Linear drivers w/ wirewound resistors - the problem?

[rhd]

I'm putting together a driver where the design would really benefit from being able to use a particular resistor that "wirewound". However, I know that I've heard warnings not to use wirewound resistors in DDL-type linear driver circuits.

Can someone refresh me on the reason for this? Is it highly theoretical, or is there a practical drop-dead "this won't work" rationale?

 

[Cyparagon]

Meh, probably because of the inductive nature that might interfere with regulation. I don't think it'd be a big deal though.

 

[BShanahan14rulz]

nvm......

 

[Flaminpyro]

Yes because they are wound with wire they tend to be inductive that can have an effect on a buck-boost type driver but for a DDL type there should be no problem, I could be wrong about that last part so let some others chime in.

 

[rhd]

My thinking also ^

They just need to behave the same way RE: the 1.25V drop I believe.

 

[Toke]

Yes because they are wound with wire they tend to be inductive that can have an effect on a buck-boost type driver but for a DDL type there should be no problem, I could be wrong about that last part so let some others chime in.

I agree.
A DDL driver gives pure DC so there should be no problem, the KHz/Mhz pulsed DC from a buck/boost would likely give problems.

 

[Benm]

You can use wirewound resistors, just make sure to observe the manufacturer specified input and output capacitors for the regulator.

The inductive nature of these resistors depends much on their resistance and power, but its not that big for low-resistance (in the order of <10 ohm) ones. It does get worse at higher resistances, but those you'd mostly find somewhere in mains powered equipment (think CRT tv's and such).

 

[NetGod3Com]

Though this is highly unlikely and purely speculative on my part, the reasoning might be along these lines, The inductive component of a wirewound resistor could pose a problem at higher resistances due to collapse of the magnetic field possibly inducing a spike which could be bad for the laser diode.

 

[Cyparagon]

We don't use high resistances in the LM317 driver

 

[Benm]

Though this is highly unlikely and purely speculative on my part, the reasoning might be along these lines, The inductive component of a wirewound resistor could pose a problem at higher resistances due to collapse of the magnetic field possibly inducing a spike which could be bad for the laser diode.

In the typical LM317 driver it actually does the exact opposite of that: When you switch the driver on, it will take time to build up the magnetic field in the resistor, during which times it acts like a resistor -higher- than its ohmic value.

Obviously this will backfire the moment you turn it off, and the stored energy in the field will be released as a reverse voltage pulse. Even a very small capacitor across the LD will catch that energy release and render it safe.

 

[rhd]

I *always* have a 10uF across the input and output of my diy drivers. 10uF - sufficient?

 

[NetGod3Com]

I understand inductive reactance, and was trying to reason out why the wirewound resistors had possibly gotten their bad rap as it were. :thanks: for clearing up the exact why for our original poster though. Electronics class was more than a few years ago now.