Low current through circuit, but don't know why! - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: March 25, 2014, 01:42:04 pm
content: OK,so here's the circuit:The parts:R1 - 100k 5%RSense - 0.68 5% / also tried with 0.5 5%Q1 - 600V/7.5A FQP8N60C MosfetDatasheetT1 - 80V/500mA MPSA06 NPNDatasheetLED - 3.55Vf / 800mA Nichia LEDSo, the problem is, for whatever reason, I am only getting 5mA of current through my LED. What gives? According to this design, I should be getting around 800mA.The 800 mA, comes from the RSense = 0.5/I = 0.5/0.8 = 0.625 Ohm. So I chose a 0.680 Ohm. I then tried a 0.5 Ohm resistor, same deal.For whatever reason, I am only getting 5mA to my LED. It seems as if for some reason perhaps the Mosfet is not turning on the way it's supposed to and letting current through?I don't really understand the mechanics of the circuit that much, but I've used it successfully in the past to power 3 of these type of LEDs.My previous schematic had, instead of the Q1 FQP8N60C, I used a IRF830PBF from Vishay (500V / 4.5A Mosfet) and a Darlington KSP13 (30V / 0.5A) from Fairchild.I also switched out the NPN for the former Darlington, same deal. I also left the Emitter pin hung and connected to GND, same thing.Anyone got any ideas?Can it be that since I'm only driving one LED as opposed to 3 LEDs, the Voltage drop over the Mosfet is too big and it's shutting down?

author: Codemonkey
author_rank: Regular Contributor
timestamp: March 25, 2014, 01:59:13 pm
content: I'd get a scope on it and make sure its not oscillating!

author: made2hack
author_rank: Frequent Contributor
timestamp: March 25, 2014, 02:05:00 pm
content: Don't have a scope @ the moment and no access to one.What would potentially cause the oscillation?

author: Codemonkey
author_rank: Regular Contributor
timestamp: March 25, 2014, 02:16:18 pm
content: Try connecting a small capacitor (~100nF) across either of the 2 resistors in your circuit. If you see a change, its almost certainly oscillating.

author: made2hack
author_rank: Frequent Contributor
timestamp: March 25, 2014, 02:53:01 pm
content: OK,I will try that. But from the specs, is there something that indicates to you that there is a potential for oscillation?

author: Codemonkey
author_rank: Regular Contributor
timestamp: March 25, 2014, 03:07:35 pm
content: Not the specs, just the circuit configuration. Whenever you have 2 transistors trying to turn each other off, there's going to be a potential for oscillation, especially if you've built it on something like breadboard or veroboard where you have a lot of extra stray inductance in the connections.

author: made2hack
author_rank: Frequent Contributor
timestamp: March 25, 2014, 03:28:41 pm
content: It is on a breadboard

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2014, 07:00:33 pm
content: What sort of resistor is Rsense?

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 25, 2014, 07:26:04 pm
content: Please correct me if I’m wrong,Your darlington needs 1.2v to start working not 0.6vTo get 1.2v across your 0.68 ohm resistor your need 1.2 / 0.68 = 1.76AMPS passing through it.So you have 12v - 1.2v - 3.55v = 7.5v across your mosfet = power = VI = 12.75 WATTSNot surprising your mosfet is 'shutting down', can your led take 1.76ampsTry changing your Rsence to 1.5 ohms minimum 1watt

author: Fank1
author_rank: Regular Contributor
timestamp: March 26, 2014, 12:52:30 am
content: The data sheet shows that the "darlington is just a normal npn transistor"I calculate to get the .62 B-E voltage to turn it on R should be .775 ohms.I suspect that the 100k resistor is taking too long to charge the gate capacitance of the mosfet.Try dropping it down to about 23K.I think you are oscillating also, it looks like the circuit was meant to do that.But with the 100 k gate resistor your "on time duty cycle" will be very short and while the peak current may reach 800 ma it will only look like 50 ma on a meter.Calculates to about 6% duty cycle.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 26, 2014, 02:34:47 am
content: KS13 is a darlington, it has a gain of 5,000 at 5v, if you look at the data sheet attached and go to Figure 3, it shows that it starts to turn on at approx 1.1v giving an Ic of 1mA and at 1.2v gives a Ic of approx 50mA. At 0.6v, 0.8v and 1.0v it has a collector current of 0mA, basically still turned off.Can the OP clarify exactly what components he is using

