Need Confirmation from Expert

Discussion in 'General Electronics Chat' started by airulfmy, Jan 23, 2016.

  1. airulfmy

    Thread Starter New Member

    Dec 15, 2015
    Hi, as the title say it all, I got myself a project that can convert 12V to 24V by simply put the pin in 5v or 0v state.

    First of all, I'm using the concept of relay with transistor instead of MOSFET/Power Mosfet to reduce the cost.
    Based on my understanding and knowledge;
    • We need to put a rated voltage(relay) on the coil pin to magnetised it
      • if rated 12V, the coil will magnetised when 12V or more/less 2v+- is supply
      • You can see the example on "relay".PNG where the coil will only magnetised when the button is pressed... that is the basic concept
    • As for transistor(NPN BC547), to make the current flow from collector(C) to emitter(E) or simply say we need to "ON" the transistor;
      • A certain current need to be supply through base(B) in order to "switch it on"
      • As for BC547, the output voltage whether on PIC or Arduino is 5v/pin(IO), so it can activate the BC547(even-though it will not fully activate it as the actual Emitter-Base Voltage is 6Vdc
      • As you can see on transistor.PNG where no voltage is applied to the base and transistor_On.PNG where there is a voltage on it.
    Alright, enough with the introduction, let's get to the REAL QUESTION...
    1. In order to switch on transistor, the Base(5v) and Emitter(gnd) need to be on the same terminal(battery), we all know that
      • What if the Base(5v from MCU) and Emitter(Gnd from the battery) ? it's totally on different terminal to begin with. Does it still work?
    2. As we need GND for the Current(electron) to flow
      • If 12V is supplied, but it connected to GND(MCU), does the electron still flow?
      • You can see this problem on transistor_LOGIC.PNG
    As the Project that I'm working on, it's actually as you can see on project_OFF.PNG and project_ON.PNG
    • Yeah I know the simulation is on PN2222, but since BC547 is the only BJT I got in inventory, so... yeah...
    • The coil is not magnetised as it should be if there is no input on the bottom transistor. So, what could be the problem right now as before this designed is fabricated on PCB, I try it on breadboard first and it WORK!
    • But when transferred to PCB, its not quite stable as it should be
  2. hp1729

    Well-Known Member

    Nov 23, 2015
    All the transistor examples show nothing to limit the base current of the first transistor. When you push the switch the transistor smokes. You need a resistor to limit the current. Maybe something in the neighborhood of 5,000 to 10,000 Ohms.
    In the last two drawings what purpose does B4 do.
    Last edited: Jan 23, 2016
    cmartinez likes this.
  3. airulfmy

    Thread Starter New Member

    Dec 15, 2015
    I know that we need to limit the base current, I just want to show how it might work. I'm not tryin' to teach something here tho', i'm tryin to solve the problem.

    What do you mean by first resistor? which one on which picture? because I do limit a 'certain' current(around ~30mA) for Q3

    This design is to make 12 volt output, became 24 volt output by simply 12v-series with-12v with a single INPUT/OUTPUT signal on Q4 which is replace by battery and push button, so... the voltage change can be trigger using MCU
  4. WBahn


    Mar 31, 2012
    Well, letting the magic smoke out of transistors is not a very good way to solve the problem.
  5. airulfmy

    Thread Starter New Member

    Dec 15, 2015
    Well alright, I admit my schematic mistake there, but no hassle as on pcb, there is a resistor 330 on Q4 to begin with, as for Q3, there is 4.7k Ohm. This schematic is not the actual device tho' as it's combine with other devices, so I have to re-design as much as I can remember the layout so you guys can get clear picture of the device.
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Here's how I'd do it, if 24V is the default output (i.e. with the relay de-energised). Note the diode for back-emf spike suppression.

    Moderators note : shown full image in stead of thumbnail
    Last edited by a moderator: Jan 24, 2016
    cmartinez likes this.
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    330 ohms is way too small. That allows a base current of over 34 mA. Small signal transistors are happier with much lower base currents. Add a 2.2 K resistor in series with Q4's base, and increase R3. How much?

    Depends on the relay. Assuming the relay coil is rated for 12 Vdc operation, what is its resistance. With this you can calculate the coil current, with that you can determine how big a drive transistor you need, and with that you can determine how much base current it needs.