Pulsed current power supply

[dedalusenviro]

I am trying to build a pulsed current power supply - the desired output is a 1 - 100 Hz square wave, peak current 1 - 1.2 A. I also want the voltage to be variable from, say, 4 - 12 V.

I've constructed a variable voltage linear power supply built around an LM 317T adjustable voltage regulator. (It works)

I'm working on another circuit based on the 555 timer. I put a separate 12 V fixed voltage regulator in it to power up the 555.

My thought is to apply the output from the 555 to the base of an NPN transistor, which will then turn on/off as the timer impulses are applied.

My questions are:

1) I imagine there needs to be a resistor between the 555 output and the base of the transistor, and a resistor between the base and ground. What are good values for R1 and R2?

2) What is the best type of power transistor to use? Do I need one of those fancy JFET ones?

3) Am I to much of an ignorant slob for this project? (hope not) If so, any recommended reading suggestions would be appreciated.

Thanks in advance.

 

[Reloadron]

The LM317 can be configured as an adjustable voltage source as well as an adjustable current source. You mention "Pulsed current power supply" so I am not sure how you are configuring the LM317 but guessing as a variable voltage supply? Can you post a schematic of what you have thus far? May take several post before an attachment gets through moderation but a drawing would really help.

Ron

 

[buteman]

Mosfets are good for switching. You can drive it from the output of the 555. A connection from the 555 directly to the gate of the mosfet with a resistor from the gate to the source of the mosfet ~ 10K to 100k would do. Mount the mosfet on a heatsink. Add a schottky diode across the drain / source of the mosfet if it will be connected to any inductive load. Choose a mosfet with a high max current capability.

 

[crutschow]

Does the load need to be grounded or can it float?

If the load is grounded then you would need to use a P-MOSFET to drive the load. The FET source goes to the LM317 adjustable output, the gate goes to the 555 output, and the drain goes to the load. You will need a logic-level type MOSFET such as this, so that it will turn on with a 4V supply setting. It has a low enough on-resistance that a heat sink shouldn't be necessary for 1.2A of current.

The MOSFET will turn on when the 555 output goes low.

EDIT: As noted, you should also add a diode from the load (cathode) to ground to protect against any inductive load transients.

 

[dedalusenviro]

I'm a little embarassed to say I don't have a schematic. I mainly work from those generic ones on the back of the cards they mount Radio Shack parts on.

I'll go buy the MOSFET today. In the next couple of days I guess I'll solder the power connections to the source and drain, and finish wiring up the 555.

How easy is it to mess up the MOSFET with stray static discharges? Do I need a wrist grounding strap?

Thank you, one in all, for taking out the time to help. I appreciate it very much.

The load, BTW, is an electrolytic wastewater treatment device. I hope that the use of pulsed current will encourage desired reactions, like metal removal, and decrease unwanted side reactions, mainly electrolysis of the water.

 

[Mr RB]

That sounds like you will need to control duty cycle too? It's easy enough to make a 555 oscillator with adjustable duty cycle, and switched capacitors to change the frequency.

 

[dedalusenviro]

I can figure out how to wire up the 555 circuit. I just needed help with the amplification stage.

No inductance in the load, not much, anyway. There might be significant capacitance, so I'll put some diodes in there to prevent current backflow/frying the electronics.

Thanks again, folks. I'll keep you posted as to results.

 

[KeepItSimpleStupid]

You might want to read this thread, since your project is "relating" to Electrolysis: https://www.electro-tech-online.com/threads/reversing-polarity-for-electrolysis-project.119652/