Use board trace as a low value "resistor"

[MJL21193]

Is this advisable? I want to do this (novelty mostly) as the base stoppers in an amp project. I found a trace resistance calculator that will give the resistance for a given length, width and copper thickness.
I have seen this somewhere else, on a board somewhere and I thought I'd pose the question.


An example of what I'm thinking:

[sam9]

Yes it works. Traces have also been used to create capacitors and inductors.

Three disadvantages:
Heat dissipation is limited, which may or may not be important. It depends on the current that will be present, so use as an emitter resistor on a the output devices of a power amp is likely a non-starter!

It may be hard to control the process. The importance of this depends on how critical the value and consistency of the value are. With large production volumes and lots of money these can be overcome.

If will be messy to change values if the need arises.

[VivaVee]

The biggest problem is the absolutely hopeless tolerance involved for practical PCB fabrication.

A real example: assume that you are happy to have a resistive trace that is 0.5 inch long and use 1/2 oz copper.

A 4 mil wide trace will give you 0.12 ohms.

The problems are:

1. You will NOT be able to make this at home
2. You will be seriously limited in your choice of a fabrication house
3. If you find a fabrication house to manufacture your pcb, then the width accuracy will be +/- 1mil typically. This is +/- 25% tolerance for the width alone.
4. The plating thickness will vary, typically +/-10%
5. It will consume more PCB real estate than a comparable discrete resistor (the resistor needs just two pads)

So you have a 0.12 ohm resistor that must be manufactured by a specialist PCB fabricator that will have an approximate tolerance of +/-35%.

If you really want to get depressed... check out the temperature coefficient of copper.

Note: the calculations get worse if you use 1oz copper which is likely for a power amplifier.

Conclusion: use regular 0.5W 1% metal film resistor!

[Jan Dupont]

Traces have also been used to create capacitors and inductors

MJL21193;
A big problem using the track example you have attached is infact what sam9 told in the quote above.....

The track will act both as a resistor, a capacitor and an inductor. You can get in serious trouble if you don't presicely calculate the capacistance and induction for such "resistor" track. You end up with what's equal to a wirewound resistor with a small cap in parallel !!!! This can cause both stabillity problems and cause very bad high frequency reproduction.

You can make it, but you have to do some practical tests and measurements to uncover all problems mentioned above.

[h_a]

All gave good points already; I want just to remember once more that most board houses do only 8mills (and larger) so the possible resistance is pretty tiny; for sure much too small for gate stoppers.

IMHO a more usefull trick is to use traces/fills for small capacitors, say a few pFs. But that's already for the well trained people

Have fun, Hannes

[unclejed613]

acd is right. the trace you show is used more often as an inductor or in digital circuits as a delay line since it's actually several lumped inductance and capacitance elements (also known as a lossy transmission line). you may find that the delay provided doesn't affect your phase margin much, but who wants to take chances? use a resistor. it may cost a few extra cents on the front end, but save the cost of smoked components and redesign on the back end.

[Nico Ras]

John,

The layout you describe makes it non inductive since the magnetic fields cancel, also parasitic capacitance is in series and effect is minimal.

Stray pick-up is minimized due to the "windings" mirroring and often used in RF amplifier stages. The value could easily be trimmed by inserting a few cob-web like tracks that can be cut or solder blobbed to tweak the value almost exactly.

Nico

[sam9]

You can get tighter tolerances than VivaVee mentions but you have to pay for it. On top of that there will be a nice NRE charge.

I think the bottom line is that the technique is used as a last resort when there is no way to get the required value or to get it at a particular location.

[MJL21193]

Thank you all for the responses. I can see the impracticality of it now.
Besides, the amp I'm working on doesn't need base stoppers, so no need for this now.

[neutron7]

Look at a computer motherboard..it must work, they do it a lot wobbly traces are the least of their problems.