Do you really need resistors on the base of transistors?

Discussion in 'General Electronics Chat' started by arthur92710, Jul 29, 2009.

  1. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    I paused on multiplexing those Dot matrix's for lack of programing experience.
    I have switched to something simpler. Multiplexing 7 segment displays.

    I have it coded and working. Now I want to make a pcb and the resistors on the bases are taking up too much room.
    Do you really need resistors on the base of transistors?
    Currently on the breadboard I have resistors on all bases.
    Also that 7805 gets really hot. How can I fix that?

    Also what else can I do to make the pcb smaller
  2. SgtWookie


    Jul 17, 2007
    Yes, you do. However, you're showing 47k for the resistors, which is far too high. You'll need about 2mA base current for 20mA collector current. Use 2k or 2.2k resistors instead.

    You'll also need resistors on the NPN transistor's collectors to limit your LED current; assuming that you have common anode 7-segment LEDs.

    You might consider using a ULN2803 instead of the seven discrete BC547A's, as it's a DIP with seven Darlington pairs and base resistors in a convenient package.

    There were also "source type" Darlington arrays made that could substitute for the eight discrete BC557's, but those are getting to be hard to find - and you probably wouldn't have the "headroom" necessary for the Darlington output.

    A heat sink, and/or reduce the 7805's supply voltage to around 8v.

    Use resistors of a lower wattage rating. You could likely use 1/10W for most everything, but calculate it for yourself. Use a wattage rating at least twice as what's required. Also, you can mount resistors vertically.
  3. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    Thanks for that.
    Just looked trough a few parts boxes and found a 3 pack of ULN2803's.

    The source for the 7805 was 9vdc. I had a small heatsink on it, with Arctic Silver 5 :)

    I will try to make the resistors vertical.

    Poped a ULN2803 in the breadboard and it works great.
    Also the 7805 does not heat up as much.
    Last edited: Jul 29, 2009
  4. flat5

    Active Member

    Nov 13, 2008
    Good song title.
    Zaxter5 likes this.
  5. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    Should I get a copyright?
  6. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    Quick question on making PCB. I am using all Through hole parts, can I route with both top and bottom layers? So far im using only the bottom but the auto router cant get 100%. If I use both it gets 100.
  7. t06afre

    AAC Fanatic!

    May 11, 2009
    Since this is a hobby project, why not use a isolated wire placed on the top layer as the second layer. Add them as 0Ohm resistors in the schematics. Also why are you using auto-router. This way you are takeing away much of the fun in such a project:)
    Ps are you somewhat sloppy with decoupling capacitors;)

    AAC Fanatic!

    May 26, 2009
    Don't they have IC's that would include a couple dozen transistors and you could have it all in one package?
  9. Wendy


    Mar 24, 2008
    Generally not, in the early days of ICs resistors were pretty common, but that has faded over time. Part of the issue is heat dissipation, part cost. Resistors are among the cheapest of components, my personal cost is around 2¢ ea.
  10. BobaMosfet

    Senior Member

    Jul 1, 2009
    Transistors are CURRENT devices, not voltage. Therefore, they require resistors both as current limiting and for pull-up .v. pull-down issues, in order to prevent you from frying the transistor or its load.

    If your 7805 is hot, have you checked a) how much current your PCB is drawing (there is a limit)?, and b) I'm concerned about how you have the 7805 wired. The bypass capacitor C3 is important for the ATMEGA168-- but I'm confused where you got the values for C1 and C2- they seem odd, whereas the datasheet suggests a different value. Perhaps you are creating an oscillating feedback issue through the 7805?