Current boost for PWM

[absolom7691]

Hey guys, I need a little help here. I have a DMX controlled PWM module (0VDC low and +5VDC high). The unit is only capable of putting out 30ma max. I need something that will boost the output current from the signal generator. The signal has to remain +5VDC though. Any ideas for a simple FET circuit that would help me out here? I don't need a lot of current, maybe only 250ma or so. I had thought of using a DMX LED dimmer but my problem with those is that every single one I have looked up uses a common anode and that won't work for what I need.

I guess it would help if I explained what I am doing with this. I am using this signal generator for speed control, utilizing the PWM feature on some 4-wire fans I have. I didn't think they would draw that much current for a signal reference but each fan puts a 5ma load on the signal generator. I will be controlling 20 fans in total so, I need a lot more than 30ma so I don't overload the PWM generator.

Any help or ideas would be fantastic!

[absolom7691]

Nevermind. Don't mind me over here! I think I found what I am looking for. I can feed my PWM signal into a 556 or two 555s. 555s are good up to 200ma I believe, which should be more than sufficient. Here is a circuit I found.

[dave]

Just a fet and resistor on the gate are all thats needed.. A 556 is a lottle bit overkill..

[mixedgas]

Overkill Hybrid....
Does not need 556, uses easier LM555 and known to work....................

Reverse Diode Across the N Channel Should be a Schottky diode rated for four times the load current and at least twice the supply voltage.
1N4001 will work, but gets bogged down at higher frequencies.

For 250 mA, IRF510 with a heat sink would be OK. Both the LM555 or NE555 and the IRF510 are Radio Shack parts in the US.

See attached. YMMV

Dave is right, most logic these days ran off 5V will drive a logic level N-Channel FET. Remember to add the reverse diode and a small gate resistor to prevent oscillations. Most FETS have a body diode that acts as a reverse diode, but why risk it.

Keywords Gate, Gate Driver, PWM, LM555

[absolom7691]

Thanks for the replies, Dave and Steve. It is much appreciated! Going the FET route.

[lightlinked]

do you want to source current or sink current? ie do you want to pwm 5 volts coming out (source with a p channel mosfet or nchannel with more complicated drive) or can you live with pwm on the ground side, the positive side of the powered device always connected to vcc with the neg to an N channel mosfet drain (sink current)

what are you powering? there are other factors that impact this design part, like pwm frequency, cable distances, if the load is inductive (that reverse diode thing)

[Draco]

Thank you for the schematics, the way i used to do it was using two audio power transistors in a darlington array , i built a model train controller using this and that was over 20 years ago and still works fine, i went overkill on the transistors for the reason it was on a train and out cat would some times derail it and or stall it, the over kill meant nothing got damaged, it just got warm and that's all, i built back when i was in high school lol
I may update it to fets now, Thanks!

forgot to ask but fets are voltage driven while transistor are current driven?

[mixedgas]

FETS are voltage driven. One caveat, unless your using the FET in a linear mode, You have to have to pull charge out of the gate capacitance fast, or the FET can die.
Believe it or not, the 555 output stage is fairly fast, less then 100 nanoseconds and can handle the inrush current on pulldown. The gate resistor, as shown, prevents oscillation. Gate capacitance is huge in larger devices, and can approach 0.1 uF.

Steve