View Full Version : looking for some help

07-05-2015, 04:09 PM
I am trying to design a 4 channel dc ssr that will be used with 12 v or 5 v DC only

as I am in the UK the standard 512-FQPF13N06L MOSFET 60V N-Channel QFET Logic Level used in the designs on the wiki are hard to get over here and most websites state they ship from the USA.

would this part be inter-changeable?

IRF540 n channel mosfet

here is the date sheet




07-06-2015, 09:05 AM
Yes, you can use it as a switch.
The gate voltage spec has a range of 2V to 4V when the switch will start to turn on. This range can be at the higher end for your units, causing your Arduino to not fully turn the lights on. In this case you may need a driver transistor between the Arduino output and the mosfet's gate. Hence the 12V supply would be better to ensure a good switch on state.
How to implement it can be varied. Some use an opto-isolator, some don't.
An opto example is here .. https://arduino-info.wikispaces.com/Brick-4ChannelPowerFetSwitch
There may be other designs you can find, to reduce parts count, if you search for it.
Or you could experiment on your own - if you have the basic tools to evaluate your results.

07-06-2015, 11:03 AM
if I used version 1 schematic would i need to change the resistor voltage between the opto and transistor

http://www.doityourselfchristmas.com/wiki/index.php?title=4_Channel_DCSSR_Assembly_Instructi ons

I am happy to go with out the optos but if it is going to struggle I will just use them :)

07-06-2015, 12:56 PM
if I used version 1 schematic would i need to change the resistor voltage between the opto and transistor ..
No, you may leave them as they are. You can also try the circuit as is, but use the IRF540 for the mosfet and see if it works for you.
You didn't indicate your lights' power supply, nor current load you plan to drive.

The mosfet is operated as an ON-OFF switch. The higher the gate voltage the harder the mosfet turns on, until it saturates. It takes anywhere from 2V to 4V to start conducting, according to the data sheet. The gate voltage to cause saturation has a variation from device to device and thus can't be given accurately. You can use the transconductance graph to get a ball park value.

If your mosfet gets hot, or the lights don't reach full brightness, it may indicate that the mosfet is not turned on hard enough. In this case you should use a 12V power supply to drive the opto transistor so that gate of the mosfet will get close to 12V of drive.

07-06-2015, 01:07 PM
My strips are 12v dc and run around 1.5 amp max

I have some 5v strips but I'm not sure if I'm going to use them this year,

If I use version one design with a 12v regulator on it I should be ok and will get full brightness :)

07-06-2015, 02:33 PM
Yes, it should.

07-06-2015, 03:36 PM
Thank you :)

now time to design in eagle should have it done by tonight

thank you for your help Lightup

P. Short
07-06-2015, 07:15 PM
You need to look at the i-v curves in the datasheet to see how well the part will work in your app. I can't do it myself now because my phone won't display pdf files. Since the part is likely in the same family as the irf530 mentioned in an earlier thread, it probably needs more than 4V to get to a sweet spot on the i-v curve.

07-06-2015, 10:08 PM
... it probably needs more than 4V to get to a sweet spot on the i-v curve.
Yes, the mosfet turn on is listed between 2V (min) and 4V (max). Hence the 12V drive supply would be a better choice, as recommended.

P. Short
07-07-2015, 11:27 PM
If you look at the 'Output Characteristics' graph, you will see that the part will typically pass only about 2A with a 4V gate-drive. I'd imagine that it's a lot worse with a 2V gate drive. It should be noted that this is a 'typical' value, not a guaranteed value. The 'Gate Threshold Voltage' parameter listed in the 'ON' section of the datasheet is a largely worthless characteristic, as the 'ON' state is defined there as passing only 250uA of current.

I think that to guarantee good results the design should go for at least 5V of gate drive. There are many ways to achieve this, although I think that the design using opto-isolators is the safest way.

07-08-2015, 02:05 AM
Yea I'm using optos and a 12v regulator, I am only using 12v this year my 5v plans have changed

Nearly finished my design just spent last night re building a go kart engine and racing around the track and tonight I'm working in a bowling alley maybe Thursday night