I humbly ask for assistance with a somewhat simple little project I am trying to finish. The project I have started is to have several reels of LED lights flash triggered by the RCA type subwoofer output of a receiver. Some specs are as follows:

ATX PSU converted – 12V @ 24 AMP Max output

On-On Rocker Switch – 12V 16 AMP (Normal Mode/ Beat Mode)

LED Strips – 12V 10 AMPS Max

LED Controller – 12V 24 AMP Handling Capacity

Subwoofer Output Line – 2 V @ 50K Ohms Max Output? Amps Unknown MOSFET? 12Volt + / 10 AMP Continuous / Gate (trigger on?) Volts Unknown - Fan Cooled No Heatsink

My troubles are that I do not know exactly what the subwoofer output RCA power is only what it would not usually be more than at full power. Specs. I were able to find on other players generally say no more than 2 Volts @ 50K Ohms. So I have no way of knowing for certain how low the voltage or amps. are at normal listening levels. If someone did know (HINT HINT) that would lead us to the second most difficult problem.

Choosing a proper switching device. I have seen videos of others using an N MOSFET (TIP30) to flash LEDs however none off of an RCA nor at the amps. I intend. Is a MOSFET not what I need? If it can be then I need one which opens at a very very low, unknown voltage. I have seen that many handle the 12V 10 Amps easily.

I have tried to contact manufactures. I have contacted distributers both national and local who are reluctant to discuss the issue and find me a $1.00 - $3.00 part. I have contacted my local shops for advice but have found my knowledge surpasses their own which is just sad. I have also tried filtering through different manufactures MOSFET spec sheets but it is all so confusing for the novice and they even seem to be switching terminology leaving me with a giant hole in my wall where I’ve twice put my head through. I try to learn, I try to figure things out on my own, but I cannot. Can someone please help me with a widely available part number for something like the MOSFET I intended that I can order? Thank you for your time.

  • \$\begingroup\$ Can you hit the edit button and try and break this into paragraphs? It's really hard to follow at the moment. \$\endgroup\$ – PeterJ Dec 22 '13 at 11:58
  • \$\begingroup\$ I agree with PeterJ - what are you trying to achieve? Stick to basics - I'm not interested in how hard it is for you to understand. Precy, shorten and summarize. \$\endgroup\$ – Andy aka Dec 22 '13 at 12:02
  • \$\begingroup\$ Sorry I just wanted to show that I did put allot of effort into finding this out on my own and I did not want to be thought of as one of those people that give up and run for help the first signs of trouble. I know the value of your education and time. Thanks \$\endgroup\$ – user34492 Dec 22 '13 at 12:17

A MOSFET will be a good choice.
The following gives a guide to directions - ask more questions.

Load max sounds like (pun notice retrospectively) LED strips at 12v, 10A
MOSFET needs to switch this load so 20V+ rating desired, usefully more than 10A - say 20A+.

Now the fun begins.
If the FET is hard on then dissipation is low. With and Rdson (= on resistance) of say 10 milliOhm the power loss at 10A due to channel resistance = I^2R = 10^2 x 0.01 = 1 Watt.
If you PWM switch the FET you add switching losses but losses are still "moderate"
If you linearly switch the FET so it acts like a resistor dissipation is much higher.
A 12V LED strip will decrease to close to 0% current by 8V (or higher) probably, so if full load is 10A you will probably be down to 5A at 10V (so driver loss = (12-10) * 5 = 10 Watts, and down to say 1A at 8V (loss = (12-8)*1 = 4 Watts.
So you can probably expect losses in linear mode to be under say 10W but allowing for 20W may be safe. Rdson becomes non-critical when used this way as resistive on losses swamp Rdson.

So, we have a MOSFET rated at ~~~= SOME OF 20V, >10A and 20W actual dissipation and Rdson = 10 milliOhm.
I say "some of" as you don't need Rdson low in linear mode but you don't need as hogh dissipation in PWM mode.
For 1 off amateur use specify leaded TO220 package for easier use and ease of heatsinking.

Searching DIGIKEY with
MOSFET 30V 20A TO220
then sorting by ascending price and looking for lowest $ in stock 1 quantity parts gives as below. I didn't specify Rdson but could have.

Infineon PSMN022-30PL - datasheet here in stock at 73 cents US in 1's. Pricing
30V, 30A, TO220, Rdson 34 milliOhm max at 5V gate drive, 41 W max dissipation,

See fig 1 for max dissipation based on heatsink temperature.
Thermal resistance is 3.6 C/W junction to case so you'd want to keep dissipation under 20W and ideally under 10W - SO probably OK.

So - how do you drive this.

  • You MAY be able to drive it straight from the amp with a potentiometer to adjust level. MOSFET in linear mode, probably.

  • Feed amp output to a comparator to make rail-rail signal and drive FET. Lower dissipation. Don't exceed max gate drive. May not look like what you want to see. May. MOSFET in PWM mode - low losses.

  • Use a linear amplifier to adjust level up or down as required. Can add simple filtering to adjust hang time, attack time, frequency response, more ... . These are all "easy enough" but lets see if this is what you want. MOSFET in linear mode.

... 30PL - slightly better Rdson, slightly more $

Infineon IPP065N03LGIN - pricing and datasheet
30V, 50A, 6.5 milliOhm Rdson, 2.7 C/W, TO220 10 milliOhm Rdson worst case at gate drive of 4.5V. $US1.05/1 in stock.
Looks very good for this task.


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