MOSFET Discussion: Difference between revisions
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(Created page with 'Question: On the replacement mosfets you recommended - http://openfarmtech.org/index.php/CEB_Automation_Prototype_II#Solenoid_Valves Given the 3A current requireme…') |
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Question: | = Question: = | ||
On the replacement mosfets you recommended - | |||
http://openfarmtech.org/index.php/CEB_Automation_Prototype_II#Solenoid_Valves | |||
Given the 3A current requirement of the 12V solenoid valve, do we need the larger MOSFET you suggested if the Arduino drivers can handle 5A? Do you think there will be startup spikes? Do you think that the Arduino driver setup is built to handle spikes? | |||
Response: | = Response: = | ||
(Fairly pure) inductors (such as solenoids and relays) don't show a current spike at turn on: The inductor resists changes of current. (see http://www.allaboutcircuits.com/vol_1/chpt_16/3.html) | (Fairly pure) inductors (such as solenoids and relays) don't show a current spike at turn on: The inductor resists changes of current. (see http://www.allaboutcircuits.com/vol_1/chpt_16/3.html) | ||
The TIP120 has two disadvantages: | The TIP120 has two disadvantages: | ||
* It has a peak current handling of only 8A, though this isn't a problem for the CEB application | |||
* If I'm reading the data sheet correctly, it has a voltage drop of 2V at 3A and 4V at 5A. That means that your 12V supply only delivers 10V into a 3A load in series with the TIP120, and the TIP120 needs a heat sink to dissipate the 6W (2V x 3A) that it's wasting. | |||
With the MOSFETs you can do without a heat sink. | With the MOSFETs you can do without a heat sink. | ||
Revision as of 22:46, 19 February 2010
Question:
On the replacement mosfets you recommended -
http://openfarmtech.org/index.php/CEB_Automation_Prototype_II#Solenoid_Valves
Given the 3A current requirement of the 12V solenoid valve, do we need the larger MOSFET you suggested if the Arduino drivers can handle 5A? Do you think there will be startup spikes? Do you think that the Arduino driver setup is built to handle spikes?
Response:
(Fairly pure) inductors (such as solenoids and relays) don't show a current spike at turn on: The inductor resists changes of current. (see http://www.allaboutcircuits.com/vol_1/chpt_16/3.html)
The TIP120 has two disadvantages:
- It has a peak current handling of only 8A, though this isn't a problem for the CEB application
- If I'm reading the data sheet correctly, it has a voltage drop of 2V at 3A and 4V at 5A. That means that your 12V supply only delivers 10V into a 3A load in series with the TIP120, and the TIP120 needs a heat sink to dissipate the 6W (2V x 3A) that it's wasting.
With the MOSFETs you can do without a heat sink.
Cheers,
Leo