I hate MM. Isn't that 25.4 mm = 1"? So .165 x 25.4 = 4.2 mm??? Adds up to me. Looks like RNE1 (1 watt). Confirm?

 

OK, so now I am hearing that the IGBT does not switch off with some level of load. I have not used those devices and so it is not clear to me just why. So another opportunity exists is to get one or more PNP power devices to use in parallel, but start with one of them. Connect the +12 to the emitter, connect the base to that sinking output through a 1K resistor, and connect a load from collector to common. 50 volt PNP power transistors should be fairly cheap and even available surplus. Probably the base resistor will need to be less resistance because enough base current is needed to keep it into saturation at the maximum load current. So you will need to look up the gain and the minimum base current versus collector current for switching applications. And then you will need heat sinking. Since the dimmer controller is a pull-down current sinking device it really should do well controlling a PNP common emitter. device.

 

OK, so now I am hearing that the IGBT does not switch off with some level of load.

Not sure where you heard that. Maybe I misspoke. Using the IGBT as a load follower, power through a light bulb to the IGBT Collector, the Emitter to ground, when I strike the Gate with 12 VDC the light bulb turns on. AS SOON AS I REMOVE THE 12 VOLTS from the gate the light bulb goes out. Rinse and repeat. The IGBT is working. But my guess would have been that once turned on with a gate voltage the IGBT would have remained on until it was grounded. So I'm definitely beyond any experience or understanding of the device.

 

Not sure where you heard that. Maybe I misspoke. Using the IGBT as a load follower, power through a light bulb to the IGBT Collector, the Emitter to ground, when I strike the Gate with 12 VDC the light bulb turns on. AS SOON AS I REMOVE THE 12 VOLTS from the gate the light bulb goes out. Rinse and repeat. The IGBT is working. But my guess would have been that once turned on with a gate voltage the IGBT would have remained on until it was grounded. So I'm definitely beyond any experience or understanding of the device.

I read that part, and also the part about it not working with the dimmer control, in that it would not shut off. At that point the whole thing is not clear as to voltages and currents.

 

OK, here's what I'm about to build:

 

it would not shut off.

Wired backward. When wired correctly it works. Apparently there must be a fast recovery diode internally.

 

OK, somewhat successful as drawn in post #125. I can dim a light from 0 to 100% with 180° of rotation of the dimmer control. The pot can actually sweep 270° rotation, but that last 90° does nothing but full power. The PWM width is not uniform. I will have to see about some photos of the scope. But as the dimmer is turned the light grows in brightness and the PWM shows a slope, growing until it reaches the top most trace. The width is about 50% (estimated), so it can definitely be improved. Will be back with photos in a bit.

 

Picture 01 Full OFF
Picture 02 First instance of light (about half brightness)
Picture 03 Near Full ON
Picture 04 Full ON
Picture 05 IGBT Gate Signal at First Instance of light

O-Scope settings: Zero volts CENTER LINE. 10 VOLTS MAJOR DIVISION. TIME DIVISIOIN 1 mS PER MAJOR DIVISION.

 

NIce, I miss analog scopes, the traces look so nice. That looks like it might work, Are you ready to put a real load on it?

 

If those are photos of the load voltage, and the power supply is staying steady, I see a problem in the future, which is a whole lot of power dissipated during the rise time, or fall time, or whichever it is with that slope.
So I am going to ask you to look at the drive voltage waveform coming out of that controller and let me know if it is a similar shape.

 

I'm surprised to see the gate voltage drop so slowly.

 

@Wolframore Not quite yet. I want it to look better than this before I put a big load on it.

@MisterBill2 I may be measuring it wrong. At first I put the scope across the load (the lamp) and you see the results. I've tried measuring the voltage across the IGBT and got an even stranger waveform, kind of inverted form of the last photo. But that IS a slow rise time. Maybe the 10KΩ is too big? I could tack another resistor across it just to see how/if the wave changes. Inside the dimmer there are two caps, 100nF and #2.2µF across Ground (negative) and Gate (inside the dimmer, see post # 12). Wondering if they're delaying the rise time or perhaps in the way they're connected, holding the MOSFET high but dropping slowly, causing the IGBT to do an inverted wave form.

The output was nice and square before I added anything to it. Nice sharp corners and near vertical rise and fall times.

@Chris65536 This is something I want squared away before I go any further forward with this.

So far the load has been either an auto light bulb or a bulb AND a small DC fan. Fan speed varies more uniformly than what appears to be with the lamp. But I don't have a calibrated eyeball, so I couldn't say that was definitely true. And I have control over 2/3 of the sweep of the pot. I'd like the full range. So far this project hasn't run into any money yet. But if need be - - - .

 

I'm surprised to see the gate voltage drop so slowly.

I'm not. The way it's wired when the PWM goes to the ground/common part of a pulse, it's not only deleting the gate capacitance but also trying to ground the + voltage of the whole circuit. This is why I keep saying it isn't going to work satisfactorily.

 

I'm not. The way it's wired when the PWM goes to the ground/common part of a pulse, it's not only deleting the gate capacitance but also trying to ground the + voltage of the whole circuit. This is why I keep saying it isn't going to work satisfactorily.

