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I am a total novice but not unintelligent. I want to use a single solar panel to switch on some lights when it gets dark and also charge the batteries for the lights. I have previously successfully done this using a JFET, but have to have TWO solar panels, one to switch off the JFET and one to charge the batteries. I have read on numerous web sites that I can use a PNP transistor. As far as I can understand it they ALL say that a PNP is ON except when a base current is applied. Example circuits are shown. I have bought quite a few PNPs, different values. I have tried all of them in the suggested circuits. I have tried all of them with just a battery and a bulb. None of my PNPs are on with no base current, they all only switch on when I apply a current to the base. This is the opposite of what all these sites are saying. What am I not understanding? Connect battery +ve to PNP emitter, connect pnp collector to +ve of LED, connect -ve LED to -ve battery NO light. Connect -v of solar to pnp collector and +ve of solar to pnp base and LIGHT. This is the exact opposite of what I want. What am I doing wrong

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  • \$\begingroup\$ PNP is on when you allow current out of the base. Otherwise off. \$\endgroup\$ – Bradman175 Feb 2 '17 at 12:21
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    \$\begingroup\$ Possible duplicate of PNP transistor to command LED \$\endgroup\$ – Artūras Jonkus Feb 2 '17 at 12:29
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    \$\begingroup\$ Dude, please use line breaks. \$\endgroup\$ – Rev1.0 Feb 2 '17 at 15:14
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    \$\begingroup\$ Don't describe schematics with free text, draw them. It takes me two seconds to read a simple schematic, but I still struggle with your description. \$\endgroup\$ – Dmitry Grigoryev Feb 6 '17 at 14:34
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To switch on any transistor PNP/NPN, you need to forward bias the base emitter junction

so for PNP emitter is +ve and base is -ve so to forward bias connect emitter to +ve supply and base to ground with resistor (see attached image)

similarly for NPN connect emitter to GND and supply +ve voltage to base

PNP-NPN

Note: the explanation are purposely made simple for understanding purpose

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PNP will work like this...

pnp connection as OFF Switch

Connect battery +ve to PNP emitter, connect pnp collector to +ve of LED, connect -ve LED to -ve battery NO light. Connect -v of solar to pnp collector and +ve of solar to pnp base and LIGHT. This is the exact opposite of what I want.

Make bit more clear explanation on this.

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  • \$\begingroup\$ I believe CONTROL need to be 11.4V and higher to make the transistor OFF. Anythging below that will open the PNP transistor per your schematics. \$\endgroup\$ – EmbeddedGuy Sep 15 '18 at 20:01
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As you describe your PNP circuit (Vs->PNP_emitter->PNP_collector->LED_a->LED_k->0V) where Vs-0V is your supply, that makes sense.

You must draw current from the PNP base for it to conduct emitter-collector current, not drive it into the base as for NPN transistors. Your base voltage must be taken below approx. Vs-0.7 V, which allows for the diode drop between the emitter and base. You will readily find plenty of text describing this on the Internet. I suspect that your solar cell's output voltage is more than 0.7 V below your battery voltage so your solar cell is drawing current from the PNP base.

Try putting a 1 K resistor between your PNP base and the battery negative, or 0V as I called it. The LED should light.

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I have read on numerous web sites that I can use a PNP transistor. As far as I can understand it they ALL say that a PNP is ON except when a base current is applied.

Well, those websites are either wrong or you have misinterpreted what they say. A PNP (or NPN transistor) is turned on (in various degrees) by the application of base-emitter current i.e. a current into the base or out of it.

None of my PNPs are on with no base current, they all only switch on when I apply a current to the base.

That's how they should work.

What am I doing wrong

Believing in crappy web sites sounds like your only crime! I would name them if you can be bothered.

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  • \$\begingroup\$ Putting aside the crappy/crime/bothered stuff, how comes you describe 'a current into the base' of a PNP when the current flows out? \$\endgroup\$ – TonyM Feb 2 '17 at 20:55
  • \$\begingroup\$ @tonym good point and I could make an argument that it flows into the base via the emitter but I shan't be so obtuse. Correction on Its way. \$\endgroup\$ – Andy aka Feb 2 '17 at 21:19
  • \$\begingroup\$ Obtuse and into confusing :-) Thanks, Andy. While editing, could you tone down the text a bit, it does read as bad-tempered, far from your normally helpful posting style. \$\endgroup\$ – TonyM Feb 2 '17 at 21:26
  • \$\begingroup\$ Now I think you are being oversensitive or a practiser of some dark art of telepathy and getting it wrong! \$\endgroup\$ – Andy aka Feb 2 '17 at 21:57
  • \$\begingroup\$ Not at all, old boy, not at all. Skin as thick as their president's arse, me. Just that, well, slagging off the world and the OP with a Ha! and a hopelessly wrong answer...that's not the way you wanna be remembered... \$\endgroup\$ – TonyM Feb 2 '17 at 22:04

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