MS3 V3.0 V12 Drawing too much current

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MS3 V3.0 V12 Drawing too much current

Postby eracerr » Mon Nov 13, 2017 7:36 pm

I've run out of ideas on why the 12V circuit drops. I have a MS3 V3 board. I build it earlier this year and had this issue with voltage dropping using the JimStim and a fused battery. The polyfuse is apparently dropping the voltage to save the MS3. I removed the U5 voltage regulator. Voltage still drops. I disconnect the S12C jumper. Same problem. Lifted Q9 and Q12. No luck. Removed Q9 no luck. Removed Q12 too. Ah Ha. Voltage is ok. Put S12C back on, voltage still ok. Put Q9 on, voltage is now 7.1V. What the heck! Lifted it, no help.

Could both Q9 and Q12 be damaged? Something else wired or connected wrong that is causing both of them to pull too much current? I'm stuck.

Thanks,
Eric
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Re: MS3 V3.0 V12 Drawing too much current

Postby billr » Mon Nov 13, 2017 7:48 pm

Have you disconnected one lead of D9? That seems most likely to me. Do be sure it is installed in the correct direction...
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Re: MS3 V3.0 V12 Drawing too much current

Postby DaveEFI » Tue Nov 14, 2017 2:44 am

I'm a bit confused. The polyfuses both protect 5v circuits, so if the 12v is low with U5 removed, it's not a 5v short.

It looks like you have a problem in one or both injector drivers. Since it's a self build, check all the diodes are the right way round and you don't have a short to ground on the tabs of the two insulated transistors. Also check the tantalum cap C14 isn't short circuit.
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Wed Nov 15, 2017 6:25 pm

Thanks for the reply.

U5 is still out but I removed D9 for good measure. Voltage on 12V still drops to 7.1 ish with Q9 in and the metal tab pulled off of the heat sink.
I removed D3 earlier to see if that brought the voltage up. It did not. Only removing both Q9 and Q12 helped. Yes, the MICA insulators are there and I used plastic screws to try and avoid any unintended grounding. This is a new build and has not been connected to anything yet except the JimStim. Could I have damaged both Q9 and Q12 soldering them in?

Just removed Q9 again plugged in the JimStim and.... the Schotky diode on the JimStim appears to have failed, voltage reg getting hot. Sigh.... now what have I done.
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Re: MS3 V3.0 V12 Drawing too much current

Postby billr » Wed Nov 15, 2017 6:52 pm

I gotta ask... are both Q9 and Q12 clearly TIP125? That is a fairly robust device, I think it would hard to damage during soldering. Did the assembly of this board go smoothly, all joints flowed nicely with just a few seconds of heat on them?
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Re: MS3 V3.0 V12 Drawing too much current

Postby Matt Cramer » Thu Nov 16, 2017 7:20 am

I have seen the tabs on Q9 and Q12 grounded with too much heat sink grease before. That stuff is conductive, even if it doesn't look like it should be.
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Re: MS3 V3.0 V12 Drawing too much current

Postby DaveEFI » Thu Nov 16, 2017 9:31 am

Check with a DVM. A dead short should be obvious between tab and heatsink.
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Thu Nov 23, 2017 8:37 am

Thanks for all of the suggestions. I have installed and removed Q9 again. Voltage on V12 drops to 7.1 with it installed, goes back up to 12V when removed. The devices is a TIP125. The metal tab with the mica insulator under it measures 60K ohms. I purchased a new Q9, the local electronics place only had TIP127 NTE equivalent NTE262 which I think would work. Cleaned all of the heat sink paste off. Carefully put a new MICA insulator and using a plastic screw installed the new Q9. Voltage V12 once again dropped to 7.1 using the STIM. I looked at all of the diodes. They look ok. Checked the resistance values on R30, R31, R32. They are correct. I have replaced C16 (tantalum cap) for good measure. Took some alcohol and cleaned the back of the board up to make sure there was no bonus flux.

Any more ideas? This has me frustrated and stumped.
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Re: MS3 V3.0 V12 Drawing too much current

Postby DaveEFI » Thu Nov 23, 2017 9:51 am

Disconnect one end of D5 with Q9 still installed.
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Fri Nov 24, 2017 7:00 am

With D5 disconnected on one end I'm getting 6.9V on 12V. What should I try disconnecting next?

Thanks.
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Re: MS3 V3.0 V12 Drawing too much current

Postby DaveEFI » Fri Nov 24, 2017 8:22 am

Have you got a Stim? Is inject 1 LED on all the time? ie, at zero RPM?
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Fri Nov 24, 2017 11:15 am

All of the testing I've been doing is using the STIM to power the MS3 V3 without the CPU board attached. The polyfuse on the STIM is what appears to be dropping the voltage due to what I assume is too much current being pulled by the MS3. None of the LED's on the STIM (other than the power LED) are on. Also, I don't have any of the STIM jumpers in use. DIP switches are all set to off.
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Re: MS3 V3.0 V12 Drawing too much current

Postby 440roadrunner » Fri Nov 24, 2017 11:43 am

Have you simply felt with your finger on various components to see what is warming up? Or for the the "modern guys" an infrared thermometer?

Are you sure it actually IS drawing too much current?
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Sat Nov 25, 2017 7:22 am

I don't feel any of the components getting hot. But that maybe because the Polyfuse on the STIM is preventing it. I used a 12VDC automotive battery and hooked it up to the STIM with a voltmeter that reads current in the circuit. When I flip the power on I get 3.24 amps that quickly drops to 260mA. Voltage will do the same thing. I assume it's the Polyfuse that causes that to ocour. D5 is still disconnected.

If I unplug the STIM from the MS3 I have 12V and 40mA.
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Re: MS3 V3.0 V12 Drawing too much current

Postby DaveEFI » Sat Nov 25, 2017 9:57 am

Think what you need is a bench power supply where you can set the current limit, and power MS direct from that. To see what gets hot. I'd not use an unlimited current power supply to see where the magic smoke comes from as it's possible to burn out PCB tracks.
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Re: MS3 V3.0 V12 Drawing too much current

Postby eracerr » Fri Dec 01, 2017 8:12 am

I disconnected one end of zener diode D6 and now have 12VDC. D5 still disconnected and U5 still out. I tried testing D6 with the diode checker on the DVM and with only one end disconnected and it shows open and 0.69V. So it should be good. Any ideas?

Reconnected D5. Still have 12V. Seems to be something with the active flyback clamp circuit. Anything else I can test?
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