Controlling negative voltage with +5 logic

Can't do ASCII schematics so I'll try to explain: Small p-channel Q1 with source to +5V. N-channel Q2 with source on -24V and drain to fan. Resistor 10K from the drain of Q1 to the gate of Q2. Another 10K from the gate of Q2 to -24V. This makes sure that Q2's Vgs voltage won't exceed 20V which is often the limit but verify the actual data for the part you select. Of course Q2 needs to be big enough for the fan. Q1 could be, for example, a BSS84.

Connect your logic to the gate of Q1. Now the fan will turn on with a logic low and off with logic high. You might need another resistor from Q1's gate to +5V if there is a chance that the logic can become disconnected.

Ahh, that works. I was thinking too hard! Thanks.

(snip)

Why? They are probably going to provide the simplest solution.

Joerg,

You mean something like this:

---------+---------------- +5V | ------||-+ logic in ||-> BSS84 ||-+ | | ---------)--------+-------- Gnd | | | .-. .-. ( F ) | | '-' | |10k | fan '-' | | ||-+ | ||

Hello Petrus,

Thanks. Yes, I'll give it a try but right now I am wrestling with rather intricate filter designs for a uC. It's like trying to fit a size 13 foot into a size 8 shoe. Plus I have to dig out all that university stuff. I aced the exam back then but it's amazing how much you can assign to short term memory as a student. A few years later and it's all gone :-(

Only a poor student assigns core material to short term memory only.

In message , dated Thu,

Indeed: core material was used to make non-volatile memory.

Why not use the N-chanel device as a high side switch, interrupting the ground connection, with the other side of the fan to the -24v? People go to a lot of trouble designing high side voltage translating gate drivers to do this so that they can build H bridges using only N chancel devices... you would seem to have it easyier since your control voltage is already on the high side. But you might need a seried resistor on the gate and then a zener diode to prevent the VGS from getting out of hand if the device is switched partway on. Do you need to PWM it for speed control, or is it enough to just turn it on and off infrequently?

It turns out that if their system is saturated with caffeine then short term memory is all they have. The caffeine suppresses the emotional response necessary for long term internalization. These days the little punks are on caffeine as well as drugs like Ritalin, so you have a lot students, who weren't all that well-endowed intellectually to begin with, coming out of school as complete ignorant idiots. They post here all the time.

And they all vote. )-;

Thanks, Rich

Hello Fred,

For a student without access to any hardware that this stuff could have been run on back then the trick is to know what is core material and what isn't :-)

So, how 'bout cerveza then?