Electronics > Beginners
Help with circuit test
(1/1)
cmumford:
I'm fairly new to electronics and I'm currently assembling a software defined radio kit called the SoftRock Lite II Combined Receiver Kit. I love the kit and after each build stage the instructions have a set of tests to verify that the stage was properly constructed. The functional tests for the Op Amp pass - sort of, and I'm not sure there is a problem or not. In short they pass, but if I leave the circuit energized for > 20 seconds the current draw steadily increases from 29 mA, up to 100 mA where my current limit is set. The voltages also drop. If I de-energize the circuit and immediately re-power it, it's right back up to 100 mA, but if I leave it disconnected for 24 hrs. or so then it's back down to 29 mA. So... a few questions:


* I know capacitors will hold a charge, but I don't see how this would result in increasing power draw. What could cause this
* Is the increased current draw indicative of a problem, or could this be normal?
The kit author is retired and no longer supporting it which is why I'm reaching out here. Here's the schematic if anybody is interested.

Any advice is much appreciated.
bson:
My immediate suspicion would be you've reverse biased a polarized cap - most likely put it in backwards.
cmumford:
Thx bson. I don't see one on the Schematic. All of the symbols are --||-- and none are --|(--
digsys:
There's really nothing in that circuit that could draw that current, IF latched accidentally. Even the oscillator is only ~10mA.
Your best bet is to check the 5V rail, see if it increases. Also look for warm / hot items. At that current, SOMETHING will be getting HOT !
Most DMMs have a temperature sensor probe, or use your pinky finger. It should be obvious. Maybe a bad track?
DC1MC:
As somebody else pointed out: capacitors, look at the 4.7uF ones, the fact that in the schematic are non-polarized ceramics doesn't mean that someone didn't put some polarized one (electrolytic or tantalums).
Also have a look if the ICs are installed in the correct position (rotation) or place, I had a rather complex board behaving exactly like this because one IC was rotated during assembly. Especially with the new packages where the orientation notch is no so large, it's very easy to overlook, especially for self-assembled boards.
Another place to look at is the voltage regulator if it's put correctly in place and produces 5V, I will decrease the input voltage to 8-9V for testing.

My bet is either an IC output shorted to ground or Vcc, or a rotated/misplaced IC, usually the misplaced capacitors are much more "energetic" in showing up their displeasure  :'(.

 Best of luck,
 DC1MC
Navigation
Message Index
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod