I have a switched circuit providing voltage to the base of a TIP. The TIP drives a load off the same battery. My problem is trying to add an led to notify me if the load has faulted (broken connection). I've tried to have the load circuit provide signal to base of a pnp transistor with an led and resistor (using the same battery) but it never works properly.

My understanding was the pnp would keep the led on constantly unless it received a signal from the base (which would be when the load is working). But there must be something I am missing since this setup does not work when I disconnect the load. I have tried using the pos and neg lines (from TIP pins) for the base on the pnp but they both really come straight off the same battery to begin with as the TIP merely connects them to run the load.

I have also tried the simple blown fuse indicator of routing an led and resistor parallel to the load but this is impossible to get the correct values since the load resistance can vary based on the specifics of the load the user chooses. At the best of the worst, this method might yield a very dim led. Hence, the dilemma.

 

Seems I forgot to initiate a question. Does anybody have any suggestions I could try to get the led to not light up with a load and to light up with the load disconnected?

 

Can you post a schematic?

 

Am I correct in thinking that a NPN transistor is an "always off" switch unless it gets a signal and a PNP is an "always on" unless it gets a signal?

Anyhow, I can describe it much simpler now. I removed the tip part of it completely just trying different things and still can not get it to work. So now I'm only working with a battery positive, battery negative, and a positive signal (using same battery). How should the PNP transistor, led w/resister, be wired up so that the led lights when the signal is missing, and the led is off when the signal is connected?

 

Actually a NPN responds to positive current, PNP responds to negative current.

 

So what needs to be done to make the led work? I am not sure how to get a negative current. I appreciate the help.

 

Where is the schematic? Words do not a circuit make.

 

A schematic of a battery and led? You have convinced me that a PNP may not be the way to go so I am back at square one.

 

Schematics are the language of electronics. We are spinning our wheels understanding what you are trying to communicate without it.

 

...to get the led to not light up with a load and to light up with the load disconnected?

Here is a simplified version that I am trying to add an LED to. Does this help?

      - Battery +
    |          _    |
    +     +----D1---+  (diode to drop voltage)
    |  b  |         |
    |  c TIP-+-load-+
    |  e  |  
    +-----+

If the load faults, I want an LED to light up. Note that the load could change and have different resistances.

 

We also need to know what the "load" is, knowing what the voltage is wouldn't hurt either.

 

The loads I'm currently working with are 2.1 ohms and 5 ohms motors (I have a preventative diode parallel with the motor to protect the tip). Using a 9 volt.

 

What tip????? I have no idea what you are talking about. You were talking about transistors and such and you provide a circuit of a battery and a diode. Can yo try to be clearer. We will do our best to help but we need clear information.

 

What tip?????

The one I mentioned in the first sentence of this thread:

I have a switched circuit providing voltage to the base of a TIP.

I had no idea getting one led to alternate would be so cumbersome.

 

Poof goes the diode. You must have a resistor in series with any LED (and not the load), or the LED (which is fundamentally a diode, conducts one way, blocks current the other) can not control the current and burns up.

A transistor BE junction is pretty much in the same boat.

A quick note about schematics, you don't need to be an artist, we have folks draw it on paper the scan it to create readable schematics for here. You will find this is a necessary thing, not a luxury, since schematics really are the language of electronics. If you scan other thread on this site you will find it is a fundamental truth.

Your schematic also tells something about the skill level, which is also something we needed to know. We help folks new to electronics all the time, it's what we do. So lets get back to basic requirements.

What exactly are you referring to as a TIP? This is not a standard word with electronics, and I don't think anyone has really understood the reference. I thought you were talking about the tip of a jack to provide electrical power. There is a device called a Tip Ring Sleeve (TRS for short). I get a strong feeling this isn't what you are referring to.

The other TIP in my vocabulary is a TIP102 transistors, which seems even more unlikely.

Reading your general description is you want a power indicator, an LED light to show if there is power being delivered to the circuit. Or maybe an LED to show if the circuit under test is drawing power (not nearly as simple, but doable).

While it is not required, you will note the established members have their geographic location posted on the corner of the posts. This is mainly to help locate parts, a secondary bonus is it can help understand vocabulary, though rare.

I tend to be one of the artists of this site, though not the only one. I'm also an experienced tech, so while I may not be the guy with the most technical background I do tend towards reasonably adequate answers.

So allow me to paraphrase what I think the question is.

You have a TSR connector feeding a load off a power supply. If the load is not connected to this connector properly then you want an indicator to show it as such.

If this is correct then we will need the following information...

1. What is the voltages involved?

2. What is the current feeding the load?

3. Is there any potential for other current paths? If the cases of the power supply and device (AKA load) touch will there be sparks? Is the case of the circuit grounded?

