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# Mosfet Switching - Constant Power

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#### petesmc

##### New Member
Hi,

I have a circuit, with a resistor between the drain and 12V, of a mosfet and I have a wave, switching the mosfet on and off. During the off periods, the resistor gets no power. I was wondering if it's possible to make it, so that when the mosfet is on, the resistor has 12V, and when off, it has 5V. Can this be done? Or should I be looking at different components?

-Peter

#### Nigel Goodwin

##### Super Moderator
petesmc said:
Hi,

I have a circuit, with a resistor between the drain and 12V, of a mosfet and I have a wave, switching the mosfet on and off. During the off periods, the resistor gets no power. I was wondering if it's possible to make it, so that when the mosfet is on, the resistor has 12V, and when off, it has 5V. Can this be done? Or should I be looking at different components?

-Peter

Just put a resistor across the mosfet - calculate the value of the two resistors to give 5V (it's a 'potential divider', you can use ohms law to calculate the values, bearing in mind the impedance it's feeding to).

#### petesmc

##### New Member
Thank you. I had this idea just randomly before but hadn't tried it, cos i thought it would be stupid. I'll try it out!

#### Nigel Goodwin

##### Super Moderator
petesmc said:
Thank you. I had this idea just randomly before but hadn't tried it, cos i thought it would be stupid. I'll try it out!

The ratio of the resistors needs to be 5/7 - could be fun working the closest preferred values out :lol:

I wrote a computer program years back (under DOS, using Turbo Pascal) which worked out resistors in parallel - you entered a value and it gave you the two preferred values that gave the closest match. I don't know what happend to it!.

#### jem

##### Member
Maybe you should consider a pot for one of the resistors. Also, do not forget to consider the input impedance of the following (driven) circuit, as it will affect the voltages.

#### petesmc

##### New Member
Ok, realised this won't work, as my first resistor is variable, so would have to use a pot, and that isn't an option. So i'm now thinking of a way, to switch between a 12V and a 5v line.

This then feeds into the resistor-> through mosfet, to ground... I was thinking relay, but the 12/5V lines will be switching up to 100Hz so this is not an option.

-Peter

#### Nigel Goodwin

##### Super Moderator
Perhaps you should post the circuit?, and an explanation of exactly what you are trying to do?.

It's quite possible that others may be able to suggest a better (or at least different) way of doing it.

#### petesmc

##### New Member
I can't really do that, though I can give explanation.

I'm trying to keep a component, resistance of 5.6Ohms constantly powered, though using PWM. At full voltage is 12V and at low voltage, anywhere between 4-5 volts roughly. So the PWM oscillates between 12V and 5V.

I have got a circuit working for the PWM from 0-12V handling the full load (roughly 2A), however, It needs power constantly. Now I'm just looking for a method, so when PWM is at 0V, it opens the 5V line to the mosfet, and when at 12V, it opens the 12V line to the mosfet. Both cannot be open at the same time.

I've been messing around with some transistor/feedback circuits trying to make something up, but having trouble. I thought about using a mosfet as an analogue switch, but don't think this will work.

#### Nigel Goodwin

##### Super Moderator
Is there some reason you can't say what the component is?.

I'm struggling to understand why anything would require such a peculiar system?.

You could use a simple diode OR gate (blocking diodes), one fed from a 5V supply directly to the load, the other fed from the 12V PWM supply to the load. When the PWM is 12V, it's diode will switch on and provide power to the load, and the other diode (5V) will be reverse biased and turn off. When the PWM switches to 0V the 5V diode will be forward biased, feeding power to the load, and the PWM diode wil be reverse biased. Obviously the diodes will need to be rated to pass the current, and you may have to increase your supplies to compensate for the forward voltage drops across the junctions.

But even now, it's hard to visualise what you are needing to do, and your original question didn't really ask the right questions.

#### petesmc

##### New Member
Hi,

I don't think that circuit will work. Here's a very simple visualisation of the circuit. Basicallly i need to convert that SPDT switch to an electronics version, depending on the ocsillation.

Edit: Thinking out it, if i could figure out how to get this working, i wouldn't even need the mosfet most likely.

#### Attachments

• circuit_983.jpg
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#### Nigel Goodwin

##### Super Moderator
petesmc said:
Hi,

I don't think that circuit will work. Here's a very simple visualisation of the circuit. Basicallly i need to convert that SPDT switch to an electronics version, depending on the ocsillation.

Edit: Thinking out it, if i could figure out how to get this working, i wouldn't even need the mosfet most likely.

As I suggested above, just use two diodes - replace the switch with two suitabled rated diodes, anodes to the top, cathodes to the bottom (joined together).

The 12V line is a PWM signal (as I understand it), you can throw the lower mosfet away, when the PWM is high the 12V supply feeds the load, when the PWM is low, the 5V feeds the load. This will give you a 5-12V PWM signal on top of a permanent 5V feed.

From what you've said so far, this is what I understand you want - but I don't have a clue why, it seems extremely bizarre!.

#### petesmc

##### New Member
Hi,

No the input to the mosfet is the PWM signal. Thats what makes it complicated.

#### Nigel Goodwin

##### Super Moderator
petesmc said:
Hi,

No the input to the mosfet is the PWM signal. Thats what makes it complicated.

I don't think I can really help any further, without knowing what it's supposed to do i'm just 'guessing in the dark'.

I can't really envisage anything that would need to do something like (I think!) you are wanting - I strongly suspect you are probably going about it in totally the wrong way.

#### petesmc

##### New Member
Ok then, don't worry about it, I'm sure I'll figure something out.

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