Converting an analog input called "SPEED" (level=24V) to a signal suitable for MOSFET

[sattar138]

Converting an analog input called "SPEED" (level=24V) to a signal suitable for MOSFET

Hi,
Can you give me some idea about how I can use an input analog siganl called SPEED (voltage level=24V) to drive a logic level MOSFET [BSS138 from Fairchild, 50V/0.22A, Vgs=+/-20V, Vgs(th)=0.8 to 1.5V]?

What should be changed or added in the attached circuit to make it work?

Please advise.

Sattar

 

[eem2am]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

what you did is right. -but put a driver like a mosfet driver there if you want to drive it on quicker.

or make your own using some transistors

 

[sattar138]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

Any other suggestion from onyone else.

 

[FvM]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

What should be changed or added...

A drain resistor instead of a short to VDD...

 

[frankliner]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

Definitely a drain resistor like FvM suggested. Also you need more gate drive. A NPN is would be cheaper.

 

[FvM]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

Also you need more gate drive.

I don't aggree. The circuit is apparently intended for level conversion of a 24 V logic signal. You may even want to slow down the response by a low pass filter because the response time is still in a µs order of magnitude now. Overvoltage protection (by a zener diode) would be a different thing.

 

[frankliner]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

1.4V nominal gate drive is too low. Especially if you consider the room temp threshold can be 1.5V, the divider tolerance could be large, and the 24V likely is +/- 5-10%

 

[sattar138]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

Hi FvM:
Can you suggest me a circuit with your ide that you mentioned above?

Hi Frankliner:
Do you want me to replace the MOSFET (BSS138) with a NPN transistor?

 

[FvM]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

1.4V nominal gate drive is too low

You're right. As you see, I argued against more gate current. More voltage is advisable however.

Can you suggest me a circuit

Connect a zener diode (e.g. 10V) between gate and source and possible a capacitor as additional filter.

 

[frankliner]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

I think the circuit could be ok with 3 changes. 470k to 400k, 30k to 100k, and a pull-up resistor of 6k. The filtering on the signal is already very high. What kind of signal is it. I assumed just a digital 24V signal.

A zener is not needed.

 

[sattar138]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

Hi Frankliner:
It's an analog signal with voltage level ~24V.
The objective is to convert this signal into 3.3V level so that it can be connected with a microcontroller.
thanks,
Sattar

 

[guti]

Re: Converting an analog input called "SPEED" (level=24V) to a signal suitable for MO

you should also take a look on the frequency response of your circuit.
It could be (it depends on the frequency of "speed") that you need an specific mos/bjt to convert it as fast as you need!