4. What is the power supply? Is it a wall wart, or a battery, for example?

5. The exact type of connector we are talking about? I used to use TSR jacks to feed power to electronics, but they are among the worst types out their, as they tend to short the power supply when making/breaking connection.


Questions #1 and #2 are the most critical, but the rest will put what you want to do in context.

Note:
I missed the second page posting this. The TIP you are referring to is not obvious, it is not part of the vocabulary of most folks.

So you want to see if a 5V circuit is drawing 2.4 amps, and if not, light an LED on the power supply side.

 

Assuming you want an LED to light up if the current is being feed out is under a set current you are going to need something more complex than a transistor. You will likely need what is known as a shunt resistor to measure current.

Rshunt is going to need to be 3 feet 3 inches of solid 18 gauge wire. This is the shunt resistor for the project. Do not roll it up, as this makes an inductor. Instead, fold it in half again and again until it is manageable, then tape it up. This will make a 0.02Ω resistor, which will drop 0.05V off the power supply (this voltage loss should be small enough it won't matter to the load). The comparator will detect this voltage and switch the LED on or off according whether it is above or below the current limit set.

Like I said, if I knew your geographic location I might have referred to Radio Shack parts. This will measure the current to the circuit, and turn on the LED if it is under value. You can adjust the resistor as needed. Since I just realized I misread your post #12 (SCHEMATICS NEEDED, ALWAYS!!!) it will need some redesign.

At this time we're waiting for feedback.

To WKilla:

Feels odd responding to a post after the fact, but I don't think so. We're just going to have to wait for the OPs response so he can tell us.

To elbc1388:

I was going to refer to post #12, but I realized I misread it too (schematics prevent this). I can modify my schematic where needed (dagnabit!).


.

 

Bill, I think after re-reading his original post he IS refering to a TIP*** transistor. But the rest of is doesn't seem to make a lot of sense.

Toober, Draw your circuit as bill suggests. Scan it in and post it here. If you don't have a scanner try taking a photo of the drawn page. Make the schematic complete not just of a battery and an LED. The more info we have the better you can be helped. We are not trying to avoid helping you but we are also not mind readers either.

 

I guess this would be the OP's problem.

Lights an LED when the cable or the motor is open-circuited.

Condition: LED as fault indicator and thus should not be lit under normal circumstances, regardless if the motor is running or not.

 

Yes, I was referring to a TIP120. I apologize for not specifying that earlier as I did not realize the larger selections of TIPs that exist and perform differently. Seeing as how by my descriptions, you have guessed that I am not an electronics engineer or I would not have had this issue to begin with. That also means I buy my parts at Radio Shack and the only one single other place in this city that actually has a selection of components. I have no connectors yet as everything is on a bread board. I know that leds need a resistor and stated that in the first few emails. I figured the two were always connected together and did not think I needed to pound the matter more with extraneous, redundant information.

Post #18 is a good representation for what is drawn. The motor load will be close to the rest of the circuit. A 9V battery (hence, changing voltages as it gets used) is regulated down to the base of the TIP120 at 5.2volts at this point I believe. The emitter goes to ground (negative from the battery), the collector goes to one end of the motor with the other motor wire going directly to +9V (positive end of the battery) as shown in the picture in post #18 (thank you eblc1388).

I need the LED to light up if the motor is disconnected. I need the LED to NOT light up if the motor is working fine. I do not need the LED to tell me if the TIP120 is working or not. I was hoping for a solution with a few discrete components to keep it simple.

Bill, I will not be able to change led resistance once this circuit is built and sent to customers. But the customers will be able to change out different resistance motors. I will try to answer your questions:

1. What is the voltages involved?
9 volts, regulated 5.2 volts(depending on changes) to TIP120 base.

2. What is the current feeding the load?
I think around 2 amps sometimes, but probably mostly ~1.2 amps, depends on motor choice.

3. Is there any potential for other current paths? If the cases of the power supply and device (AKA load) touch will there be sparks? Is the case of the circuit grounded?
Other current paths may exist if it could draw from the regulator part. There is no case at this point and it may not be metal in final production. I do not think there could be sparks as the motor unit will be attached to this battery power supply all in one box (user changeable motor).

4. What is the power supply? Is it a wall wart, or a battery, for example?
It is a 9 volt alkaline battery right now.

5. The exact type of connector we are talking about? I used to use TSR jacks to feed power to electronics, but they are among the worst types out their, as they tend to short the power supply when making/breaking connection.
No connectors yet, it is assembled on a breadboard. I have not found a good place for connectors, Radio Shack is lacking in this area.

Hope this gives you a better idea of the circuit.

 

I don't think you will be able to use a 9V battery to drive a motor, they can not provide much more than 200ma, and then for a short while. You'll need to use X4 C batteries more than likely.

The circuit I showed will work at 12VDC or 6VDC, or what ever. I will redraw it to show a LM339, which Radio Shack carries.