High Current Homemade Driver

[HydroSean]

Hey what's up guys,

I want to build a 2.2A driver that outputs 4.9V from 2x 18350 batteries. I am reading the datasheet for the LM117 "click here" and it says on page 6 in section 7.6 that the typical current limit is 2.2A for (Vin - Vout) <= 15V. This is perfect right? What does "TO-3 Package (over full operating temperature range)" mean? Is this what I should use? If so, where is the schematic?

 

[Benm]

The TO-3 package is a very large one, meant to be mount on a heatsink. You may have seen them on the back of audio power amplifiers from the 70s/80s. The usual package the LM317 comes in is a TO-220, the familiar one with 3 pins on the bottom and a tab with a hole though that on top.

In any case it's a poor choice of regulator really: it allows to pass 1.5 amps continous current only, will be operating far outside specification in your setup, and dissipate a lot of heat. At these power levels you really want to go for a switchmode design for anything portable, and a proper purpose built constant current source for something bench mounted.

 

[HydroSean]

The TO-3 package is a very large one, meant to be mount on a heatsink. You may have seen them on the back of audio power amplifiers from the 70s/80s. The usual package the LM317 comes in is a TO-220, the familiar one with 3 pins on the bottom and a tab with a hole though that on top.

In any case it's a poor choice of regulator really: it allows to pass 1.5 amps continous current only, will be operating far outside specification in your setup, and dissipate a lot of heat. At these power levels you really want to go for a switchmode design for anything portable, and a proper purpose built constant current source for something bench mounted.

Okay yeah I see now the TO-3 is too bulky for a handheld driver circuit. I do want the driver to be portable so I'll take your advice and do some research on these "switchmode" drivers. Thank you for your advice

 

[Benm]

No problem, i suppose you can find some suitable drivers for your application on here - depending a bit on the form factor you want it to fit. In your situation you will need what is mostly known as a 'buck regulator' - a switching power suppy that outputs a lower voltage than the batteries provide.

 

[Illuminum3415]

If you absolutely must use TO-220, whats stopping you from using LM350 instead? Same pin out and reference as the LM317, Good for 3A

 

[HydroSean]

No problem, i suppose you can find some suitable drivers for your application on here - depending a bit on the form factor you want it to fit. In your situation you will need what is mostly known as a 'buck regulator' - a switching power suppy that outputs a lower voltage than the batteries provide.

Yeah so what I've come to understand is that a capacitor can be used solely as the voltage regulator and the buck driver can be used to limit the current, right?

If you absolutely must use TO-220, whats stopping you from using LM350 instead? Same pin out and reference as the LM317, Good for 3A

I looked up the LM350 and the data sheet said its minimum current is 3A. I can't go over 2.4A and I'm shooting for 2.2A.

 

[Illuminum3415]

LM350 has the same pin out as LM317, but has an upper [not lower] current rating of 3A as opposed to 1.5A as the LM317. In fact the minimum load current of an LM350 is the same as the LM317, about 10mA.

Wire the LM350 the same way you would wire an LM317 for constant current. The reference voltage is the same: 1.25V.

 

[Benm]

It is, more or less, a drop in replacement for the LM317 in TO220 package.

It also requires a considerable forward drop and burns 1.25 volts in the resistor, the latter being completely unavoidable in this type of setup. When running at 2.5 amps that resistor alone will dissipate upwards of 3 watts and needs to be quite large and have airflow around it - not to mention the heatsinking requirements for the LM350.

If you are building this as a portable device, just consider that you are wasting about half of the energy in the regulator. This is not good for runtime, but if you want it to be compact it also is bad news for cooling and required host size/form.

Also you are still operating it outside of specification regardless: 2 cells provide 7.4 volts, minus 1.25 for the resistor, minus 4.9 for the laser diode, leaving only 1.25 volts for the regulator when the batteries run low. At 2 amps or so it -requires- 2.5 volts to work reliably which is hardly there when the batteries are fully charged at 4.2 volts each.

It may work, it may not, and it may under certain conditions and with specific individual regulators. I don't discourage experimentation at all, but if this was a project for any electronics study it'd be a fail.

 

[HydroSean]

I've researched a lot about various buck mode switching circuits and it looks like there is just too much information to handle at the moment. I think I will attempt a buck driver build after much more research.

I am now really leaning towards making the current limiter out of two parallel LM317s. Is this bad? or will it work?

 

[Benm]

In this setup it doesn't matter if you use two LM317s or one LM350 really - you have insufficient voltage drop for either solution.

What i predict will happen is this: The laser will light, but at a current lower than you designed the driver for. You can verify this using a multimeter in 20 amp connection in series with the batteries.

LM317s and such pass whatever voltage/current they are able to even if it is not sufficient. They do not shut down on too low drop out voltage suddenly.

Good thing is that this will not damage your laser diode, so by all means try building it and let us know what the results are. I presume it will be enough current to be over the diode threshold so you get a working laser, albeit at lower power and a power level that drops off when the batteries go down.