Mystery voltage - what am I missing. - Page 1
Forum section: electronics / beginners

author: LiftedTrace
author_rank: Regular Contributor
timestamp: November 06, 2020, 10:26:44 pm
content: I got this BT radio that has strange charging behaviors. It wont start charging until i press the power button for what ever reason. I wanted to see if I could figure out what might be wrong so I started to get an idea of some of the circuit and whats going on. I started with the power switch which has 1.2V present on it to ground. I traced the power switch to two diodes. The voltages on the other side of them diodes to ground are .006V and .56V.So what I am confused on is where on earth this 1.2V is coming from. I looked all over the board for a trace that shoots off from the switch or the diodes and I can find none. I used a meter and did a continuity check between the switch and all visible solder joints over the board and cannot find anything in common other than the two diodes.The diode that has .56V on the anode is connected to a BT module. What am I missing? What I have drawn is impossible correct? Unless there is another source of voltage that I cannot seem to locate that is connected to the switch?

author: Manul
author_rank: Super Contributor
timestamp: November 06, 2020, 11:35:54 pm
content: If schematic is really traced correctly, and button node does not have any other connections, then my only idea is that it is driven by a pulsed voltage throught the schottky.0.56V is the average or RMS value (depending on your meter) of these pulses, and 1.2V is rectified peak value minus schottky forward drop (because there is some diode and meter capacitance present, so the circuit acts somewhat like a rectifier with a filter). Of course the measured voltage also depends on signal source vs multimeter impedance.Needs more tracing. Voltage does not come from nowhere. Just keep in mind, that it may also be short pulses, not DC.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: November 07, 2020, 12:41:59 am
content: QuoteWell after reading this I was hopeful you were on the right track. I put a scope on the schottky and there was no pulse I could find, just a steady half a volt. I must have traced the circuit wrong, but I cannot for the life of me find what else is tied to that switch.Very confusing.EDIT:So i used the scope to check the DC value at the switch and the diode and they were the sameImage3 is the switch side of the diode, and Image 4 is the other side going off to the BT module. What I don't understand is why the scope is seeing the same voltage on both sides when the meter shows more on the switch.

author: Manul
author_rank: Super Contributor
timestamp: November 07, 2020, 01:23:59 am
content: Well, I really don't know how it might be implemented. As you know schottky diode usually have significant leakage, so it might be that is what you measure on the anode. I guess I would just temporarily remove both diodes and look what happens with 1.2V.Also the technique I like for quick tracing is to measure the current limited voltage with a multimeter (aka diode mode) on a point of circuit. Both current directions (multimeter negative to ground and then multimere positive to ground). It is like a basic IV signature of that point. If I get a reading similar to diode forward voltage, then, while measuring, I go quickly with a hot air (low temp, like 150C) over IC's and semiconductors. If I hit the IC to which this trace is connected, the measurement drops suddenly. There are of course exceptions and traps, but still usefull. Be sure to remove all power and these two diodes if you try. Also you may try DC resistance on that point. Everything which might give information.Sometimes it is hard to trace PCB's, in some complex cases I was even doing X-ray.

author: Manul
author_rank: Super Contributor
timestamp: November 07, 2020, 01:32:24 am
content: Quote from: LiftedTrace on November 07, 2020, 12:41:59 amThis should be because your meter's input impedance is lower then the scope's. So your meter is loading that point more and voltage drops. It also means that voltage on anode side is really high impedance. Probably just backwards leakage (some nanoamps or low microamps of current).Edit: Oh, so it is both like 0.6V on a scope? No 1.2V? Sorry, I have not noticed immediately. So then I guess the scope is loading more.