Mosfet opperated by Signal generator.

[Armagdn03]

So heres the deal,

I have been designing several circuits which require pulse power. To do this I obviously chose mosfets as the switch of choice. I could use 555s, or many other PWM chips to accomplish my goal in driving the mosfet, however it is tedious to measure the duty cycle and the frequency every time I need to make an adjustment, which is alot since I am working with ressonance of tank circuits.

So I thought, perhaps I could use my signal generator to run the mosfet (ir510), but I was worried that having one source for the sig gen (mains power) and one source for the circuit to be driven (18v 3amp power supply) I would possibly damage the signal generator. After a bit of searching I found a Youtube video, where a man used the negative side of the sig gen connected to the gate of the MOSFET and had the source connected to the other two leads with the load inbetween as standard. At first this seemed a bit odd to me, since there is no completed circuit with the sig gen since only the negative lead is connected, and neither source has a common ground.

So I tried it and it worked, I suppose its a sort of capacitive coupling like with one wire power transmission. But.........its opperation was wierd. For example I was testing it with a small dc motor. For starters, the motor would only run when the frequency neared 3Mhz, which would make sense if it was a capacitive deal with the gate. I would think that if it were opperating correctly, I could turn it down to 1hz and have it go on for a sec and off for a sec....not so. Also, when I turn down the potential of the signal generator, the motor slows down, and when I turn it up it speeds up, which is more consistent with a transistor, but weird for a mosfet. so....

1) WHATS HAPPENING!?!?!
2) is there a safe way to run a mosfet with a signal generator without ruining the sig gen? if so please proceede to question 3.
3) Would anybody be willing to draw me up a simple little circuit?

 

[blueroomelectronics]

Can you post a schematic of your 555 & FET circuit?

 

[Armagdn03]

No need really, I will if you want, but its super simple

Signal generator negative to mosfet gate,

then the power supply positive and negative are connected to mosfet source and drain, with the motor inbetween. all that there is too it.

 

[Sceadwian]

IRF510s have pretty bad gate drive characteristics. If you only have one wire connected from your generator to the FET at a low frequency there won't be enough voltage transfered to cause the FET to even start to turn on let alone turn on fully it is capacitive. What voltage does your signal generator put out and what is the load (resistive) on the FET? I have a spice model for an IRF510 that I use that I can plug numbers into if you can give them to me. The signal generator may be mains supplied but depending on the type of 18V power supply you have they may share a common ground, otherwise you can simply connect the signal generators ground to the 18 volts ground without worry, depends on what the signal generator uses as a ground itself, if it has it's own internal DC power supply connecting the two grounds together will solve your problem. You really need to supply a lot more information about the equipment you're using. You should not use a MOSFET without a freewheeling diode when driving a motor, the parasitic diode on a mosfet is slow recovery and will not work above a very low frequency, which will at best waste power, and at worst destroy the fet. Where the motor is connected is also important as Mosfets don't work so well when driving loads attached to their source. They're best used when the source goes straight to ground and the load is placed on the drain.

 

[Armagdn03]

ok, perhaps I posted too much information to get the response I needed ironically. (moderators "read this before posting" eat your heart out )

To simplify the matter, would anybody be willing to post a circuit diagram that would allow me to run mosfets off of a signal generator reliably? output of sig gen is around 12v .5 amps. .001hz - 5Mhz

 

[Armagdn03]

Nobody can suggest a good way to run a mosfet off of a signal generator? I would think it wouldnt be too hard, I mean with a PWM chip off of the same source it could be done easily, but I really like the idea of having a dial and readout of what frequency and wave form im using while doing testing.

Thanks again.

Andrew

 

[jpanhalt]

Possibly one concern is the lack of information we have about your signal generator. I have used mine (it has variable duty cycle) to drive a power mosfet many times, but always with some sort of interface. That interface can be a dedicated mosfet driver, a single transistor, or totem-pole driver. Check out the IR application note AN937 for several examples. John

 

[Sceadwian]

You attach signal ground to the circuit ground and the signal line to your Mosfet gate, if your signal generator only has one wire out then you have to figure out how to share a ground between the circuit you're playing with and the signal generator. These two circuits are most likley being fed from transformer power supplies (Your signal generator probably has it's inside) So you have to tie the grounds between the two circuits together. You really should explain a little bit more of your setup though as this is likley going to cause the mosfet to self destruct or just cease to function, IR510s switch very slow so above a few 10khz it's operating in it's linear region all of the time and wasting power, perhaps to the point where you destroy the FET. Also without a flyback diode across the motor load the inductive spike is going to trash the FET eventually if not the gate, the parasitic diode on the FET is not suitable for being used as a flyback. If you leave the Mosfet gate floating while the signal generator isn't attached to it a simple static zap is going to end the FET quickly.

 

[erosennin]

Try a logic level FET... Put a 1k resistor inline with the gate. The gate voltage usually shouldn't exceed 5V.

 

[gramo]

A logic mosfet sounds like the way to go, like erosennin said

**broken link removed**

The above example is a "safe" design for when the Gate is connected to a high impedance (with the tie down resistor) - a typical condition if the driving device is switched off for some applications

Without it, and if the target device had a separate power supply, then the mosfet would switch hard on due to leakage current..