and, from what I have read, it would not protect T1 anyway. Instead, I outran my headlights and installed D3 (1N4004) in parallel with the coil so it could act like a flywheel when the coil de-energizes.
So, why is Bill's TDM working great and mine failed after a few cycles? The supply voltage is typically closer to 14vdc so I will use that number. Current to T1 base is 1.6mA. The relay coil resistance is 85 ohms so T1 collector is seeing 165mA. D3 should be protecting T1 by clamping transient voltage when T1 breaks ground connection through T1 emitter. In looking at the specs for a 2N3904, max collector current is only 200 mA and collector/base gain is less than 100 so I wonder if the currents killed T1 instead of the transient voltage spike?
Hopefully you endured this long-winded story and can help me understand what went wrong and how to fix it. Thanks!
Here is the TDM schematic from the tractor Service Manual:
https://lh5.googleusercontent.com/-4CaKNQyx6ag/UF3lWcsp1xI/AAAAAAAAAy0/Ki1o79iPsPg/s562/%21cid_887DC29CB5F8481E86285FE760C88CDC%40WilliamVAIO.png
Bill exposed his TDM circuit and discovered the Service Manual had R1 in the wrong position. He also identified the components. Here is his sketch:
https://lh5.googleusercontent.com/-...q7Mi2jh4m78/s783/time+delay+interlock+(2).png
Here is a picture of Bill's TDM:
https://lh4.googleusercontent.com/-...AAzk/pmR4FbtQEj4/s631/IMG_20120922_170143.jpg
I exposed the circuit on my TDM. It did not have D3. My transistor is a MPSA06 instead of a 2N3904. My R1 is 6.8K, not 7.5k ohms. I had already bought parts so I built mine with R1 =8.6k ohms, T1 = 2N3904, D2 = 1N4004, C1 = 10 microF electrolytic. I did not install D3 as shown in the Service Manual because my TDM did not have it and, from what I have read, it would not protect T1 anyway. Instead, I outran my headlights and installed D3 (1N4004) in parallel with the coil so it could act like a flywheel when the coil de-energizes.
So, why is Bill's TDM working great and mine failed after a few cycles? The supply voltage is typically closer to 14vdc so I will use that number. Current to T1 base is 1.6mA. The relay coil resistance is 85 ohms so T1 collector is seeing 165mA. D3 should be protecting T1 by clamping transient voltage when T1 breaks ground connection through T1 emitter. In looking at the specs for a 2N3904, max collector current is only 200 mA and collector/base gain is less than 100 so I wonder if the currents killed T1 instead of the transient voltage spike?
Hopefully you endured this long-winded story and can help me understand what went wrong and how to fix it. Thanks!
Last edited: