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Topic: Tip 122 vs. ULN2003 (Read 2 times) previous topic - next topic

anderscrib

I have a project that I need to control 6 lengths of LV flex strip LEDs independantly. I have seen and read about using a tip122 to switch the ground but I hav also seen a ULN2003 chip used; My longest length of lighting strip will be 11.5' which is about 17.25 Watts. Can anyone provide some benefits on either? or some suggestions to help?
Many thanks

CrossRoads

How much current are you sinking? Probably need to with hefty really low Rds N-channel MOSFETs.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

dc42

#2
Oct 12, 2012, 12:29 am Last Edit: Oct 12, 2012, 06:38 pm by dc42 Reason: 1
TIP122: like all power darlingtons, pretty much obsolete these days. Darlingtons have high saturation voltage, so at high currents (e.g. > 1A) they dissipate too much power and a mosfet is a much better choice. At lower currents, a normal NPN transistor will do the job with less power dissipation, if you choose an appropriate transistor.

ULN2003: it has darlington outputs too, so has the same problems as the TIP122. Its redeeming feature is that it integrates eight transistors plus base drive resistors into a single package. However, if you try to sink more than about 70mA from each of the 8 outputs at the same time, it it likely to overheat because of the relatively high saturation voltage.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

anderscrib

Crossroads,
"How much currrent are you sinking?"....be patient with me..
P/V=I:  my longest length is 11.5' and about 17.25 watts at 12 volts. 17.25/12= 1.43 Amps (?)
(that is a question..) the other lengths will be shorter.

CrossRoads

Math looks good.
I think you'll want indiividual logic-level low Rds N-channel MOSFETs then.
Use to connect the - of the strip to Gnd, connect + to +12V.
A part like this for example
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
Drive from Arduino pin with 150 ohm resistor in series so Gate capacitance does not damage anything (MOSFET internal cap looks like a short to Gnd, resistor limits the current to ~35mA.)
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

anderscrib

Thank you CrossRoads.
I ordered a very similar MOSFET to the one you suggested. I got the MOSFET-N-CH 60V 16A TO-220..I am going to post my schematic in a bit and would love for your input..
Thanks CrossRoads.

dc42


I ordered a very similar MOSFET to the one you suggested. I got the MOSFET-N-CH 60V 16A TO-220..I am going to post my schematic in a bit and would love for your input..


Did you make sure it was a logic-level mosfet? The key is the voltage at which Rds(on) is quoted. This needs to be 5V or lower.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


CrossRoads

Yes, that first one looks really nice if you have a way to mount it. Rds of 0.00036 ohm at 5V.

The 2nd one, all the testing is spec'ed with Vgs=10V, so its not a "logic level" part. I did specifically say a logic level part.
You can still use it, you just need an NPN transistor in front of it to pull the gate low to turn it off when the NPN is turned on, and a pullup resistor to your 12V source to turn the MOSFET on when the NPN is off. You probably want a pullup to 5V on the NPN base so it is turned on at reset to hold the MOSFET off until you sketch can take control of the IO pins (all are set to inputs after a reset/power on).
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

anderscrib

Thanks CrossRoads, great input. I do want to get the right one and not add components.
I found these guys local in Miami to avoid shipping.. They have a list here of logic mosfets:
http://www.nteinc.com/Web_pgs/LL_MOSFET.html

I am trying to figure out which one... My longest strand of lights is 16' @ 36w and 3Amps..I think 2980 or 2981 would be ok..(?)
Thanks Crossroads, this has been a big help.

CrossRoads

Do some calculating for power dissipation: I^2* Rds

3A*3A*.2ohm = 1.8W - so you'd probably want to heat sink that.
vs
3*3* 0.1, 0.05, 0.028
Makes a difference.
Use a low value gate resistor to drive max current into the Input Capacitance so it changes state fast.
5V/.035A = ~150 ohm
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

anderscrib

trying my best to digest that..
Thanks

CrossRoads

Power dissipated by the MOSFET when turned full on is:
Power = Voltage x Current
Voltage = Current x Resistance, subsitute into the power equation:
Power = (Current x Resistance) x Current
commonly written as P=I^2*R
So you want 3 amps of current, you have Rds of 0.2 ohm, Power = 3 x 3 x 0.2 = 1.8W
Find a part with lower Rds and the MOSFET will dissipate less power:
3x3x0.1 = 0.9W
3x3x0.05 = 0.5W
3x3x0.028 = 0.25W, will be nice & cool

Put a 150 ohm resistor between your arduino output pin and the mosfet gate, will keep the arduino pin from being damaged.
Add a 10K pulldown resistor from the gate to ground to ensure the mosfet stays off while the ardiuno is power up, and the arduino pins are not yet driven low by the sketch.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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