resistors for paralleling

[ejn327]

I am building a gainclone with 2 LM3875's per channel, and as I understand it I need to run 0.22 ohm resistors in series with the outputs of the amps.

I am also not sure about the watt rating of them, using the overture design spreadsheet i get 4.94 Amps. This going through a 0.22 watt resistor would be 5.5 watts (P=I^2 * R)

I also understand that I will need 1% tolerance, does anyone know what type (i.e. wire wound, metal oxide), to get or where to get them?
I see Digikey has an aluminum housed wire wound 0.2 ohm 10 watt, would this work?
It looks tough to find a 0.22 ohm resistor with a high enough wattage rating.

Any help would be appreciated

[motherone]

These are current sharing resistors. If you are going by the BPA 200 Application Note from National, they should be .1 ohm/1%/5 watt resistors.

The resistors spec'd on the datasheet are available here:

http://www.mouser.com/index.cfm?hand..._pcodeid=71010


If you still want to go with the .22 ohm resistor, the closest you'll find is this:

http://www.mouser.com/index.cfm?hand..._pcodeid=71010

Basically mouser carries the .2 or .25 ohm/5 watt Dale's.

Good luck.

[BrianGT]

I am using Vishay Dale LVR5 0.1 ohm resistors to go with the LM4780 in parallel configuration, and I expect that this will work good for the LM3875/3886 in parallel configuration also.

Here is a link to the part that I choose:
http://www.mouser.com/index.cfm?hand...oductid=372142

--
Brian

[azira]

I would say that matching is more important than the actual value in this case. If it was me, I'd go with the .25 so that I could be slightly less careful about matching the chips.

Edit: Then again, since the DS calls for .10, maybe you should go on the smaller side. Any reason you're choosing .22?
--
Danny

[BrianGT]

Originally posted by azira
I would say that matching is more important than the actual value in this case. If it was me, I'd go with the .25 so that I could be slightly less careful about matching the chips.

Edit: Then again, since the DS calls for .10, maybe you should go on the smaller side. Any reason you're choosing .22?
--
Danny

The resistors that I referred to were 1% 0.1ohm 5w, which should be fine. When I first got them, I put them on an expensive benchtop LCR meter, and they were quite close in value to each other. (less than 1%)

--
Brian

[ejn327]

I just looked through the BPA 200 application.
I have 2 of the briangt kits ordered to parallel. Is there a reason that he specifies using 0.22 ohm resistors for parallel configuration and the BPA 200 specifies 0.1 ohm?

[BrianGT]

Originally posted by ejn327
I just looked through the BPA 200 application.
I have 2 of the briangt kits ordered to parallel. Is there a reason that he specifies using 0.22 ohm resistors for parallel configuration and the BPA 200 specifies 0.1 ohm?

With regards to the instruction manual, this was created before the LM4870 kits, and the 0.22 ohm resistor was listed in the manual, since it was what Jeff Rowland used in his earlier Concentra amps (bridged parallel setup:
http://www.jeffrowland.com/review16b.htm

My current recommendation is 0.1ohm resistors, as they seem to work good with the LM4780 kits, and are recommended by National. I will change the manual to reflect this when I complete the next revision.

--
Brian

[ejn327]

Figures that while i am posting a reply there are 3 new answers.
Thanks for the help guys, I ended up getting the Vishay/Dale 5W 1% Low Value Power Resistors. I got a few extra to get a better chance at matching.
How are you measuring the values? I have a fluke at home but I don't think it has a high enough resolution.

[thomas997]

I have lots of Dale .22 ohms, something like 1% (reads 0.220 on meter).

They are surface mount though, but tabs could be pulled out. About 80mm long..