What are you trying to switch, and how are you measuring this voltage? The difference between 7mV and 0V is often unimportant in switching applications. I can't tell from your message whether the 7mV is on the gate or at the load, but I doubt it matters either way. If it does matter, you're probably using the wrong switching technique altogether.
If you're just measuring the voltage at the output of the switch, especially if there's no load on it at the time, don't sweat it. Other than the fact you're measuring a little voltage there, is everything working properly (I
*think* I understand the answer to be yes)?
Zero only exists inside computers. There's no such thing as zero in the real world.
"Tuurbo46" wrote in message news:d2ukkc$eso$ snipped-for-privacy@news8.svr.pol.co.uk...
Hi.
power because its in a remote location. The FET
I believe it is doing just that and agree with Mr. Harley's take on the importance of what you see. A MOSFET with 7 mV gate-to-source is effectively off if it is the usual (enhancement mode) kind. (If it was a depletion mode device, it would be on.) I suggest you put either a current meter (if you have one) or a 100K resistor (with your mill-voltmeter across) in series with your nominally off circuit and measure the actual current when the MOSFET is supposedly off. That should show whether there is a real problem or or not.
and the source. Currently there is 0.007V on the
wasting power).
Your premise is wrong. Lookup the meaning of "threshold voltage" in relation to MOSFETs.
and source) is connected and therefore my FET driven
and there is no mains power supply and the FET
and when the radio is supposed to be off it isnt
this small error to 0V.
Is the radio observably not off? If it continues to draw power, the cause is something beyond the 7 mV you mention. In that case, it is time to show some kind of schematic.
No worry, all par for the course.
[quotes and sig only follow]
fet and the driven circuit works correctly, but when
completely to zero and therefore the switching circuit
and this doesnt work. Also i have tried putting a 10k
voltage from micro through a precision voltage supply
difference between 7mV and 0V is often unimportant in
the gate or at the load, but I doubt it matters
technique altogether.
if there's no load on it at the time, don't sweat
everything working properly (I *think* I understand the
world.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
If this small gate voltage is accompanied by measurable drain current, I suspect you have blown the gate insulation with static and the mosfet is no longer functional.
Hi Im currently having a switching problem. Im applying 5V to the gate of this fet and the driven circuit works correctly, but when the gate is switched off, the micro which is driving the gate does not go completely to zero and therefore the switching circuit stays on slighty (0.007v).
I have tried putting a 1N4001 diode in series with the gate and the mirco and this doesnt work. Also i have tried putting a 10k resistor from the gate to the source(0V) and this also does not work.
At this point im stuck. Would my gate voltage read 0V if i ran the output voltage from micro through a precision voltage supply (e.g 3 pin chip)?
Im trying to switch a radio on and off at certain times by a micro to save power because its in a remote location. The FET circuit is switching the radio on and off.
The gate voltage has to be 0V, else there is still a circuit between the drain and the source. Currently there is 0.007V on the gate and therefore the drain and source are still in use (therefore radio is wasting power).
When the gate to the FET is 5V the ground 0V to my radio ( through the drain and source) is connected and therefore my FET driven circuit is working. The problem in having is my radio is in a remote location and there is no mains power supply and the FET circuit is switching the radio on and off. At the moment this is not happening and when the radio is supposed to be off it isnt because the gate voltage is 0.007V above 0V. I am therefore trying to make this small error to 0V.
All mosfets have a threshold voltage below which they don't conduct. 7 mV on the gate is never going to turn on an enhancement mode mosfet., so that certainly isn't your problem.
It's not the 7 mV.
What's the supply voltage for the radio that you're switching ? What's the load current for the radio ? What is the mosfet's drain voltage in on and off conditions ?
I read in sci.electronics.design that Pooh Bear wrote (in ) about 'FET (TOSHIBA 2SK941) SWITCHING', on Tue, 5 Apr 2005:
It's difficult to understand what you are doing. Have you put the FET in the negative supply to your radio? Is the radio grounded? Is the power supply grounded?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Like Graham said: it's not the 7mV that's your problem. The problem is either that you've got the circuit configured wrong, or that you've blown the FET, or both.
Can you draw a simple schematic here to show what you're doing? (Google "aacircuit" to find a simple, easy, free ASCII schematic editor.) Also, is the 2SK941 an N-channel or a P-channel FET? (And as Graham asks, can you point us to a datasheet somewhere?)
FETs are analog devices, not digital. They don't have a well-defined "on" and "off", even though we sometimes talk about them that way. Rather, the max current from drain to source is determined by the voltage from gate to source (or drain, but that's another story) in a fairly smooth way. Below a certain Vgs, very little current flows. As Vgs approaches a "threshold" voltage of a couple of volts (depending on the FET), more and more current is allowed to flow; and then, as Vgs continues to increase, at a certain point the max current hits its limit. If you look at some FET datasheets you can see graphs of Id versus Vgs that will show you what I mean.
The point being, 7mV is way below threshold, so you should expect a small amount of current to flow but (a) no different than if Vgs was zero and (b) not enough to power the radio.
However, there are certain very common mistakes that newbies make in designing FET circuits, and I suspect (and so does John, I think) that you've made one of them. Thus the request for you to show us a schematic.
Sounds like you have Drain/Source leads reversed on the MOSFET...have you verified with an ohmmeter- there is an internal body diode with anode at Source and cathode at Drain...
Yes you are all correct and i made a silly error(school boy error). 7mV on the gate is erelavent! I disconnected the wire from the FET to the radio and put an amp meter in series with it and when the gate was 5V the radio was using 150mA, which is correct. When the gate was 7mV the wire to the radio was 0mA. The thing which confused me yesterday was that the (on led) on the radio was dimly visable when off (7mV on gate). This was attributed to a big capacitor in the radio. This can also be seen when all leads are disconnected from radio and the led is still dim for a short period( 1 min ).
The lesson i have taken from this is dont try and short cut jobs and use a multimeter more.
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