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Troubleshooting surface mount

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jnnewton

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Hello, I have recently started suface mounting all of the components on my projects. I haven't had much of a problem until now. I have built 3 fairly large circuits (for me, approximately 50-60 sm parts per board ). Now that it is reflowed, and doesn't work (can't connect to microcontroller using programmer on one w/o p/s whine, low resistance from power to ground (boards (both boards), power supply whines when plugging it in (1)) I need some type of method for finding the problem area(s). I have two switching regulator circuits from NI, a dsPIC304013 microcontroller, and all of its I/O / rs232 / CAN peripheral components, a couple of analog circuits going to inclinometers.. etc. I have no idea where to start...any suggestions would be appreciated, and if anyone would like to see any of the circuits (they are separate drawings for each) let me know.
 

Diver300

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It sounds like the supply is shorted. If the power supply is protected so that a short won't damage it, leave it connected, and measure the supply voltage.

If it is less than 0.1V, you have a short. Find a voltmeter that reads to 1mV, or 0.1mV if you can, and look for the point on the circuit board where the supply voltage is the smallest. That should be where the short is.

At high currents, there will be a small but measurable voltage drop on the power supply lines, and that is what you can use to trace the current to the short circuit.
 

jnnewton

Member
Ok, The voltage measures 4.98 v on the 5v circuit, and 11.96 v on the 12v circuit, great....until i put the scope on it....The voltage levels are oscillating like crazy, the spikes on the 5 v circuit are as high as 12 volts and it oscillates all the way down to almost 0 volts. I think that I have a problem with the switching power supply.... I used a design directly from NI so that i would have a solid starting point... What can I do?
 

ericgibbs

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jnnewton said:
Ok, The voltage measures 4.98 v on the 5v circuit, and 11.96 v on the 12v circuit, great....until i put the scope on it....The voltage levels are oscillating like crazy, the spikes on the 5 v circuit are as high as 12 volts and it oscillates all the way down to almost 0 volts. I think that I have a problem with the switching power supply.... I used a design directly from NI so that i would have a solid starting point... What can I do?
hi,
Are the +12v and +5v power supplies from the same SMPs or do you have an onboard +5Vreg.

The way you describe the problem, 'when you connect the scope', suggests a common/grounding problem.

Is there a circuit diagram you could post?
 

jnnewton

Member
yeah, i found the problem with that one, there was a short through an IC on the board, only took all night. Thanks for the info, now on to the next problem....diode pops/sparks/catches fire when i apply voltage to a coil. p/n 1N5819HW-7 across an omron pcb mount relay. this part of the circuit is all 24 vdc, the relay is rated to 30 volts and i have tested it without the diode and may continue to use it without , but I thought I might protect the mosfet driving the coil. It seems to work without the diode, but I was wondering if there is something I am not seeing in the datasheet for the diode?
 

ericgibbs

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jnnewton said:
yeah, i found the problem with that one, there was a short through an IC on the board, only took all night. Thanks for the info, now on to the next problem....diode pops/sparks/catches fire when i apply voltage to a coil. p/n 1N5819HW-7 across an omron pcb mount relay. this part of the circuit is all 24 vdc, the relay is rated to 30 volts and i have tested it without the diode and may continue to use it without , but I thought I might protect the mosfet driving the coil. It seems to work without the diode, but I was wondering if there is something I am not seeing in the datasheet for the diode?
hi,
Stating the obvious, you do have the 'bar' symbol on the diode, towards the +24Vdc supply? ;)

Its possible with a 24V relay coil to get well over the 40v blocking/28Vrms rating for the diode.

You could fit a 1N4001 in order to check the circuit with a diode in place.
 
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jnnewton

Member
yes, the 1n4001 worked, thank you. Do you know if it is available as surface mount, i searched for 1n4001 on digikey and only got the DO-41 package option?
 

dknguyen

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DL4001-DL4007 from Diodes Inc. Basically the same package but without leads (it's still round and same size and all).
 

jnnewton

Member
Looks tough to suface mount being that it is cylindrical and all, do you know of anything in like an sot-23 (sorry for being picky, but I am trying to keep from having any unknown problems arize next time.
 

dknguyen

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I do not, I was just passing by this thread and thought "Oh! I know the answer to that!" It'd be more difficult to track down if it was something else. It doesn't actually state this one is the same as the one you wanted, but it's so very obvious from the number and packaging.
 

ericgibbs

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