EEVblog #102 - DIY dummy load - why does the current cap out at 1.3A? - Page 1
Forum section: electronics / beginners

author: waspinator
author_rank: Contributor
timestamp: August 06, 2014, 12:57:28 am
content: I'm trying to understand what limits Dave's design to 1.3A. The MTP3055 MOSFET has a drain current of 12A. Is that not the specification that's important for how much current can go through it? Is there another transistor or opamp that can be used to increase the maximum current draw, or is this more complicated than that?Thanksedit: which graph in the datasheet corresponds to what Dave shows?

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2014, 05:00:16 am
content: The transistor and the sense resistor have power dissipation limits as well. At high voltages, the transistor is likely to have either secondary breakdown limits or something similar.

author: mij59
author_rank: Frequent Contributor
timestamp: August 06, 2014, 05:34:19 am
content: Take a look at page 6 figure 12 of the data sheet.Assuming static operation, the current and voltage must be below the line DC.

author: waspinator
author_rank: Contributor
timestamp: August 06, 2014, 02:26:24 pm
content: Quote from: mij59 on August 06, 2014, 05:34:19 amso it looks like if you connect a 10V power supply you should be able to pull 4A. or am I reading the graph wrong?If the opamp could provide a gate-to-source voltage of 6.5V would I be able to pull 12A?Thanks again

author: mij59
author_rank: Frequent Contributor
timestamp: August 06, 2014, 04:00:01 pm
content: Quote from: waspinator on August 06, 2014, 02:26:24 pmyes, but only with a case temperature of 25 C.Quote from: waspinator on August 06, 2014, 02:26:24 pmyes, but for static operation you'r outside the safe operating area

author: waspinator
author_rank: Contributor
timestamp: August 06, 2014, 08:59:32 pm
content: oh okay, thanks. so do you know a transistor that could handle more current off the top of your head? (that could be used in the original circuit)the FQP30N06L seems like a good candidate, but I'm not sure.

author: mariush
author_rank: Super Contributor
timestamp: August 06, 2014, 09:17:34 pm
content: I'm using fqp50n06l in my own load, but i've used the slightly modified circuit shown here :http://www.sleepyrobot.com/?p=136And it's not a transistor, it's an n-channel mosfet

author: waspinator
author_rank: Contributor
timestamp: August 07, 2014, 02:06:10 am
content: Quote from: mariush on August 06, 2014, 09:17:34 pmso how was your performance? could you get 3 or 4 amps at 12V?um, a mosfeT isn't a transistor? what's the T stand for?

author: mariush
author_rank: Super Contributor
timestamp: August 07, 2014, 03:35:53 am
content: Well, yes.But it's like saying "a vehicle" instead of "a bus"... when most people think of regular cars when they hear the word "vehicle".I think most people would think of npn or pnp transistors when you say "transistor".

author: Leestons
author_rank: Contributor
timestamp: August 07, 2014, 04:21:45 am
content: Pretty much. When I think transistor the last thing I think of is a MOSFET.

author: mij59
author_rank: Frequent Contributor
timestamp: August 07, 2014, 04:46:18 am
content: Quote from: waspinator on August 06, 2014, 08:59:32 pmDissipation of 80W is not possible, the temperature of the junction gets too high, the thermal resistance is too high.A TO247 package transistor would be better, e.g. IRFP460.

author: waspinator
author_rank: Contributor
timestamp: August 07, 2014, 07:05:19 pm
content: Quote from: mij59 on August 07, 2014, 04:46:18 amthanks, I'll try that.