Current limitation circuit

But your counter circuit is probably the limits of what you would want with analog circuitry.

If I somehow got a contract that was intended for you and needed to make a gazillion of the things for less than a penny apiece then I'd take your approach. If I needed ten or a hundred, or if I needed more features, I'd think of using the PIC.

Nick Park's "Wallice and Grommit: A Grand Day Out"; the scene where Grommit drills a hole...

Microprocessors are like that. But they're also often the smallest, least-cost way to go for a surprisingly large (and growing) set of problems.

If you can do it reliably with analog circuitry, meet all your performance, price and manufacturability goals _and_ not be too big then that's the way to go.

For you.

Most of my customers so far build around 100/year/board and would like to be able to do field upgrades or at least have flexible manufacturing capabilities; for them the engineering time makes a big factor in the amount they have to charge per board. Add that to the fact that with a digital system you only have to sweat the precision of the ADC and DAC circuits and microprocessors start to look very attractive.

For the current limit function, I like 10:1 self-adjusting resistors. These are sold in many venues marketed as 'light bulbs'. A 100 watt bulb has a cold resistance of about 10 ohms, goes to about 100 ohms hot, and, oh yea, dissipates 100 watts if necessary.

Hi all,

I'm considering to build my own PWM power supply for my mini drill. My first idea is to use a NE555 driving an N Type Mosfet and that seems ok. However, I would like to put some sort of "locking" protection into the circuit. If I'm correct, when the effort increases onto the drill, the current goes up, and can go very high if the rotor is blocked. So this condition can be detected, but I have no clues as to how to do that. Any leads are welcome.

In the end, I'd like to get the functionnalities of this project:

but using more up to date components, especially Mosfet(s) instead of transistors to minimize the dissipated heat.

Thanks a lot for your help. Olivier

Hello Tim,

Agree.

Analog ain't that bad. It's possible with some cheap logic chip like a CD40106. I did that for an industrial angle grinder. It would let the user go through a tough spot for a second but when the load persisted would regulate it down to safe levels, all the way to "off" in a complete stall. The circuit also had a "soft start" so if would wind back up more gently. The formerly common rapid wind up from zero had placed a lot of strain on the wrists of users.

Possibly something like that might help prevent injuries because a sudden and fast revving after getting unstuck can send stuff or the whole tool on a rampage.

Regards, Joerg

Hello Tim,

Most of my stuff is in the gazillion category. Maybe that's how I got to be a cheapskate when it comes to electronics. Other times I don't like simple apps to return "the program has performed an illegal operation...". But that's rare nowadays with uCs, which is why I started looking at the MSP430. Before it ends up in a design though they have to make good on their promise "starting at 49 cents". At the current Dollar something a pop, no chance.

Hey, I splatted a fly with a wood splitter a couple weeks ago. It got sucked into the intake of the engine.

Problem is, often that's only financially feasible with Chinese four-bitters. Anything eight bit seems to be 30c or more.

Regards, Joerg

Hello OBones,

Unfortunately not. Also, this was not for DC but for 240V AC. Anyway, all you need to do for jobs like this is figure out what kinds of ramps and delays you need. Then take a Hex-Schmitt and create those. "Poor man's logic" with dual-diodes and transistors does the interaction. Most of the time I don't even need all six inverters.

Regards, Joerg

Hello Tim,

Yes, for 100qty the uC makes perfect sense. Sometimes even for large qties. Like a strobe that has to double flash at very precise intervals. A uC with a versatile timer can run circles around an analog solution for such designs.

Regards, Joerg

Hey, I like that approach. Are you allowed to post a schematic of that part of the project?

Yeah, but for my project, it feels like using a hammer to splat a fly...

Hello OBones,

In case you want to learn about micro controllers, and I guess only then it would make sense: Check out the Texas Instruments MSP430 series. They can be programmed via an RS232 protocol and the interface is rather simple. Of course, you'd also have to make sure your computer still has one of those. Unfortunately many "modern" laptops don't.

If you go that route the MSP430F1232 is a pretty decent one because it has ADC capabilities.

Regards, Joerg

I agree with that, but thing is, this is a once in a lifetime project (most probably), and this would require me to have a PIC programmer, which I don't have. I could build one (I have the schematics), but it increases the costs for something that I consider to be simple. I don't need high precision, just some sort of a protection. Thanks for the help, I appreciate it.

Ok, that's what I thought, NDA and all that stuff coming into play. I'll look around and think hard about it. Thanks.

Put a current limiting chip/circuit between the DC supply and the mosfet. For complete turn off if too much current is drawn (such as would happen in a rotor locked condition), add a double pole relay and a pushbutton. One set of contacts on the relay is in series with the supply, and, when the relay is energized, connects the relay coil to the output side of the current limiter. The other set of contacts is in series with the input to the current limiter. The pushbutton momentarily connects the relay coil to the power supply. When too much current is drawn, the current limiter drops the voltage and the relay drops out, de-energizing everything. I'll try a drawing: __ +--o o-------------------------+---relaycoil---+ | | | | +--o/ o--+ | | | |

• --+--o/ o--+--Limiter----+-----mosfet-----motor---+ | | | +--------------------PWM---------------+ |

- --------------------------------------------------+

Ed

Oh but I know about microcontrollers, I used them when I was a student, two years ago. But I don't have access to the uni resources anymore...

Thanks.