Do watch out here - it is true that MOSFETs have a positive temperature coefficient of the "on resistance" when in the OHMIC region (close to being fully ON), but in the LINEAR region (where Vdg is less than the threshold voltage), the temperature coefficient of CURRENT is positive. SO, MOSFETs can be paralleled as long as the LINEAR region power dissipation is kept to a minimum, but if you have multiple MOSFETs in a partially-on state, they cannot be paralleled without ballast resistors (just like a BJT). In other words, for a given gate-source voltage, there is a current threshold, call it I1. Below I1, the current is linearly proportional to the applied voltage (Vds), hence the OHMIC region. If the current goes above I1, it becomes independent of Vds (LINEAR region, called LINEAR because this is the region of operation typically for linear amplifiers). Paralleling FETs is OK if they stay within the OHMIC region, but requires extra caution if they dissipate any significant power in the LINEAR region. I've even seen this become an issue for FETs in switching applications where the switching losses were comparable to the I2R losses in the FET. Sean On Thu, Apr 14, 2011 at 11:33 AM, Michael Watterson wro= te: > On 14/04/2011 15:58, V G wrote: >> Hey all, instead of using a wire wound resistor for testing battery drai= n, >> I'm thinking of using a heat sink bound MOSFET and control the drain-sou= rce >> current with the gate. I'm looking to get around a 0.5 - 2 ohm resistanc= e >> between drain and source. Is this possible? > > yes, but measure the source current with a 0.01 to 0.5 Ohm resistor as > the "resistance" will increase as it warms. This is why FETs can be > paralleled, unlike bipolar which give INCREASE in current with temperatur= e. > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .