Amp's power MOSFET died. Can I just remove it?

"current-hogging".

If they are in parallel it is quite necessary. If they have source resistors, it is less necessary, because those provide a form of current feedback.

I built myself a very simple rig to do the matching, but I don't remember offhand the component values I used. I'll try to post the schematic later tonight. It basically hooks the MOSFET up in a two-terminal current-sink configuration (gate connected to drain) and sees how much current it draws. In principle you're trying to match two curves, so you want to measure Ids for a lot of different Vgs and get the best overall match. But in practice, using a single point has worked okay for me.

Do you have a dummy load to test the amp against, once you've got it fixed?

Yeah, it's hard to find TO-3 replacements. You probably won't do better than Profusion PLC. (And, I don't have any matched equivalents for those, sorry!)

On Tue, 18 Nov 2003 11:55:49 -0800, Walter Harley wrote (in message ):

Thanks!

Someone suggested a couple of Walmart electric heating coils in series. Sound good?

Profusion now says that the MOSFETs they will sell me are, due to good QC, closely matched, and are good enough for parelleled service.

Thanks,

I read in sci.electronics.design that DaveC wrote (in ) about 'Amp's power MOSFET died. Can I just remove it?', on Tue, 18 Nov 2003:

No. You need an **8 ohm** resistor, of at least 250 W rating. (Assuming you amp needs an 8 ohm load, which most of them do.)

On Tue, 18 Nov 2003 23:56:52 -0800, John Woodgate wrote (in message ):

The person suggesting this solution stated that the heating coils (I think they are actually electric tea kettles, IIRC) were each about 4 ohms. Not exactly, but close, and a resistive load. Two in series seems quite a good idea. And I can fill them with water to dissipate the energy, and make a pot of tea, as well :-)

I'll look around for a large wirewound resistor, but 200 W? I don't hold out much hope, even here in Silicon Valley...

Thanks,

You should find it easily. This sort of resistor is a lot more common than you may think. However thick film types are tending to superceed wire wound designs. Vishay, for instance. list a RPS500DH, which is a 500W metal film module, commonly used in things like power inverters, and a 1000W wirewound part. Generally though, in 'one off' quantities, kettle elements are cheaper!.

Best Wishes

I read in sci.electronics.design that DaveC wrote (in ) about 'Amp's power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003:

Are these for use on vehicles, from 12 V, then? I didn't see that article.

You can buy high-power resistors from Farnell/Newark and they are not costly. They are even in the horrible UK Digikey catalogue, as 'Aluminum Housed Chassis Mount Resistors. Eight 1 ohm TMC-50 type in series give you 8 ohms at 400 W if you bolt them to a big sheet of 1/8 inch 'aluminum'.

The fixture I use to match MOSFETs looks like this:

Vdd

| .-. 150R | | 25W | | '-' | .-------o '----||-+ | | === GND created by Andy´s ASCII-Circuit v1.22.310103 Beta

I use Vdd = 15v, for MOSFETs comparable to what you're using. Measure Vgs at the indicated point (or at the gate, but I like to keep the gate resistor physically close to the gate). You should see something around 3 or 4V, depending of course on your particular MOSFETs.

Note that the value will CHANGE substantially as the MOSFET heats up - for TO247's I actually mount a bunch of them to a big hunk of aluminum, and then test them all in succession a couple of times and throw out the outliers. But for TO3's that's harder, so basically you just need to be aware of the effect and try to treat them all the same.

What everyone else said. Personally, a while back I managed to find a deal on a bunch of 50W 10R resistors, and soldered 'em up in series and parallel to get two 500W 4R loads, which I can then put in series for a single 1000W

8R load if I want, or use as a stereo load. I mounted the whole deal in a box with a fan on it so that it can do something with the heat. Nice hair dryer. Use high-temp wire and be aware that it'll be thermal-cycling so connectors may loosen.

It's worth a shot. I'd still check the matching myself.

John Woodgate wrote: || I read in sci.electronics.design that DaveC wrote || (in ) about || 'Amp's || power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003: ||| The person suggesting this solution stated that the heating coils (I ||| think they are actually electric tea kettles, IIRC) were each about ||| 4 ohms. || || Are these for use on vehicles, from 12 V, then? I didn't see that || article.

A 115V 1500W electric kettle apparently has 7.6 Ohms (when warm probably). But you have to get an American one, difficult in Europe. Anyway, a simple multimeter can verify this. Eventual inductive behaviour can be compensated with a parallel R-C combination, but I do not think this will be needed in the audio range. Of course the 200W will not heat up a lot of water, so put just a minimum amount into it. Then you can make a nice Nescoffee after a reasonably timed test. (Only English make tea). Precision will not be possible, but for an amplifier test it will suffice.

ciao Ban

On Wed, 19 Nov 2003 10:14:48 -0800, Walter Harley wrote (in message ):

Thanks, Walter.

I'm a bit confused about the connection of the FET:

The drain and gate are tied together (per your earlier post). These are connected to Vdd through the 150 ohm resistor?

But then you provide a gate voltage via 470 ohm resistor?

Source is tied to ground?

I think the ASCII is screwing me up...

View the ASCII with a fixed-width font. It should look fine.

Source is tied to ground. Gate is tied to drain through a 470R resistor. Drain to power supply through a 150R power resistor. The only reason for the 470R gate resistor is to eliminate high-frequency oscillations that could skew the results. It's probably not necessary, but I just don't like connecting a MOSFET gate directly to something like a test lead that could have arbitrary inductances, capacitances, etc.

This circuit is a "two-terminal current sink". It is a self-contained negative-feedback regulated system. If current flow increases, that makes the voltage drop across the 150R increase, which drops Vgs, which decreases current flow; and vice versa. So, Id is constant; all you have to do is

*measure* Vgs, you don't independently control it.

The number you get doesn't really tell you a lot about the MOSFET. But two MOSFETs with the same Id-to-Vgs curve will show the same Vgs in this rig, and that's all you care about.

If you wanted to actually plot the whole curve of Id versus Vgs, you would need to separately control Vgs. But there's no reason to do that, because you're not going to buy enough MOSFETs to be able to do matching on that level, so you'd only make yourself unhappy :-)

-walter

You could build a dummy load in a paint can (available at hardware stores) filled with mineral oil to dramatically increase the power handling of the resistors. The kettle elements sound like a good idea to me though.

I like the Dale NH-250 series but they were not cheap last the time I priced them. A freind of mine used to use toaster coils from the realy old ones that used ceramic insulators. They would glow pretty good on a larger amp. Jeff

Interested in professional sound reinforcement and distribution? Then go to