Power MOSFET as a voltage controlled resistor

In summary, the conversation revolved around using a Power MOSFET as a varying load resistance to match an internal resistance for peak power. The main question was what properties to look for when selecting a power MOSFET for this purpose. Suggestions were made for specific types of MOSFETs that could handle the required power and vary in resistance, and ideas for using an op amp to automate the impedance matching process. The conversation also touched on the possibility of using a DC-DC converter instead.
  • #1
atlbraves49
81
0
We need to have a load resistance to match an internal resistance for peak power.. so we've decided to use a Power MOSFET as the varying load resistance, but my question is.. when I'm looking for which power MOSFET to buy, to use, what should I be looking for in terms of properties?

This is what we need:

The Power MOSFET...
1) Has to be able to handle the ~25W of power that will be fed into it
2) The resistance has to vary between a couple ohms and 25 ohms.

Any ideas?
 
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  • #2
BUZ and IRF series.


BUZ11A 90 Watt device. (with suitable heat sink).

IRF540 100 Watts 22 Amp.



IRF640 125 W 200 v.

All quite cheap.
 
  • #3
and they would all be able to vary in resistance from 1ohm to 25 ohm, depending on the "voltage control" ?
 
  • #4
I think all the BUZ and IRF types go below 1 ohm and the max resistance is pretty high. All enhancement mode I think. Need a postive voltage between gate and source to turn them on (low R).

To prevent the resistance from ever going too low you could put a low valve high power resistor in series.. 0.5 Ohm say.. In the source supply and apply the control voltage between the power supply side of the resistor and the gate (this will stabilise the current through the Mosfet to some extent).


http://www.datasheetcatalog.org/datasheet/philips/IRF640_S_1.pdf
 
Last edited:
  • #5
thanks for the help; one more question, what should i be looking for on datasheets that will tell me it can reach 1-25 ohms? On the datasheet you posted, the only thing I see related to resistance is "Drain-Source(ON) resistance" of hundreds of mOhms (which of course is too small), so what am I missing.. in terms of knowing whether a power mosfet can reach 25 ohms?
 
  • #6
With no bias voltage on the gate the MOSFET will be more or less an open circuit falling to near zero with a certain positive voltage on the gate so will cover the range you want. You will have to experiment and see what bias voltage is required. As I say if you put a fixed resistor in series of 0.5 or 0.75 Ohm it will ensure you can never go below those values even if the MOSFET is switched hard on.

I will try and draw a diagram.
 
  • #7
Select R2 to give the max require resistance. Select on test. I would guess 50K or more.

DIAGRAM is WRONG. R2 to be below the 10 K potentiometer. In fact you could have resistors top and bottom to get your required range.

I can't guarantee this is all correct. You need to experiment.
 

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  • #8
Select R2 to give the max require resistance. Select on test. I would guess 50K or more.

DIAGRAM is wrong. R2 to be below the 10 K potentiometer. In fact you could have resistors top and bottom to get your required range.

I can't guarantee this is all correct. You need to experiment.
 
  • #9
atlbraves49 said:
We need to have a load resistance to match an internal resistance for peak power.. so we've decided to use a Power MOSFET as the varying load resistance, but my question is.. when I'm looking for which power MOSFET to buy, to use, what should I be looking for in terms of properties?

This is what we need:

The Power MOSFET...
1) Has to be able to handle the ~25W of power that will be fed into it
2) The resistance has to vary between a couple ohms and 25 ohms.

Any ideas?

Hi atlbraves. Just wondering, are you trying to use this power for any purpose othe than creating heat in the mosfet?
 
  • #10
uart said:
Hi atlbraves. Just wondering, are you trying to use this power for any purpose othe than creating heat in the mosfet?

Yep, it's for impedance matching (for peak power).

We have a thermoelectric module, generating power, with some internal resistance, and we want a load that will automatically match that impedance to get peak power.
 
  • #11
atlbraves49 said:
Yep, it's for impedance matching (for peak power).

We have a thermoelectric module, generating power, with some internal resistance, and we want a load that will automatically match that impedance to get peak power.