Did you see the latest version of the circuit? (Post #125) I'd be interested to see the scope display of the sinking dimmer output. Maybe a load resistor from the dimmer to + would help the PNP turn off better?

 

@Chris65536 The fifth trace photo is the signal at the gate of the IGBT. The problem I'm running into is the transistor I used doesn't handle a ton of wattage, so I can't go much lower to shut the transistor off any quicker. Maybe a push/pull arrangement might facilitate shutting it off quicker, but even the signal at the base of the transistor isn't sharp like it was without a load on the dimmer (sink pin).

@shortbus I was originally thinking of NPN transistor applying power to the IGBT, then a resistor to pull the gate back to zero when the pulse goes away. I don't recall (without going through all the posts) who suggested a PNP transistor. Was that you? I originally thought an NPN would pull the IGBT Gate low when the dimmer was "NOT" low. I say "NOT low" because it does not have a positive going edge. Just an intermittent sink, depending on the brightness setting. Let the NPN transistor go high and provide the IGBT Gate a hard voltage and then quickly shut off. It will act like an inverter and I'm not needing all that high of a wattage to control the base. The pull down resistor could be higher, as long as it pulls down fast enough. There's not a lot of capacitance on the IGBT Gate, so it should shut down fairly quickly. Given it shuts off quick enough without a pull down. That is to say, the IGBT conducts when the gate is powered. But as soon as I remove the +12 at the gate the iggy shuts down quicker than the filaments in the bulb can cool, so it's not possible to see any capacitive voltage drain-off of the gate. I haven't set up to scope that aspect yet. It may be a while before I can get back to the project. Looking at a job assignment in Arizona for a few weeks. Don't know yet.

 

I said in an earlier post that I've never seen any one trying to trigger a mosfet or IGBT in this way and stand by that. A sinking alone of the PWM module is not going to get this done. A dedicated low side driver with an inverted input would. Something like the UCC27517A. There are probably others that can do it but I know about that one.

There can only be one connection to the gate(other than a pull down resistor) and that connection has to both source and sink, and most are 1AMP or higher, so the gate is charged and discharged fast. Doing it any other way and you will just spend time and not get any real good switching from it. Sorry if this is thought of as changing the project but facts are facts.

 

facts are facts

Granted, they are.

The end goal is to have a working circuit. If I learn something along the way then I learn something. And like I said before, sometimes stubbornness can be overcome by stark reality. When something doesn't work - or doesn't work satisfactorily - then it doesn't work.

I'm now considering going the route that was suggested early on and build a circuit using a 555. But also as I've said before, I'm not in any hurry. I'll dig through my treasure trove of circuits and find one that suits using a 555 for a full sweep through the pulse width. All I can think of right now is one that can go only 50%. I'm sure I have one. If not - the internet is full of schematics. I'm sure I can find something.

 

I've decided to try a 555. Found this circuit in my treasure trove and made a few modifications. Let me know what you think. I may have to go with a smaller C1 cap.

OK, have at it. Let me know what you see wrong with this schematic:

 

Said I wouldn't respond but have very little impulse control.

Why the connection between the 555 pin #3 and the pot? Not a normal thing, but then again none of this is normal.

The 555 only sources(though when it is in the output on mode they call it sink?) less than 200mV. Not only that it can't sink and source at the same time, so your IGBT still won't switch. http://www.555-timer-circuits.com/using-the-output.html

If I learn something along the way then I learn something.

But yet when I try to tell you something, things that don't go along with the other people here that some admit they have never done this, you only listen to what they say. So I'll ask you what you asked me earlier, what did I do to you???

 

Why the connection between the 555 pin #3 and the pot? Not a normal thing, but then again none of this is normal.

Pin 3 goes high and low. Pin 7 - the "Discharge" leg goes open or to low (ground).

Pin 3 goes high to charge timing cap C1 through the pot and D2. When it is low it drains the cap through D1. This way the pot should produce a full PWM from near zero to near full speed. No? I'm in unfamiliar territory now - so don't think I won't be listening. One reason why I didn't want to build a 555 was because of lack of familiarity with the chip. Had a working dimmer and thought it should be easy enough to go forward.

The 555 only sources(though when it is in the output on mode they call it sink?) less than 200mV. Not only that it can't sink and source at the same time, so your IGBT still won't switch.

Really? I thought with the high from pin 3 the gate of the IGBT would turn on and when pin 3 went low the IGBT would turn off. Hard and sharp transitions with very steep slopes to the on and off wave forms. Again I ask - NO ? ? ?

when I try to tell you something, things that don't go along with the other people here that some admit they have never done this, you only listen to what they say. So I'll ask you what you asked me earlier, what did I do to you???

It's hard to listen to everyone. Like driving a car and one person is telling you to turn left at the light and someone else is telling you to turn right. Who do you listen to? Sometimes it's a crap shoot. Partly because I had a notion that my circuit would work. Given that assumption, the people telling me how to move forward with what I already had sounded to me like the right thing to do. Don't take offense because I followed what ended up being advice for a non-workable solution. To be honest, I have no idea who knows more and who knows less. People talk a good game and those are the ones that are listened to more often than not. And when someone (you) disagree with them (the others), you don't always win.

Look for a PM.