I've discovered that the max voltage for a device isn't always so. Sometimes
an i.c. is sensitive to over-voltage, but not always. I like linear circuits, use
the 555 frequently, and it seems it is not sensitive to supply voltage. I'll post
the circuit I am working with:

My question is, can I use this at 32volts, or is their a substitute that will work? I know that I can isolate
the drive from the Mosfet switching side, but apart from that as a solution, is there a device replacement for
the NE555 that I can reliably use at 32v? This schematic, as-built, does not immediately go to smoke at 32v,
but I know I am pushing the envelop with regards to reliability.

 

My question is, can I use this at 32volts, or is their a substitute that will work?

No and none that I know of.
For reliable operation, the 555 should not be operated above its maximum of 18V.
Just use a simple zener or LM317 regulator to reduce the voltage to the desired value.

 

okay, 1/2w, 18v zener on input, what value of resistor should I put ahead of the zener?

 

While at it I call into question the use of an 1N4005 as a damper diode because of its slow recovery time. A UF4007 might be a better choice.

 

While at it I call into question the use of an 1N4005 as a damper diode because of its slow recovery time. A UF4007 might be a better choice.

Thanks Dick, I already replaced it with a Schottky diode.

 

On the subject of damper diodes, that connection for the diode (in parallel with the MOSFET substrate diode) will not do any damping, as the inductive spike will generate a positive voltage at the MOSFET drain.
It needs to be connected across the inductor (cathode to plus power) to absorb the inductive spike and allow the PWM current to keep flowing through the inductor when the MOSFET is in the OFF part of the PWM cycle.

okay, 1/2w, 18v zener on input, what value of resistor should I put ahead of the zener?

I assume you are putting the Zener from the 555 plus power input to ground.

The 555 will take a maximum of about 15mA, and running 5mA through the zener, the series resistor should be about
(32v-18v) / 20mA = 700Ω.
Add a 100nF ceramic cap from the 555 power pin 8 to ground for decoupling.

 

Why use such a high supply voltage on the 555?
The datasheets for the LM555C and NE555 show that their absolute maximum operating supply voltage is 16V and their spec's are guaranteed only if the supply is not higher than 15V. Use 12V.

 

While it is quite simple to provide a reduced voltage for the 555 device, you may need a higher voltage to switch the RFP50N06b device on completely. I have not investigated the specifications, but it may take more voltage to switch fully on.

And as for device ratings, there are "abs max" ratings and above those ratings the device may fail and there is no claim that it would not fail.

 

That 10k gate pull down is probably not needed.

 

You can simply add a voltage regulator for the 555 supply, such as an LM7815, in place of that 100 ohm resistor. But do not leave out the two 0.1 mFd capacitors as the application notes state are needed. three new parts and losing one resistor and you should be all set. Not a bad way to go, and with a regulated voltage the frequency will be a bit more stable.

 

Nobody has yet pointed out that, even if the 555 could handle 32 volts, you'll blow up the gate oxide on the MOSFET. Most are limited to 20 volts gate-to-source.

If your 32 volts is reasonably stable, you could also use a zener diode in series with the positive supply (pin 8) to drop the voltage. In your diagram, the zener diode would replace the 100 ohm resistor. The bypass capacitor should stay on the 555 side of the zener, as you have it.