EntryModel::populateModels throws a 500 error I am retrieving a data set with about 5000 records and I am using the EntryModel::populateModels($results) to populate the model, this is causing a 500 error. When I limit the number of records returned to 100, it is working fine but when i set the limit to null it throws the error.

There isnt any error logged in the logs, and nor can it be a timeout because it only takes about 5 secs to return the error.
There's probably an error in craft/storage/runtime/logs/phperrors.log and there's a 99% chance the problem is you're hitting your php.ini file's memory_limit setting.

Populating 5,000 models is going to take a significant amount of memory.