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Elvis II variable power supply
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Chiffy242
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09-16-2011 08:14 AM
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Not sure if this is the right forum for this question. I am new with the NI products and just got in the Elvis II Instrumentation, Design, and Prototyping Platform. I wanted to build a quick voltage regulator circuit I have built before and had working on another breadboard. The circuit is attached. So using the variable power supply source on the Prototyping Board as the power source set to 9v and for the switch, I just used a jumper wire from the + source to leg 1 on the LM7805. So turning on the power by running the vi, I verified I had 9v. I then moved the jumper to leg 1 and noticed the LED flickering. After doing this several times with the same results, I then measured both the source supply and output voltage. Before connecting the jumper, Source = 8.84v. After connecting the jumper, Source dropped to ~ 3v and the output voltage jumps between 1.6v – 2.3v. So here is the interesting thing: If I leave the jumper wire connected and stop the variable power supply vi and then restart it, the circuit works correctly. Also, if I remove the wires to the variable power supply terminals on the Elvis II proto board and connect a 9v battery, I can connect and remove the same jumper used as the switch before and the circuit also works. Any ideas on what is happening?
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Accepted by topic author Chiffy242
AnalogKid2Digit
Trusted Enthusiast
09-16-2011 10:01 AM
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My guess is that an initial current inrush occurs when the switch is closed that is caused by charging C5 (100uF). This may be causing the Elvis supply to go into foldback or protection mode. I do not know any of the details of that supply.
Try eliminating C5 as a quick test. If that works then reinsert C5 with a lower value on the range of 1-10uF.
-AK2DM
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"It’s the questions that drive us.”
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"It’s the questions that drive us.”
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09-19-2011 09:52 AM
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AK2DM, thanks for your reply. I changed C5 to 10uf and the circuit worked correctly. Still not sure why the Elvis VPS acts this way. I have attached 3 scope pictures of the LM7805 output in case anyone is interested.
ScopeView1.jpg : Output of LM7805 – C5=100uf . Triggered by stopping VPS vi then Running VPS vi
ScopeView3.jpg : Output of LM7805 – C5=100uf Timebase 500us. Triggered by connecting LM7805 input to 9v Power
ScopeView4.jpg : Output of LM7805 – C5=10uf Timebase 500us. Triggered by connecting LM7805 input to 9v Power
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