SMPS repair tip

I worked at a pro audio company using wirewound power resistors to test Class D power amps. I couldn't convince them to consider the reactance effects at hypersonic frequencies, not only the switching artifacts but the Power Factor Controllers too, create a rich VLF stew. It won't hurt speakers but it may affect efficiency and heat load. Am I nuts?

unless these power resistors were very weird, as long tube shaped things they'd still have less inductance than a speaker winding.

"dave"

** The lower the resistor's value, the more significant inductance becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a dummy load - wired in series for 16 ohms and in parallel for 4 ohms. The ceramic tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz, the error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH. However the ringing frequency was around 3MHz making the inductive reactance almost 4 ohms - four times more than the value of the resistor !!!

.... Phil

However the ringing frequency was around 3MHz making the inductive reactance almost 4 ohms - four times more than the value of the resistor !!! "

You twitwitted unsuccessful doppelganging lowlife untermention !

No idea of the parts you are putting in MY equipment ? No wonder they need consumer watchdogs organizations.

Actually I am just joking dude. If I had that problem, I would have found it btu likely it would take longer. they do not give you the specs on a print for the voltage across that usually. They CERTAINLY don't give you a spectrum ananlysis.

Thanks for the tip - I've learned something today! Mind you most of the stuff I deal with doesn't work in those frequencies, but still, good to have in the back of one's mind when troubleshooting or installing replacement parts in items like switching power supplies or other medium to high (and RF of course) frequency things...

John :-#)#

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*** Yep.

" 1W wirewound, low inductance type, KNP ....... "

Because it looked so similar, it was stored along with MF types. I actually suspected the resistor pretty quickly, got another and tested it with my Bob Parker ESR meter - it read spot on. I did a 200kHz square wave test too and still missed it.

The particular SMPS operates at 100kHz while the ringing frequency is

3.5MHz - it has many outputs and runs ALL the low power electronics including a valve !!

FYI:

it's internal to a Chinese " Ampeg SVT-7 pro" bass head, which has a bridged pair of class D amps rated at 1000W into 4 ohms and a 1.2kW SMPS on the same PCB as the small supply.

The original fault was a mid air arc-over between the leads of two 1W resistors, spaced 8mm apart. So from +340VDC ( from the main SMPS's electro bank ) to the IC end of the 1ohm sense resistor. Blew 200mm of 2mm wide track right off the PCB, among all the other carnage.

.... Phil

A very complex reactance anyway. The amps were ca 10KW 12KW bridged into

4 Ohm loads made of 8 Ohm cabs series parallel. There were a lot of wirewound power resistors, in metal enclosures per OSHA, "switched" with Banana plugs. I like the resistors you mount on heat sinks like power transistors.

Can't wait to see one of those. I fixed an SVT3Pro with fried amplifier stage (everything north of the tube). Learned a few things. That was over a year ago and it still works. Knock on MDF.

I couldn't count all the times I ordered low value resistors and put on the page "NON INDUCTIVE". but rewally, just HOW non-inductive do I want it ?

Wire wound resistors already are wound with a bucking configuration. If you want a coil they make all the turns go the same way, if you want a resisto r they do not.

Alot of times we were taliking equipment that was not really working at all that high a frquency. I have always been careful about components that go into decent audio equipment for example. I do not use ECGs or SKs or wahtev er, which are fine for TV deflection circuits usuall, a few other things. B ut for the audio path I pay alot of attention to what I use.

But now this comes up. We knew it would happen. I mean we knew if you used a wirewound resistor in a certain application this might happen. What is ju s a bit surprising is that the voltage eveloped across your resistor is not filtered. I see such designs and kinda wonder, do they really need this cu rrent response so fast that they can't use an integrator to smooth it out ? that method allows alot more configs for current limiting.

Of course that is not the trend anymore. there are SMPSes that damnear shut down in one cycle in case of a short. This of course allows them to avoid overbuilding them. Need 20 watts diddipation, get a 20.0001 watt device. th at is engineering. You can't be a wasteful engineer - for long. None of my projects are ever likely to see a production line, but they will probably o utlive the designer and builder.

But that's not what sells.

diddipation ? lol

dissipation of course

It's getting bad folks

There's a price to pay for film type R's that are low inductive, they usually can't handle a very short high peak pulse. POP!

Carbon composite is just about the best in both worlds, they handle transients and are not very inductive. We use these types for a HV divider string from 2Mev down to a few uA for a reading. The signal is at 100Khz so can't have induction there and also can't have the R's popping when we get a now and then snap of voltage that causes a short transient.

Jamie

I couldn't count all the times I ordered low value resistors and put on the page "NON INDUCTIVE".

**Only way to get that is to order carbon composition types.

Wire wound resistors already are wound with a bucking configuration.

** That is so rare I have never come across one.

If you want a coil they make all the turns go the same way, if you want a resistor they do not.

** Fraid they do, resistors are nearly all coils - but it hardly ever matters.

.... Phil

"Phil Allison"

** See link to good pic of the insides of one:

Bottom of page, click on image to enlarge.

The high power SMPS uses just two TO220 pak IGBTs while the 1000W bridge amp uses only four TO220 Mosfets.

There are no class D amp chips and the mode of operation of the output stage is mysterious.

.... Phil

Just for short duration testing purposes I suppose, or back to the bad old days of barbecue charcoal laying in wait, in sustained overload situations

I hope you used special gold plated, cyogenic magical copper cables for all that.

I just toured a plant that had gigantic resistor arrays mounted into what appeared to be bread racks. All the resistors were sealed into oil filled glass tubes, in house. Any like anything used on a shop floor, it had lots of banana plugs, but with upto tens of thousands of volts across them when energized. Each jack for each tap into the string was Dymo labelled. I can't imagine how much it cost to make each one.

The really scary stuff was done in rooms with dented expaned metal cages.

Carborundum resistors are used for RF.

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** Sure - but this is a current sense resistor in SMPS circuit where most of the current is at 100kHz but with a strong harmonic at about 30 times that frequency. Problem being, the over-current trip was being activated prematurely because the peak value of that harmonic was being exaggerated by a factor of 3.

While 1W metal film resistors have low enough inductance to work OK in the circuit, typical wirewound types do not.

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound to bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W film type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2 ohms corrects it to beyond 5MHz if that was ever needed.

.... Phil

What is a Tripath amp?

No. 14g stranded usually. 12g for the woofs.