Timeline for Odd reading when replacing an LED with a voltmeter on a PNP's collector
Current License: CC BY-SA 3.0
15 events
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| Oct 20, 2016 at 20:21 | comment | added | Spehro 'speff' Pefhany | @MartinRosenau True, but it seems like a real anomaly to me, and I'd want to get to the bottom of it. Up to the OP of course. I've seen this issue with acid core solder and it made a real mess in the field, cost the company a lot of money when 120VAC was there rather than 12V. | |
| Oct 20, 2016 at 20:17 | comment | added | Martin Rosenau | @SpehroPefhany We don't know the condition of the board; it may be dirty and/or wet. Some companies in automotive industry say that 100 kOhm is realistic. 200-300 kOhm is the resistance of the skin. Maybe the user holds the board in his hands while doing the measurements. We also don't know if his multimeter has a 1 MOhm shunt or more (e.g. 2 or 5 MOhm). All in all I think it makes no sense to me to measure this voltage not having a "real" load connected. | |
| Oct 20, 2016 at 20:05 | comment | added | Spehro 'speff' Pefhany | @MartinRosenau Wow, I don't think so. Maybe if the OP is using acid-core solder. I usually see G ohm leakage at worst on clean boards. | |
| Oct 20, 2016 at 20:01 | comment | added | Martin Rosenau | @SpehroPefhany Assuming the multimeter uses an 1 MOhm shunt a parasitic resistor in parallel to the transistor of 150 kOhm would lead to 10.2 Volts. 150 kOhm caused by dirt, humidity or similar is at least realistic. | |
| Oct 20, 2016 at 19:54 | comment | added | sribasu | Noted the point of R1. It is a good learning. | |
| Oct 20, 2016 at 19:49 | comment | added | jonk | Yes. Any time someone says "12 V LED" I start thinking now of those long strips or else of the high power modules. Either way, lots of current. His \$R_1\$ suddenly scared me and I wanted to know what he was really doing. | |
| Oct 20, 2016 at 19:47 | comment | added | jonk | @sribasu I understand that you think you want to understand it, but you really don't need to. Not for such a circuit. If you did want to manage this behavior, adding a resistor parallel to your load might be one approach. Then it would become predictable. But in practice this isn't important because the load itself makes the result predictable. The lack of a load is a situation that doesn't occur in practice. So you don't need to study it. | |
| Oct 20, 2016 at 19:44 | comment | added | Spehro 'speff' Pefhany | Your resistor value R1, as @jonk suspected, is way high for a 450mA load. It should be more like 500 ohms. And you should control the LED current with a resistor if there isn't one as part of the LED assembly. I would expect Q2 to be in some distress under those conditions - and it could be damaged. | |
| Oct 20, 2016 at 19:42 | comment | added | sribasu | Actually, I am. I am little serious to understand the behavior, you know. Because I am planning to use this circuit in a different project. But I need < 1V moments (with Arduino LOW) for that project. I am doubtful, if this circuit is correct for that or not. | |
| Oct 20, 2016 at 19:35 | comment | added | jonk | @sribasu I completely agree with Spehro's answer, though. He's right that it would take a lot of input impedance to show the voltage you say you read, given tiny leakages and the circuit. But perhaps the answer here to you is that "it's not expected" but it's also in an area where the circuit isn't under design management, either. So it's not necessarily important unless you are serious about tracking down the behavior as a matter of gaining a detailed physical understanding for the observation. | |
| Oct 20, 2016 at 19:23 | history | edited | Spehro 'speff' Pefhany | CC BY-SA 3.0 |
added 42 characters in body
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| Oct 20, 2016 at 19:23 | comment | added | sribasu | I replaced with another one. Same result. No, not sure of the input impedance of my meter. | |
| Oct 20, 2016 at 19:22 | comment | added | Spehro 'speff' Pefhany | How do you know it is not damaged? Did you try swapping it for another part? And what is the input impedance of your meter? | |
| Oct 20, 2016 at 19:21 | comment | added | sribasu | Q2 is neither damaged, nor hot. I verified. And it works fine when on load. I am using a simple digital Multimeter to measure voltage. No special equipment as such. | |
| Oct 20, 2016 at 19:19 | history | answered | Spehro 'speff' Pefhany | CC BY-SA 3.0 |