What is this resistor?

[leonzak]

I am working on my PMP5000 Behringer. The speaker outputs got shorted and I've got a few bad transistors on one channel and on the other channel this resistor is burnt out as well. Here's a picture of the channel with a good resistor. Before I order and replace it I've not worked on surface mount devices in a while - is this R137 a 30 meg ohm or is it coded and something else? And what or how do you interpret the 0E03 next to it?
Thanks.

[dmeek]

It's 30 milliohm .03 ohm.

m = milli
M = Mega

[leonzak]

Thank you for your response - I saw the 30m on the part - but (old dog trying to learn a new trick) how do I get that from 0E03 on the print? And is the wattage in that or is just done by size?

[Helmholtz]

0E03 = 0.03 = 30/1000 = 30 milli.
For wattage go by size.

[leonzak]

Is the E a multiplier or other designation? What would 30 or 300 or 3k look like?

[J M Fahey]

E represents the decimal dot/divider, for two very good reasons, both trying to avoid confusion.

1) Americans (and guess British too) and Imperial in general use a . as a decimal divider, as in 1/10th of X is written 01X Notice the almost invisible dot.

Europeans and Metric guys in general use , as a decimal divider. So 1/10th X is written 0,1X

That alone is confusing enough, and possibly VERY dangerous.

2) we now keep everything as graphic files which can get *exact* copies but for over half a Century the standard was a Photocopy

Which is quite a crude operation, famous for removing small detail (such as dots, fine lines and small print) and adding its own fake on e, such as random dots all over the place.

So Europeans started to use a letter as decimal separator. Most often an R (from resistor) so they wrote 0R1 , you will find lots of those.

Some use an E. Not sure where it came from, but interpretation is clear.
Only guessing, R seems to be used for regular values, say 1 ohm, (1R), 10 ohm (10R) etc. while E appears in less than 1 ohm resistors, say E33 , E22 , etc.

So some write 0E1 or similar, as here.

So 0E03 means 0.03 ohms or 30 milliohms, same thing.

I thought it would be used for current sensing and I am right.

It must trigger some kind of protection.

Overcurrent over it triggers T16, an MPSA92 PNP mid voltage (150V) transistor.

How much current?

A general purpose transistor needs abut 065Vbe to be turned ON

IF Vbe were straight across R137b that would mean: 0.65V/0.03 ohm= 21.7A (always rounding a little to keep numbers simple) but we have a voltage divider: R75 and R112, so actual trigger current is 1.3*21.7A=28A

This amplifier is clearly a beast.

[Chuck H]

Ok... I don't know this circuit and I'm not an engineer. I only work with tubes and very rudimentary SS circuits. So "I" need to ask, what is the point of a 30 milliohm resistor? I didn't even know they spec'd resistors at such low values.?. In my (much simpler world) you might read this across a piece of lead wire. Provided you had a meter that could even do that accurately. Seriously, thirty TOUSANDTHS of a single ohm.?. Somebody please tell me that this is intended as a shorting resistor in the circuit.

[Chuck H]

My post was made while Juan was posting so now I have his to reference. So this resistor is basically a fuse? Why not use a fuse?

[leonzak]

Thanks Juan, great explanation. Chuck does have a good question - why not a fuse?

And yes - this amp is a beast with 600 watts per channel - a great piece of equipment. During a gig it got knocked over and the outputs where shorted.

I've ordered new parts - got them at newark.com - the resistor is a 2 watt - I did find it by size.

[x-pro]

A fuse is needed to completely turn off the unit when the current is exceeded. And it is a non-linear element.
The resistor with the entire circuit around it is current protection. The element is constant, normalized by resistance. The current limit can even be monitored further by the circuitry and displayed as "prot", and/or fed into a power limiting circuit.
Why did he burn out?
It is possible that the outputs were short-circuited during high signal power. In this case, the protection did not have time to work. Any protection has a response time. Also, the control circuit, if any, may not have worked.

[leonzak]

Thank you x-pro - it did happen while playing so very feasible during high power. The speaker plugs got smashed and most likely it was a dead short.

I appreciate all the help from everyone.

leon ...

[J M Fahey]

It is better than a fuse, which is quite a crude device.: no precision, can take from milliseconds to hours to blow, a mess. We use them only because they are cheap, simple and cheap, did I mention cheap?
While this circuit will trigger "something" (a relay? a Triac?) at exactly 28A.
Say within 10% (parts tolerance), also Vbe changes with temperature, but in any case immensely more precise than any fuse.
We should follow the path to see what is actually being triggered, of course.