Constant Current Dummy Load - Page 1
Forum section: electronics / projects

author: mnmlistmktg
author_rank: Newbie
timestamp: July 29, 2013, 11:06:06 pm
content: Hello all,I'm working on the constant current load as seen in Dave's video, but am running into weird op amp action (weird to me at least, I'm pretty green at this stuff). I opted to include the front end follower (had a quad LM324) and am using an IRF820 mosfet. Front end of the circuit is working fine... inverting and non-inverting voltage are equal on the follower, however, when I come to the second op amp I'm getting very low inverting voltage and constant output voltage (see measurements below, had PSU at 10V):@5V supply @2V supply1in+ : 5V 1in+ : 2V1in- : 4.4V 1in- : 2V1out : 4.4V 1out : 2V2in+ : 4.4V 2in+ : 2V2in- : 228mV 2in- : 236mV2out : 4.33V 2out : 4.33VI did take out the voltage divider between the op amps for these tests, but had the same issues with them in. Full range of values for 2in- is 0 to 235mV; 2out range is 3.08V to 4.34V. I've checked, double-checked, and triple-checked the wiring and everything is as it should be (pretty simple circuit, or so I thought). Any ideas at all? I know that the only time op amp inputs don't match is when saturated, output changes faster than slew, or at very high frequencies... but I don't think that's it.Any help or resources would be greatly appreciated.

author: mnmlistmktg
author_rank: Newbie
timestamp: July 29, 2013, 11:35:23 pm
content: Correction.... supply was constant 5.6 volts, the @ indictions reference the 1in+ set from the potentiometer.

author: SebG
author_rank: Regular Contributor
timestamp: July 30, 2013, 05:07:49 am
content: Hi I would be very helpful if you showed the circuit you implemented.

author: SebG
author_rank: Regular Contributor
timestamp: July 30, 2013, 05:40:04 am
content: Ok Circuit aside, if V at 2in- is 228mV (and if i'm correct) is the voltage across the 1 Ohm resistor that means the MOSFET is conduction 228 mA. I've looked at the data sheet for a Vishay IRF820 part which showed that the minimum Gate voltage is 4V to begin turning the device on. If i am correct of my assumption of the circuit then a 2in- would only range as you said (0 - 235mV) because the opamp can output a maximum of 4.4V or so and the mosfet needs a Gate Voltage or 4V to 10V to conduct current. Two things can fix that. Use a much higher voltage at the op-amp rails, say 12 Volts (i think the LM324 can support it) or replace the mosfet with a logic level mosfet (for example FQP13N10L from fairchild).

author: mnmlistmktg
author_rank: Newbie
timestamp: July 30, 2013, 02:30:00 pm
content: Thanks SebG, I appreciate the feedback. Switched to a 12V supply and everything is working as expected. I used the IRF820 because it's what I had, but the gate-source threshold had the same specs as the MTP3055 that Dave used, so thought it would operate at logic level. (schematic attached)Thanks again, have a logic level mosfet on the way.

author: perfect_disturbance
author_rank: Regular Contributor
timestamp: August 02, 2013, 07:28:06 pm
content: What do people think of the PHP79NQ08LT for this application.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 03, 2013, 12:34:35 am
content: I just builtthisfromherebecause I wanted to try making one with an LM10 I had. I just swapped in and out a few mosfets to see what worked better than others. The one I settled for is marked on the schematic. Also, I made my own .13 ohm resistor using 8.25 feet of 22awg wire for R2. I figure you could just try using different mosfets and see if they work. That being said, the ST one I used is 60V but I only wanted to go up to 1.5A to test the linear chips I use.

author: perfect_disturbance
author_rank: Regular Contributor
timestamp: August 03, 2013, 01:37:37 pm
content: Here is a link to the spec sheet.http://www.mouser.com/ds/2/302/PHP79NQ08LT-118172.pdf

author: Rerouter
author_rank: Super Contributor
timestamp: August 03, 2013, 01:44:52 pm
content: same flaw as every other constant current load design i have seen on here so far, you want a resistance, say 100 ohms between the op amp output and the gate, and you want a capacitor between the inverting input and the output on the order of 500pF to 5nF to slow it down to prevent oscillation,op amps don't like driving capacitive loads,

author: IanJ
author_rank: Unknown
timestamp: August 03, 2013, 02:00:35 pm
content: Hi,I used the LM8272 op-amp in mine.Output current is 65mA and specifically designed for driving high capacitive loads like TFT LCD panels, power mosfets etc.I testing it alongside the usual suspects and it is indeed significantly better.Still need the R/C's per Rerouter though.Ian.