MOSFET madness - Page 1
Forum section: electronics / beginners

author: skysurf76
author_rank: Regular Contributor
timestamp: July 18, 2013, 08:25:36 pm
content: I'm building a circuit that takes sound from a condenser mic, sends it through a low pass filter sandwiched between two op amps as buffers, and the output will go to an LED string so that the light will pulse with the bass. I really got hung up on the MOSFET setup at the end though. After having done a ton of research for the last week I think I finally have it down conceptually and just want to make sure my understanding is correct before I go and build this thing.I tested the light strip and at its specified voltage of 12 volts it draws 1.35 amps. After my research I've learned that I will be operating the MOSFET in the triode region and in reality the light strip itself is what will limit the current. The gate-source voltage will be 5 volts, which for the selected MOSFET will be well above the threshold voltage, therefor the MOSFET will be trying to increase the current to get into saturation. In the process the voltage across the light strip will be rising as the current increases, the voltage across the drain-source will be going down, and eventually almost all of the 12 volt supply voltage will be dropped across the light strip (as desired) and eventually the intersection of the load line for the light strip, and the exponential curve of the MOSFET will intersect and equilibrium will be maintained. As long as the selected MOSFET has a low Rds, then the final voltage drop across the drain-source should be minimized and the voltage drop across the light strip should be maximized which is the desired behavior.Am I correct in my understanding of the operation?Also, another conceptual question I would like to have verified is the following... for a given MOSFET, the lower the Rds, the farther left the Vgs traces will be shifted on an Id-Vds graph?Thanks for taking the time to read this.

author: madires
author_rank: Super Contributor
timestamp: July 18, 2013, 09:07:51 pm
content: Is the light strip just a bunch of LEDs or does it include a controller to limit the current? Do you got a datasheet of the light strip?

author: skysurf76
author_rank: Regular Contributor
timestamp: July 18, 2013, 09:40:44 pm
content: Here is the type of light strip I'm using....http://www.ebay.com/itm/5M-5050-SMD-LED-RGB-Waterproof-300-LED-Strip-Light-Lamp-44Key-IR-Remote-New-/151053414307?pt=US_Car_Lighting&hash=item232b7c2fa3The strip is made up of individual segments that contain two smd LEDs on either side between a 330 ohm smd resistor. It has two hookup wires with no connector attached. Just a black and red lead.

author: madires
author_rank: Super Contributor
timestamp: July 18, 2013, 10:06:41 pm
content: Quote from: skysurf76 on July 18, 2013, 09:40:44 pmSo we can assume that the light strip is designed for a specfic voltage (12V?) and the current is limited by the resistors?The next question is if you just want to switch the light strip on and off (based on the bass) or also like to vary the brightness.

author: skysurf76
author_rank: Regular Contributor
timestamp: July 19, 2013, 04:39:49 pm
content: Yes, the light strip is specified for 12 volts. When I applied 12 volts to it it drew 1.35 amps. As far as whether I want to vary the brightness or just pulse it, I haven't really given that much thought yet. I'll probably just experiment with some trim pots in the op amp circuits to see the results.

author: madires
author_rank: Super Contributor
timestamp: July 19, 2013, 05:34:07 pm
content: Quote from: skysurf76 on July 19, 2013, 04:39:49 pmFor switching the light strip you would drive the MOSFET into saturation mode to get R_DS_on, i.e. lowest possible resistance, to minimize the voltage drop and the heat generation. For example, if R_DS_on is 0.3 Ohms the voltage drop would be 0.3 Ohms * 1.35A = 0.4V and the max. power dissipation 0.4V * 1.35A = 0.54W. Since the light isn't switched on all the time the average power dissipation will be much less. The PSU provides 12V for the light strip und the current is limited by the resistors integrated in the light strip.For varying the brightness of the light strip you have to limit the current, i.e. drive the MOSFET in the triode mode. The MOSFET will act as a variable resistor limiting the current while also heating up much more. But there's another way using a PWM signal to switch the light strip. By varying the PWM ratio the light could be dimmed from off to fully on and you would have the advantage of the MOSFET in saturation mode (low resistance, low power dissipation). You could use a PWM IC or built a simple PWM with a comparator. The PWM ratio is driven by a voltage, i.e. by the output of the bass filter. If you like you could also add a threshold stage to supress low bass volumes.

author: skysurf76
author_rank: Regular Contributor
timestamp: July 20, 2013, 10:59:13 pm
content: Now I'm confused again. If you wanted the MOSFET in saturation when you were using it as a switch, that would mean that assuming you had a specific gate voltage you were going to be using in your circuit that you would have to select a specific MOSFET that went into saturation at the exact input voltage you were using AND that would give the exact drain current that you needed at that gate voltage.I'll use the following graph for reference...http://www.electronics-tutorials.ws/transistor/tran56.gifGoing from the characteristic curves on that graph of a MOSFET it would appear that by using the MOSFET in the triode region you would give yourself the option of choosing from a large number of MOSFETS, as the load line of the circuit you were attaching the MOSFET to would be what would control your final current. And by staying in the triode region you would have a small drain source voltage as well.

author: madires
author_rank: Super Contributor
timestamp: July 21, 2013, 02:16:54 pm
content: Quote from: skysurf76 on July 20, 2013, 10:59:13 pmWhy do you want to control the current if you just switch the load on or off? The light strip has some resistors built in to limit the current for 12V. To select a MOSFET you would look for I_D (>1.35A), a low R_DS_on (low losses) and if V_GS is low enough for your circuit. For 12V V_DS shouldn't be any problem. We don't have to deal with high speed switching, so we can skip several other parameters. If your circuit runs with 5V or less let's select a logic level MOSFET. This will give us plenty of choices.Saturation mode means that the FET is fully conducting and it can't get any better. That's what I would expext from a switch. The triode mode (ohmic region) is used for amps or electronic loads where you need control over the output based on the input. And because the MOSFET is in the triode mode it's resistance is larger than R_DS_on, i.e. larger voltage drop and more heat.If you simply switch a load or control it by PWM (which is also based on switching) you would use the MOSFET in saturation mode to get as less power loss as possible to minimize heat (heatsinks are expensive and large).

author: hlavac
author_rank: Frequent Contributor
timestamp: July 21, 2013, 03:22:42 pm
content: You said you have RGB LED strip but just two wires - are you using the "receiver box"?If so, it will not work.There are probably 4 wires in the strip itself for red, green, blue and common. You may want to drive these directly.And use PWM, it is more efficient and does not change color of the LED with brightness.