I need help with PNP transistor switch - Page 1
Forum section: electronics / beginners

author: ksrele
author_rank: Contributor
timestamp: September 06, 2015, 11:03:38 am
content: Hello,I have (let's call it "input") AC voltage source of 11V (I will put bridge rectifier at input so it will be DC) and I need to turn ON some device when input AC voltage drops below 5V (I think, I must check it again). Device that need to be powered ON have it's own power supply (15V DC). Grounds are floating, so I can connect negative terminal of input voltage (after rectifier) with negative terminal of other power supply.My idea was to use PNP transistor to monitor (rectified) input voltage and to switch transistor on when voltage drop, but I have problems with calculations of resistors... Can someone help me?Transistor is BC 327 25, but I find in datasheet this "Emitter-Base Breakdown Voltage: (min) -5Vdc". What this mean? Can I damage transistor if I put large positive voltage at base input (relative to emiter)?Device that I need to power ON draws very little current but it needs minimum 12V, it is small transmiter.

author: Landrew2390
author_rank: Regular Contributor
timestamp: September 06, 2015, 03:14:40 pm
content: Using a PNP transistor with a voltage divider will work, but it's going to be difficult to dial in a 5V threshold. If it was me, I would use a low voltage detector fed with a voltage divider to keep it within spec. You can take the output from the voltage detector and feed it into a P-channel MOSFET. That way it will turn on the circuit whenever the voltage falls below the threshold you specify.This part is kind of what I'm thinking of.BD48xxx seriesI've attached a schematic. It may be easier to understand than my explanation.

author: ksrele
author_rank: Contributor
timestamp: September 06, 2015, 04:59:31 pm
content: My mistake, 5V is not important threshold, I just need to to power ON other device when input AC voltage drops by few volts.OK, I will explain what I need in more detail so then maybee it will be clearer.I need to make wireless door bell on secundary house in backyard. In main electrical cabinet (english is not my main language, sorry) I have transformer for door bell in main house. It is old inductive door bell that works on 12Vac. When I measure that transformer it is providing 11Vac on stand by and it drops to about 4.3 when bell is ringing. That same transformer have 2 more outputs and their values are: 4.8-1.7V and 6.4-2.2V. Maybe it is better to use some other output...Then I bought basic wireless door bell and I wan't to put transmiter in main eletrical cabinet and reciever in secundary house. How can I do this?Tomorow I will go to store and by OP and MOSFET transistor and build this circuit, but I was hoping that I can do this today (I'm from Serbia, here is sunday 7 PM now) with parts that I have at house... But...Thanks any way.

author: Landrew2390
author_rank: Regular Contributor
timestamp: September 06, 2015, 06:25:06 pm
content: I think we're on the same page now. What you need to do is setup a comparator to monitor the voltage coming off the diode bridge. I would use a potentiometer for the inverting input on the comparator to make it adjustable, but you can simply choose appropriate resistors.A couple of things to watch for:- Make sure to do the math on the voltage coming off the diode bridge. You don't want to fry your comparator.- You may have to use a voltage divider on the non-inverting input of the comparator. This will only be necessary if the input would be above the voltage supplied from the second circuit.- Some integrated circuits have low voltage limits. If you can, check that your chips can handle the voltages involved.Let me know if you need a schematic. Since you don't need a specific trigger voltage, this design will work and uses parts that should be easier to get.

author: ksrele
author_rank: Contributor
timestamp: September 06, 2015, 06:54:08 pm
content: Thank you.I know how to do it with Op Amp and then NPN transistor of (better) MOSFET at output but I didn't have those in my house, I will buy it tomorow.