Transistor and multimeter current

[Jared231]

Hi, I'm pretty much a newbie to electronics, but I've gotten a little way through a project and come across a problem (or at least an oddity) that I can't find an answer to. I built a simple 555 timer switch, that I used to control a flashing light. I added a NPN transistor(TIP 3055), then added a PNP transistor(NTE 2537). Everything worked fine while just the light was hooked up, but after that I wanted to see if there was any discernible power drop. I have a power supply rated at 30 volts and 4 amps. I checked it from after the PNP with a pretty common multi-meter. The volts read fine, very little drop. Then I checked the amps, and I don't know what to make of it. On the meter, the amps started to rise, past four amps. At the same time, the PNP started getting hot within a few seconds, so I don't know just how far up the amps would have risen. This transistor is rated at 110 volts and 40 amps and stayed cool to the touch for several minutes just running the light. The NPN appeared unaffected. Why would it start to climb like that and why is the transistor getting hot? Is there a way to fix this? I've used the same meter to check the power supply directly, and it works just fine; no buildup and no noticeable heat. Any advise is appreciated.

 

[kellys_eye]

Schematic?

Sounds like thermal runaway - although the amps can only rise to as high as your power supply is capable of delivering.....

 

[BobK]

How did you check the current? It sounds like you replaced the load with the ammeter. This creates a dead short. The ammeter must be placed in series with the load.

Bob

 

[(*steve*)]

If you're lucky you haven't damaged anything.

You can't measure amps the same way you measure volts.

It sounds like you have made the newbie mistake which often damages meters and destroys equipment.

 

[Jared231]

Schematic?

Sounds like thermal runaway - although the amps can only rise to as high as your power supply is capable of delivering.....

I think you're right. When I can, I'll try to mitigate, but at the moment I'll have to let it be. Thanks for your input.

 

[Jared231]

How did you check the current? It sounds like you replaced the load with the ammeter. This creates a dead short. The ammeter must be placed in series with the load.

Bob

Yes I was doing it that way, thanks for the info. I wasn't aware of that. Unfortunately the problem persists even if the load is still there. Something off about how my multi-meter measures. It still works fine for everything else, including amperage. Just something about this configuration.

 

[Jared231]

If you're lucky you haven't damaged anything.

You can't measure amps the same way you measure volts.

It sounds like you have made the newbie mistake which often damages meters and destroys equipment.

Luckily , I seem to have gotten away without any damage to either the transistors, or the meter.

 

[BobK]

Yes I was doing it that way, thanks for the info. I wasn't aware of that. Unfortunately the problem persists even if the load is still there. Something off about how my multi-meter measures. It still works fine for everything else, including amperage. Just something about this configuration.

No, you are using it wrong. You are placing the probes across the load. That shorts it out and shorts the power supply. You must break the connection to the load and reconnect the load THROUGH the ammeter. I.e. break the connection of the load,say at ground. Then connect the + load from the meter to the load and the minus lead from the meter to the ground. The current has to go through the meter in order to measure it.

Bob