@SamR Yeah, I have a couple tins to toss stuff into every now and then. More like every "then".

 

Probe @ neg, Near full On pic & scope
File size for picture of the board and connections is too large.
Since scope probe is at negative, the IGBT is almost always on, pulling the voltage across the bulb to ground. That's why it looks like it's nearly full off. It's nearly fully grounded.

Scope settings may have been change during the test. Horizontal position was adjusted. @ 5V below center line is zero for parts of this test.

 

Near full off. Hmmm. Some ringing going on. Don't know why.

 

 

IF I reduce it to 100Ω then that'd be 120 mA and the 555 is rated for 200 mA if I read the data sheet right.

Here's the thing about an emitter follower - the emitter voltage will rise with load current, and so Vbe will fall to the point where the transistor has almost shut off. You will not have the full voltage across the base resistor. With a capacitive load, the transistor will shut off completely and the base resistor ends up with 0 volts across it. You might even delete it from the circuit. In the simulation in post #16, with a 250ohm base resistor, the gate is charged and NPN transistor is turned off by 0.2us. The discharge is essentially 0 ohms through the 2N2369, and takes only 0.1us. Your 1K base resistor might stretch these out to 0.5 - 0.8us in both directions.

 

Since scope probe is at negative, the IGBT is almost always on, pulling the voltage across the bulb to ground. That's why it looks like it's nearly full off. It's nearly fully grounded.

What is meant by Probe @ "Negative" and "Collector"? Aren't these the same node? It's labeled "Out" in the schematic in post #16. Please clarify.

 

My mistake. It should have said "Emitters". I was looking on my white board where I initially drew out the circuit the way I built it. I then drew it out the way you suggested when you said they should be swapped. I did it on the white board. When I was touching up the photos I mistakenly looked at the diagram for the way you suggested them. Yes, collector and ground are the same things. Sorry for the confusion.

The scope probe was placed in the following locations: 1)Across the negative node for the light bulb. 2) On the EMITTERS (not collectors - it wasn't built that way). 3) On pin 3 of the 555 (not shown in this circuit). If need be I can post the picture - but with the 100K pot, not the actual 10K that is there now (temporarily).

What do you think about the ringing? What can be done about that?

 

Well, here's the whole diagram, less the motor. Also DP1 is not yet in circuit. The only load on the board is the test lamp.

 

Earlier I tried to upload the file. Instead, I opened a preview of the photo and then copied and pasted it below. You can see the test lamp in circuit. This photo shows the scope probe attached to the negative node of the test lamp. The lamp can be lit to near full brightness or dimmed to the point where it extinguishes. When a small computer fan is placed as a load the fan goes from near full to near stopped. It continues to spin slightly.

 

The lamp can be lit to near full brightness or dimmed to the point where it extinguishes. When a small computer fan is placed as a load the fan goes from near full to near stopped.

It sounds like it's working pretty well. That ringing is crazy though. It hits -12V! I wonder where the inductance is coming from?

 

Could it be my power supply? It's an ATX computer power supply modified into a bench top supply. It has 3.3V, 5.1V and +12V as well as -12V, all referenced to ground (or common or whatever its zero point is).

 

Any reason why you are not driving the gate directly from the '555?

Bob

 

I don't see any decoupling capacitors.
The addition of those, especially one from the blower motor's V+ connection to the IGBT's emitter, should reduce the ringing.
Make sure the high ground current has a direct path to the supply and does not go through the circuit ground
Also minimize the length of all the leads carrying high current or use twisted-pair wire, which reduces inductance.

 

Looking at the data sheets for the 2N3904 and 3906 transistors.

What does "Delay" "Rise" "Storage" and "Fall" times mean? Aside from the obvious - they have to do with timing. Most curious about "Storage". (see below for my issue)

_3904 ________________ 3906_
-35 nS ---- Delay --- 35 nS
-35 nS ---- Rise ---- 35 nS
200 nS -- Storage - 225 nS
-50 nS ---- Fall ----- 75 nS

This circuit (in part) (below) is operating at 12 volts. R2 is a square wave input (high / low). The 3906 is getting warm. After a minute of operation I'm measuring a case temperature of 99˚F. This is concerning! The red line is 12 VDC (no resistance) and the black line out of the bottom of the 3906 goes direct to ground. This should only be charging and discharging an IGBT Gate.

The IGBT gate capacitance is:
Cies --- Input Capacitance --------------- 5770 -- pF VGE = 0V /// VCC = 30V
Coes -- Output Capacitance ------------- 400 --- pF
Cres -- Reverse Transfer Capacitance ---- 100 --- pF

View attachment 197363Cut and pasted from a larger diagram. Available if necessary - but many have already seen it.

Cies --- Input Capacitance ------------ 5770 pF VGE = 0V /// VCC = 30V
Coes -- Output Capacitance ---------- 400 pF
Cres -- Reverse Transfer Capacitance - 100 pF

It takes time for voltage to rise and drop- nothing is instant.

That's from the 2N3904 datasheet. Notice how the voltage has to rise, holds for 300ns, and then falls in 1ns?

Get something like this book. Seriously. Learn before doing, then you understand what you're doing. If you think electronics is simple, you're being fooled, lied to, or mislead. If you hope to do anything significant, you have a great deal to learn, first.

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

Any reason why you are not driving the gate directly from the '555?

Bob

Tried it. Don't know why it wouldn't work.

I don't see any decoupling capacitors.

This is the scope after putting a 102 cap (1000 pF) across power and ground as close as possible to the chip.
5VDC per major. 2µS sweep. Zero dead centered. PWM set to near zero.

 

Some of the ringing can be on the ground.
Connect the probe next the ground connection on the circuit and view the voltage.
The scope probe ground connection needs to be as close as possible to the probe signal connection (you are using a 10:1 probe?).

 

(you are using a 10:1 probe?)

Yes. But set to 1X.

The ground clip is very short - about 4 inches.

 

Yes. But set to 1X.

Why?
It should always be set 10X to minimize the probe impedance and its effect on the signal, unless you need the maximum sensitivity at the most sensitive oscilloscope vertical gain setting.
At 1X the probe likely adds upwards of 100pF to the node capacitance, which can significantly add to the observed ringing.

 

Put a 104 (0.1 µF) cap across R2. There's been some improvement. I'd say the ringing lasts 3 minor divisions whereas the former (without the 104) the ringing lasted 1 major division.

 

@crutschow I'll give that a go.

[edit] that improved matters greatly. Still getting a negative shoot through of -5V.