Switch Help for Current Limiting

Discussion in 'General Electronics Chat' started by cgha20@yahoo.com, Feb 23, 2010.

1. cgha20@yahoo.com Thread Starter Well-Known Member

Oct 21, 2009
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I need to know how to connect an N or P Channel Enhanced Power Mosfet so that at first, the 10 ohm resistor attenuates the inrush current. Then the RC configuration turns on the MOSFET at about 2ms. And the final goal is that the 10 ohm resistor is then bypassed or shorted out of the circuit. All I have available is above mentioned Mosfet. The input Voltage is 8 to 40V. See attached picture. Thanks.

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2. Bernard Expert

Aug 7, 2008
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565
Try resistor gate to ground, cap + V to gate. MOSFET P ch., S to + V.

3. cgha20@yahoo.com Thread Starter Well-Known Member

Oct 21, 2009
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Thanks for your response. Are you saying that I should use another resistor from gate to ground? What is the reason for this and what value? i take it that the drain is connected at after voltage drop of the 10 ohm res.? Thanks.

4. SgtWookie Expert

Jul 17, 2007
22,198
1,800
This is something close to what you're looking for:

R1 is your 10 Ohm inrush limiter.
C1 is the timing capacitor.
D1 is a 10v Zener diode; it limits the maximum Vgs to -10v; most power MOSFETs have a limit of +/-20v for Vgs, beyond which they will be destroyed.
The LM317 and R2 make up a 10mA (approximate) constant current sink. If the input voltage exceeds 40v, the LM317's maximum ratings will be exceeded. The constant current sink charges the timing cap C1 until the Zener voltage is reached, then the Zener prevents the cap from further charging.

Q1 is currently a standard P-ch power MOSFET; it really should be a logic-level MOSFET if you want to operate the circuit at less than 13v. If Q1 is a logic level MOSFET, you can then operate down to 8v, adjusting C1 to compensate for the change in gate threshold voltage and changing the Zener to one rated for 4.7v to 5.1v. (edited x2)

D2 discharges C1 when input power is turned off.

One problem with this circuit is the slow MOSFET turn-on after the 2mS time elapses. Depending on your load, power dissipation in the MOSFET may be excessive during that time.

• 2ms Inrush Limiter.PNG
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Last edited: Feb 23, 2010
5. Bernard Expert

Aug 7, 2008
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Using Sgts ckt & suggestions, if expotential decrease in 10 ohm current is acceptable, then substitute a 300 ohm for LM317 & R2, I think.

6. cgha20@yahoo.com Thread Starter Well-Known Member

Oct 21, 2009
78
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thanks for your help. But it seems we have introduced alot more parts. The other restriction I have is that this circuit needs to work for very high temperatures. 175C and above. So my component list is small. Regulators are not too forgiving at those temperatures. Also, I do not want to draw much power. What is the most basic circuit based on the drawing I placed. Would a FET be better?

7. Ron H AAC Fanatic!

Apr 14, 2005
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If you are commenting on an existing thread, append your comment to that thread. Otherwise, we may have no idea what you are talking about.

Oct 2, 2009
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Apr 20, 2004
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10. cgha20@yahoo.com Thread Starter Well-Known Member

Oct 21, 2009
78
0
thanks for your help. But it seems we have introduced alot more parts. The other restriction I have is that this circuit needs to work for very high temperatures. 175C and above. So my component list is small. Regulators are not too forgiving at those temperatures. Also, I do not want to draw much power. What is the most basic circuit based on the drawing I placed. Would a FET be better?

Oct 21, 2009
78
0