Putting the "dummy" in load.... - Page 1
Forum section: electronics / beginners

author: int2str
author_rank: Regular Contributor
timestamp: February 19, 2015, 09:24:37 am
content: I put together an "EEVBLOG Dummy Load" using these parts:1x LM324 quad op-amp (Datasheet)1x NTE2395 N-channel Mosfet (Datasheet)1x 1 Ohm 10W resistor1x 10K, 10 turn potentiometerPlaying with it, I am running into a bunch of problems...1. Setting the "control" voltage to 5v, I can only get about 80mA of load.2. The 10 turn pot is only useful for about 45 degrees out of it's 3600 degrees range.What am I doing wrong?- With the load voltage set to 10V and the pot turned down to 0mA load, the gate voltage is 2.9V already.- Cranking the pot 45 degrees moves the gate voltage to 3.85V, at which point it's drawing 80mA of load. Turning the pot more has no effect.- The pot at 45 degrees produces 0.095V at the output of the first voltage followerIs this the wrong mosfet somehow?If so, what parameters am I looking for?I've watched Dave's video 100x and mjlorton's video series on this.Yet this dummy can't figure out why mine isn't working

author: int2str
author_rank: Regular Contributor
timestamp: February 19, 2015, 09:33:32 am
content: Oh, one more observation:- If I change the "control voltage" to 9.6V, I can get 3A+ of load. At which point the gate voltage is about 8.5VSo clearly, the control voltage is affecting things.How do I know if I'm over driving the mosfet?How do I calculate the "safe" or ideal gate voltage for the mosfet?What am I looking for in the data sheet?Thanks!

author: electr_peter
author_rank: Unknown
timestamp: February 19, 2015, 09:49:35 am
content: How are you powering your e-load? Assuming circuit is correct, there is few things to be aware of: LM324 is not rail-to-rail output opamp; MOSFET needs some minimal voltage to work; load has to provide some minimal voltage level.If supply voltage for e-load is too low, MOSFET can not turn on hard enough (and thus pot has limited range).

author: tombi
author_rank: Regular Contributor
timestamp: February 19, 2015, 09:50:33 am
content: I'm relatively new too and built one of these.The data sheet says Vgs can be +-20V so 10V at the gate is no problem.Also the specs say the Vgs threshold (the gate voltage where the mosfet turns on) is between 2 and 4. If you have 3 amps flowing then there is 3V on the load resistor so your Vgs is 8.5-3=5.5V which isn't much above this range.So does your voltage on the load resistor match the voltage on your op amp input?I had trouble with mine oscillating and had to add a feedback capacitor.Another forum member wrote this guide which might help.http://m.instructables.com/id/Arduino-Programmable-Constant-Current-Power-Resist/Tom

author: int2str
author_rank: Regular Contributor
timestamp: February 19, 2015, 10:02:59 am
content: Quote from: tombi on February 19, 2015, 09:50:33 amThe input to the second op amp?If so, then yes. Both inputs to the 2nd op amp read 100mV when the load resistor has 100mV across it.QuoteMy scope has not arrived yet, so I can't characterize any ripple...

author: rqsall
author_rank: Regular Contributor
timestamp: February 19, 2015, 10:15:10 am
content: The mosfet needs to be a logic-level mosfet. The Vgs doesn't only need to turn the mosfet on at around 2.5 Volts, but also has to make the mosfet go full open to several amps before Vgs reaches 4 volts or so, only logic-level mosfets do this. There is no plot on the datasheet showing Vgs vs Id, so can't tell on this one, but the datasheet doesn't mention that it is logic-level.

author: Seekonk
author_rank: Super Contributor
timestamp: February 19, 2015, 10:57:51 am
content: Is that all the parts you used? Do you have a reference voltage in the circuit? I think a schematic would show an obvious problem.

author: rdl
author_rank: Super Contributor
timestamp: February 19, 2015, 03:25:31 pm
content: Quote from: rqsall on February 19, 2015, 10:15:10 amYes, this ^.That said, I've built Dave's circuit several times, using the exact parts he did, and it oscillated madly every time. I guess I just can't hold my tongue at the proper angle.