mosfet trouble

Archiver · 2002-02-26 04:16

Hello,

I'm trying to turn the power of an adjustable regulator (LM317 type)
on and off using a mosfet (IRF-510) as described in the stamp faq on
Al Williams' web site. http://www.al-williams.com/wd5gnr/stampfaq.htm

The mosfet gate is connected to a stamp pin, the source is connected
to +12 and the drain goes into the positive lead of the regulator.
The problem is that the mosfet is always conducting regardless of the
signal coming out of the stamp. I've tried a pulldown resistor on
the gate pin, and i replaced the mosfet with a new one, but no
success.

Can someone tell me what is going wrong here?

Thanks,
Stewart

--
Stewart Mayer, stewlist@k... on 2/25/2002

Archiver · 2002-02-26 16:18

For a Stamp I/O pin to switch a MOSFET on and off the source needs to be
connected to ground, the same ground as the Stamp (Vss). I'm not sure where
you want to connect the drain of the MOSFET in your application. Could you
explain your circuit in more detail?
--
Greg

Stewart Mayer wrote:
>Hello,
>
>I'm trying to turn the power of an adjustable regulator (LM317 type)
>on and off using a mosfet (IRF-510) as described in the stamp faq on
>Al Williams' web site. http://www.al-williams.com/wd5gnr/stampfaq.htm
>
>The mosfet gate is connected to a stamp pin, the source is connected
>to +12 and the drain goes into the positive lead of the regulator.
>The problem is that the mosfet is always conducting regardless of the
>signal coming out of the stamp. I've tried a pulldown resistor on
>the gate pin, and i replaced the mosfet with a new one, but no
>success.
>
>Can someone tell me what is going wrong here?
>
>Thanks,
> Stewart
>
>--
>Stewart Mayer, stewlist@k... on 2/25/2002

Archiver · 2002-02-26 20:58

That sounds like the problem. The source pin of the mosfet is connected to +12
and the drain goes to the voltage input of the regulator. Essentially I have
the mosfet working as a switch because normally the input of the regulator
would be directly connected to +12.

The regulator is not directly connected to ground, it has Vin, Vout, and an
Adjust pin that is connected to ground through a resistor. If it was a fixed
78xx regulator then it would have a path to ground and might work. I suppose I
could interrupt the output power at ground instead of at Vin. I would also
have to interrupt the weak ground connection to the adjust pin so power isn't
sucked when it is off. I'll try that and see if it works.

Thanks,
Stewart

On Tue, 26 Feb 2002 10:18:19 -0600, Gregory D Wagner wrote:
>For a Stamp I/O pin to switch a MOSFET on and off the source needs to be
>connected to ground, the same ground as the Stamp (Vss). I'm not sure where
>you want to connect the drain of the MOSFET in your application. Could you
>explain your circuit in more detail?
>

--
Stewart Mayer, stewlist@k... on 02/26/2002

Archiver · 2002-02-26 22:50

If you connect the MOSFET drain to the LM317 adjustment pin, the source to
ground and the gate to a Stamp I/O pin you can almost shut the regulator
down. A +5V on the gate would turn the MOSFET on and essentially make R2=0.
The minimum output voltage is 1.25V since Vo=1.25(1+R2/R1).
--
Greg

Stewart Mayer wrote:
>That sounds like the problem. The source pin of the mosfet is connected
>to +12
>and the drain goes to the voltage input of the regulator. Essentially I have
>the mosfet working as a switch because normally the input of the regulator
>would be directly connected to +12.
>
>The regulator is not directly connected to ground, it has Vin, Vout, and an
>Adjust pin that is connected to ground through a resistor. If it was a fixed
>78xx regulator then it would have a path to ground and might work. I
>suppose I
>could interrupt the output power at ground instead of at Vin. I would also
>have to interrupt the weak ground connection to the adjust pin so power isn't
>sucked when it is off. I'll try that and see if it works.
>
>Thanks,
> Stewart
>
>On Tue, 26 Feb 2002 10:18:19 -0600, Gregory D Wagner wrote:
> >For a Stamp I/O pin to switch a MOSFET on and off the source needs to be
> >connected to ground, the same ground as the Stamp (Vss). I'm not sure where
> >you want to connect the drain of the MOSFET in your application. Could you
> >explain your circuit in more detail?
> >
>
>--
>Stewart Mayer, stewlist@k... on 02/26/2002
>
>
>
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Archiver · 2002-02-27 08:13

>That sounds like the problem. The source pin of the mosfet is
>connected to +12 and the drain goes to the voltage input of the
>regulator. Essentially I have the mosfet working as a switch because
>normally the input of the regulator would be directly connected to
>+12.

It sounds like the circuit you have (or should have) uses a p-channel
mosfet switch. (not n-channel) The trouble is, you can't turn off
that p-mosfet directly from the stamp pin, which outputs a maximum of
5 volts, because it takes 12 volts on the gate to turn it off (OFF
happens when gate voltage=source voltage, and ON happens when gate
voltage is less than source voltage by more than about 3 volts).
The usual trick is to use a second mosfet or transistor in open drain
mode to translate the voltage from the Stamp up to 12 volts:

p-ch _______
+12--o

;-; ;----| LM317 |-o-- out
| =====

|
| R | gate | |
`-/\/\-- o o-/\/\-'
| | R
||-' \
stamp ||-; n-ch / R
P0---||-| \
| |
Vss Vss

-- Tracy

Archiver · 2002-02-27 16:40

<HTML><HEAD>
<BASEFONT FACE="Arial" SIZE="4" COLOR="#000000">
</HEAD>
<BODY bgcolor="#DFDFDF">
<div>Thank you Tracy, I'm just getting into mosfets and transistors, and this
helps very much. I like your ASCII art!</div>
<div> </div>
<div>Stewart</div>
<div align="center">
<table BORDER="1" width = "100%"><tr><td WIDTH="50%"> 
>It sounds like the circuit you have (or should have) uses a p-channel 
>mosfet switch. (not n-channel) The trouble
is, you can't turn off 
>that p-mosfet directly from the stamp pin,
which outputs a maximum of 
>5 volts, because it takes 12 volts on the gate to turn it off (OFF 
>happens when gate voltage=source voltage, and ON happens when gate 
>voltage is less than source voltage by more than about 3 volts). 
>The usual trick is to use a second mosfet or transistor in open drain 
>mode to translate the voltage from the Stamp up to 12 volts: 
> 
>p-ch _______ 
>+12--o

;-; ;----| LM317 |-o-- out 
>| =====

| 
>| R | gate |
| 
>`-/\/\-- o
o-/\/\-' 
>| |
R 
>||-' \ 
>stamp ||-; n-ch /
R 
>P0---||-|
\ 
>| | 
>Vss Vss 
> 
>-- Tracy 
 
</td>
<td WIDTH="25%" COLSPAN=2> </td>
<td WIDTH="13%"> </td>
<td WIDTH="13%"> </td>
</tr>
<tr><td WIDTH="50%"> </td>
<td WIDTH="25%" COLSPAN=2> </td>
<td WIDTH="13%"> </td>
<td WIDTH="13%"> </td>
</tr>
<tr><td WIDTH="50%"> </td>
<td WIDTH="25%" COLSPAN=2> </td>
<td WIDTH="13%"> </td>
<td WIDTH="13%"> </td>
</tr>
</table></div>
<div>-- </div>
<div>Stewart Mayer, stewlist@k... on 02/27/2002</div>
</body></html>

mosfet trouble

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