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Tip41 Emitter voltage issue — Parallax Forums

Tip41 Emitter voltage issue

stampedstamped Posts: 68
edited 2006-10-18 14:09 in BASIC Stamp
Hi all,

I am building a basic H-Bridge and I have been having some issues with the output voltage of a Tip41 NPN power transistor. I put my H-bridge together and found that the output voltage is always ~5v, when the voltage applied to the collector is 10v. I have no problem switching the emitter "on"·by applying power to the base, but its always ~5v that is output from the emitter. Any ideas why this is so? I thought the Tip41 would output what is applied to the collector. Maybe this is wrong? I am no electrical engineer..

PS: I do not think there are any issues with the Tip41. I tried a fresh one with a simple circuit, 5v on the Base, 9v on the Collector, and the Emitter output ~5v.

I need ~9v from the Emitter.

Any help is appreciated.

Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2006-10-15 12:20
    Can you give us a sketch of your H-bridge along with where you're measuring the voltage?

    These transistors are current driven, not voltage, so I'm a bit concerned when you say you've put 5 volts on the base.

    Also, with npn's its usual for the emitter to be tied to 0 volts.

    Cheers

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tom Sisk

    http://www.siskconsult.com
    ·
  • stampedstamped Posts: 68
    edited 2006-10-15 13:38
    Thanks for the response Tom. Its a little hard to get the schematic onto the computer (its on p260 - 18.6 of Robot Builders Bonanza). It appears to be a fairly straight forward H-Bridge (4 Tip41's, 4 1-3k resistors, 4 diodes), which has two inputs one for forward and one for reverse. The actual H-Bridge base is getting around ~.7v which is switching on the emitter, which is outputting ~5v. The emitter when the base is not "on", is sitting at 0v.

    When I mentioned applying 5v to the base, I was referring to a seperate "experiment" to see if it would output (Emitter) the voltage on the Collector, when it was "switched on". It also output ~5v, and somewhat proved that my H-Bridge was ok.

    My understanding was that a power transistor is used to switch on a voltage seperate from the control electronics. Maybe I am way off here, but that is what I thought. But no matter what I do, the emitter is always ~5v.

    Generally speaking how would I get the emitter to ouput the same voltage as is on the collector? Even a simple circuit would be helpful here. In my "experiment" (not the H-bridge), I have a couple of volts going to the Base, 10v on the Collector, but the emitter is ~5v. Any newbie mistakes here (apart from the volts on the Base)?

    Does it sound like I have something like my ground tying wrong? Such as transistors in the wrong direction (cathode)?

    Thanks again. I appreciate you taking the time to respond.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-10-15 13:54
    This should have been posted over in The Sandbox.
    Schwabe_Hbridge.JPG
    1015 x 773 - 168K
  • stampedstamped Posts: 68
    edited 2006-10-15 14:14
    Thanks PJ. Next time I will post in the Sandbox.

    I have a few H-Bridge schematics already. What I was looking for was a very simple schematic showing how to get 10v out of a Tip41. I have seen an H-Bridge like the one you posted (NPN and PNP).

    The thing I am searching for is; I have ~10v on the Collector, shouldnt that be enough for it to output 10v on the Emitter? I am out of ideas as to why it is dropping 5v somewhere along the line...
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-10-15 15:40
    I don't know what you're trying to describe.· If I can use Paint to illustrate a point, then anybody can (hint.)

    Perhaps this (see attached) is what you're on about?

    [noparse][[/noparse]Note -- RLcould be a motor.· Try experimenting with a high-value resistor at first, 10K or more.· RB could be, at this point, 1K or so.]
    emitter follower.JPG
    415 x 415 - 11K
  • stampedstamped Posts: 68
    edited 2006-10-15 22:29
    Thanks PJ for the schematic. This is what I was looking for.

    I will hook this up later today and let you know how I go. My "experiment" circuit was similar and that was getting ~5v. That said "similar" is not close enough, so I will hook this up exactly and see how many volts I get on the Emitter.
  • stampedstamped Posts: 68
    edited 2006-10-17 12:06
    Attached is the schematic that has my transistors smoking (they really stink!). There has to be something obvious that I am doing. This is not the original schematic. I have taken a step back from the H-bridge to see if I can get a single transistor setup working. Then I will look at the H-bridge again.

    Any help is appreciated!
    transistor smoke.jpg
    414 x 611 - 63K
  • BeanBean Posts: 8,129
    edited 2006-10-17 12:19
    I believe the TIP41 is an NPN transistor. You have it drawn as a PNP.
    Are you sure the diode is not in backwards ? That would cause a large current to flow through the transistor (assuming it IS an NPN).

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cheap used 4-digit LED display with driver IC·www.hc4led.com

    Low power SD Data Logger www.sddatalogger.com
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    "People who are willing to trade their freedom for·security deserve neither and will lose both." Benjamin Franklin
    ·
  • stampedstamped Posts: 68
    edited 2006-10-17 12:42
    Thanks for noting that Bean. I am not sure where the NPN drawing is in Visio. I could only find the PNP. I am definately using an NPN and hooked it up as it is drawn. I do not want to fry another Tip41.. I can switch the diode around and pray it does not fry another one wink.gif
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-10-17 14:32
    Hello,

    ·· One of the most common mistakes I have seen with the TIP41 is that some people connect it like a 2N3904, where the leads are Emitter, Base, Collector.· On the TIP41 though (from left to right) they're Base, Collector, Emitter.· Do you have them wired correctly?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
  • stampedstamped Posts: 68
    edited 2006-10-18 06:57
    Thanks for the response Chris. I have the Tip41 wired up with Base (first pin on left), Collector (middle pin) and Emitter (right pin). I checked the Tip41 docs before wiring it up and it is all around the right way with the pins wired correctly.

    From the schematic (transistor smoke.jpg) I posted do you see any newbie mistakes, because it is smoking the Tip41's?

    Edit:·Just tested·switching the direction of the Diode (1N4004) and its still smoking...

    Post Edited (stamped) : 10/18/2006 8:12:56 AM GMT
  • BeanBean Posts: 8,129
    edited 2006-10-18 11:19
    Can you post a picture of your setup ? Maybe we'll see something ???

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cheap used 4-digit LED display with driver IC·www.hc4led.com

    Low power SD Data Logger www.sddatalogger.com
    SX-Video Display Modules www.sxvm.com

    "People who are willing to trade their freedom for·security deserve neither and will lose both." Benjamin Franklin
    ·
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-10-18 14:09
    Aside of the wrong transistor part in the schematic I don’t see anything wrong with it.· Everything else looks correct to me.· I agree with Bean, perhaps a picture will tell 1000 words (or at least 999).

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
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