So you've got a thermo-electric module converting heat to electric power and then you want to convert the electric power back into heat in the mosfet. Is this just a measurement exercise or are you trying to do something else. Are you sure you're not really looking for a DC-DC converter to match the thermocouples output characteistics and produce electric power rather than heat?
 
  • #12
uart said:
So you've got a thermo-electric module converting heat to electric power and then you want to convert the electric power back into heat in the mosfet. Is this just a measurement exercise or are you trying to do something else. Are you sure you're not really looking for a DC-DC converter to match the thermocouples output characteistics and produce electric power rather than heat?

yes this is completely a testing / measurement exercise, we are NOT interested at all in utilizing the power that is being created by the thermoelectrics. The setup is for testing various conditions related to thermoelectrics, so what happens to the power is irrelevant. Right now it is being dumped across resistors that are manually matched (with rheostats) to the internal impedance. We are looking to automate that "impedance matching" so we don't have to sit there and tune rheostats.
 
  • #13
I was thinking of using an op amp to drive the gate of the MOSFET. Use the voltage across the internal resistance as the inputs. This way, the op amp will automatically adjust the gate of the FET to such a level that you have the best matching. Just an idea.
 
  • #14
Pumblechook said:
BUZ and IRF series.


BUZ11A 90 Watt device. (with suitable heat sink).

IRF540 100 Watts 22 Amp.



IRF640 125 W 200 v.

All quite cheap.

I know this was a long time ago but I have a similar need for a variable resistance (0-240 Ohms) at 12 volts, which can be controlled with a varying voltage from a micro. I looked at the spec sheet for the above devices and it's not clear how to read the salient specs. Any help would be appreciated.
 
  • #15
You can purchase a dynamic load, but that costs money...

Or, you can try this one :)

Most any op amp will do including 741 or TL061, T071, TL081.

R1 needs to be BIG, i.e 10 watt and M1 needs to be fastened to a fairly big heat sink. The little heat sinks that hang off the part are about 1/20th what you'll need. A sheet of 1/8" 6" square aluminum should work nice. Or, even better, you can get a big computer heat sink with integral fan.

Best Luck,

Mike
 

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  • #16
Mike;

Thanks for the input.
 

Related to Power MOSFET as a voltage controlled resistor

1. How does a Power MOSFET function as a voltage controlled resistor?

A Power MOSFET (metal-oxide-semiconductor field-effect transistor) is a type of transistor that can be used as a voltage controlled resistor. By varying the voltage applied to the gate terminal of the MOSFET, the resistance between the source and drain terminals can be controlled. This is due to the fact that the voltage at the gate creates an electric field, which affects the flow of current between the source and drain, effectively changing the resistance of the MOSFET.

2. What are the advantages of using a Power MOSFET as a voltage controlled resistor?

One of the main advantages of using a Power MOSFET as a voltage controlled resistor is that it allows for very precise and fast control of the resistance. Additionally, MOSFETs have a high input impedance, meaning they require very little current to control the resistance. They also have a low output impedance, resulting in minimal signal distortion.

3. Are there any limitations to using a Power MOSFET as a voltage controlled resistor?

One limitation of using a Power MOSFET as a voltage controlled resistor is that it can only be used for relatively small signal levels. Additionally, MOSFETs have a limited voltage and current handling capacity, so they may not be suitable for high power applications.

4. How does the resistance of a Power MOSFET vary with changes in the gate voltage?

The resistance of a Power MOSFET is inversely proportional to the gate voltage. This means that as the gate voltage increases, the resistance decreases, and vice versa. This relationship is typically described by a transfer function, which can be used to predict the resistance at a given gate voltage.

5. Can a Power MOSFET be used as a variable resistor in a circuit?

Yes, a Power MOSFET can be used as a variable resistor in a circuit by connecting it in a specific configuration, such as a common-source amplifier. By varying the gate voltage, the resistance of the MOSFET can be adjusted, effectively changing the gain of the amplifier. This can be useful in applications such as audio amplifiers, where precise control of gain is necessary.

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