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| Is there a way to tell max volts an LED will take from a CC power supply? |
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| bob91343:
Generally the photo resistor would connect from the collector to the base so that when it's illuminated the transistor conducts. Converely it could be between base and emitter and shut the transistor off when illuminated. The main problems with mixing up collector and emitter is that the transistor has lousy gain and voltage limit if backwards. And of course you won't know if it's NPN or PNP. You can try it with a low enough voltage so it won't get damaged. |
| Beamin:
--- Quote from: bob91343 on June 02, 2019, 04:46:45 pm ---Generally the photo resistor would connect from the collector to the base so that when it's illuminated the transistor conducts. Converely it could be between base and emitter and shut the transistor off when illuminated. The main problems with mixing up collector and emitter is that the transistor has lousy gain and voltage limit if backwards. And of course you won't know if it's NPN or PNP. You can try it with a low enough voltage so it won't get damaged. --- End quote --- I think you typed this as I was uploading the img, so you are saying the schematic on the top with the red X is how it should work? |
| bob91343:
Yes that should work. The one with the green check cannot work. |
| Beamin:
--- Quote from: bob91343 on June 03, 2019, 12:03:52 am ---Yes that should work. The one with the green check cannot work. --- End quote --- Now that I think about it the circuit I drew at the top of the page should work. When ambient light is bright the ohms of the CdS should go down making the least resistive path through the CdS and around the transistor. When its dark the CdS has a high resistance making the current flow through the base of the transistor. It was the fact that light=lower resistance not higher that was throwing me off. So when this is off in bright light shouldn't it pull at least 1 ma? The meter says zero and if it was 1ma the meter can show that. |
| bob91343:
Base current for the transistor is provided by the photocell. When resistance is high in the dark, the base current is small. When illuminated, there is more base current and then we have collector current, through the LED. This is not a good circuit because it depends greatly on the gain of the transistor. A high gain transistor will light the LED with very little illumination. In fact, it could do it with no illumination. A lower gain transistor will work unless its gain is too low in which case insufficient current will flow even in bright light to make the LED glow. |
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