author: made2hack
author_rank: Frequent Contributor
timestamp: March 26, 2014, 06:30:12 pm
content: I have both a MPSA06 (see datasheet) and a KS13 (from previously successful build).Previously, I used the KS13 together with the IRF830PBF and 3 LEDs in series, this worked fine and gave me around 950mA using a 0.47 Ohm Rsense.Currently, I have the MPSA06 (NPN) with the FQP8N60C (Mosfet).I have also tried the KS13 together with the FQP8N60C.I don't have the previous Vishay mosfet to test.In both situations, I get the same 5mA on the LED. I am using this schematichttp://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/, where it states that the current for the LED is set by RSense, or by using the formula 0.5/I (in my case 0.8A), so 0.5 / 0.8 = 0.625 Ohm for Rsense. Thus the two values I tried, 0.5 Ohm and 0.68 Ohm.When I was successful, I had 3 LEDs in series. At the moment, I just have one. I suppose I could connect 3 in series, in order to reduce the voltage drop across the Mosfet.

author: made2hack
author_rank: Frequent Contributor
timestamp: March 26, 2014, 07:24:44 pm
content: OK,So it seems the problem was the fact that I was using only 1 LED.3 LEDs in series fires up ok!Also, when reducing the load into the LED with a much larger resistor, I can run only 1 LED.When I run the 3 LEDs in series, all is OK as only 0.25Volts is dropped over the Mosfet (or there about).

author: made2hack
author_rank: Frequent Contributor
timestamp: March 26, 2014, 07:30:13 pm
content: Can anyone explain why it is that whether I connect the Emitter of the NPN to GND or whether I leave it floating, the result is identical?I mean, it works the same way?What am I missing?

author: Neverther
author_rank: Regular Contributor
timestamp: March 26, 2014, 08:44:02 pm
content: My guess is the fet.Look at the datasheet, from first Vds - Id curve your max theoretical amperage is 6A. At your setpoint the wattage across fet is about 6.3W, theoretical Rds at 9.9ohm.Lets look at thermals: junction to ambient at 6.3W comes around...392 deg above ambient, better have a heatsink or that thing is dead.Internal limiting: Figure 10, 0 amperage at 150 deg Celcius case temperature, and Junction temp will have reached it long before that.Even with heatsink you have junction at 9 over, and not the smallest (13C/W) to220 heatsink would be at +82 from that. So junction sits around 120 deg C@ room temp, thats not a nice thing for a fet.With 3 leds the wattage at the fet is far less than the calculations above.9.6 watts in, where 2.84 goes for the led.Pretty inefficent, slap a chinese lm2596 with current limiting there if youre lazy and got time.Or use a power mosfet with beefy heatsink to warm up your surroundings.Or lower the input voltage/design your own efficent voltage dropper.

author: drtaylor
author_rank: Regular Contributor
timestamp: March 26, 2014, 10:47:49 pm
content: This design will never work with a MOSFET. The problem is that the MOSFET is a switching type. The gate channel requires so little current to turn it on that when the BE of the transistor closes on .6V the MOSFET just shuts off, and then the cycle completes. I can practically guarantee that this is oscillating between off and the current limit value, giving the average current much lower than hoped for. The well know circuit using two BJTs works because the linear region has a much wider dynamic range. It does not work with a switching MOSFET. There is no reason to use a MOSFET as it is going to have to dissipate all the excess wattage anyway. However, that circuit is better used in protection circuits as a current limit, than used as a constant current source as it, in this simple form, will have a lot of variation in current regulation.A better circuit would be a LM317 in current source mode. Or the previous suggestion of a constant current IC.This is another example of why MOSFETs are hard to apply in linear circuits. High voltage MOSFETs and those specifically designed for linear operation have wider linear regions. But again, a MOSFET in this simple circuit just will never work well.