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Question about transistors?

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Plasmaarc452

Mon Sept 21 2009, 10:47PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

Ok its been forever since I posted here but I'm revamping my electronics hobby so I will be posting here much more often now.

I was going through a whole bunch of small mouser/digikey bags of parts and came across a new 600v 40amp NPN IGBT. I hadn't done to much with transistors and the arduino yet so I decided to give it a shot. I hooked it up to my micro controller using this diagram. (I took out the led and the 2nd diode!)

This was hooked up to a 16v wall power supply and the device it was driving was a 60mm computer fan. The code that I put on the arduino simply toggled a D/O pin HIGH and LOW every 10 seconds over and over. The circuit worked and the micro controller was turning the fan on/off. But here's where I was puzzled.

I hooked a voltmeter up to the output leads going to the fan and when the fan was off I was reading 16 volts! When the micro controller turned it on the voltage dropped to 14.5v because of the load and the fan started to run.

So my question is why was there voltage on the output leads when the transistor was off? Shouldn't it be 0 volts?

Sorry if this is a newbie question!

Myke

Mon Sept 21 2009, 10:57PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

It should be 16V because the transistor looks like a lot higher impedance compared to the motor. Try connecting the probes across the transistor. When the transistor is on, you should read a low voltage and when it's off, you should read 16V. The transistor drops some voltage when it's on so that's why you read 14.5V. Hopefully your power supply's voltage isn't sagging too much.

EDIT: Wait... you measured 16V across the fan when it was off?

Plasmaarc452

Mon Sept 21 2009, 11:11PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
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Posts: 111

Yes, I measured 16 VOLTS across the fan when it was OFF as in the fan was not moving at all. When the transistor turned on the voltage dropped to 14.5v due to the power supply (its cheap) and the fan started moving fast like it should. Is this a sign of a bad transistor?

Now that I think about it, it may be a bad transistor as a few years ago when I bought this stuff I think I may have put these on a breadboard and hooked them up extremely wrong. I guess I must have put it back in the bag lol. When my parts from Sparkfun get here later this week (just shipped) I will have a parts kit (and a crap load more) which contains small transistor that I will try and hook up.

kell

Tue Sept 22 2009, 02:23AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
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Posts: 102

You say with 16 volts across the fan it doesn't run, but with 14.5 volts across the fan it runs.
This does not compute.
Draw a diagram for us.

Voltwad

Tue Sept 22 2009, 04:09AM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

It sounds like you may have measured the voltage from the +V fan lead to Gnd instead of +V fan to fan Gnd (IGBiT collector). That would make sense if the transistor dropped 1.5v when turned on (14.5v across the fan).

TheMerovingian

Tue Sept 22 2009, 10:47AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

It is not driven in saturation probably, but remember that 1.5V for a high voltage IGBT is a very low voltage drop, so don't be surprised, maybe driving in saturation you can lower it a bit (1.4V) but not too much

Plasmaarc452

Tue Sept 22 2009, 07:31PM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

kell wrote ...

You say with 16 volts across the fan it doesn't run, but with 14.5 volts across the fan it runs.
This does not compute.
Draw a diagram for us.

I know that's why I asked the question in the first place it seemed very odd for it to do this. I think the transistor is just stopping most of the current from reaching the fan and when I juice it up it allows the current to flow. I am starting to think more and more that it is in fact a bad transistor. I will also try disconnecting it from the arduino next.

Here is a diagram of my setup and yes when the fan is off it reads 16volts.

EDIT!!! I put battery in the diagram lol it is actually a wall power supply if that makes any difference.

IamSmooth

Tue Sept 22 2009, 09:14PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

why don't you just try a different transistor and see what happens?

Better hurry, because I don't know how long this guy -->

can hold his breath.

wylie

Tue Sept 22 2009, 11:51PM

Registered Member #882 Joined: Sat Jul 07 2007, 04:32AM
Location:
Posts: 103

Long-time reader, 1st post. Gonna try an answer for this one to test my knowledge.

Sounds like business as usual.

With the switch off, the full supply voltage will appear across it. Once the switch turns on and current flows, the voltage reduces according to the drops from the load and the switch.

If your transistor was broken, it wouldn't be switching the fan. The current would be stuck on/off (ie: failed short/open).

Reading from both leads on the fan, you should see 0V in the off condition, but as Voltwad pointed out, if you measure from 1 lead on the fan to the circuit ground, you'll still measure the supply voltage (minus drops in the on-state).

Also, there seems to be an error in your diagram, with the mcu output attached to the collector (going with "collector" based on the red/black color coding and an NPN device). I assume the arrow+label should be pointing at the base connection in the middle heading south off the page.

Plasmaarc452

Thu Sept 24 2009, 12:36AM

Registered Member #1394 Joined: Sun Mar 16 2008, 06:18PM
Location:
Posts: 111

Ok I think part of the problem was that I had it hooked up the wrong way to the micro controller due in part to the schematic and a data sheet for a similar transistor (it was the wrong one lol). I tried using a 6v battery with no mcu and hooked it the other way. This time it worked the same but I was reading a very low voltage like 90 millivolts.

I am fed up with this transistor tomorrow morning fedex will be at my doorstep with a nice package from sparkfun. I got a parts kit along with a bunch of other stuff so I will have at least 10 transistors to play with that I know work.

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