# Simple current limiter is programmable via resistor

Available integrated circuits can let you program a current limit, but they generally involve microcontrollers or data converters (or both). As an alternative, the circuit of Figure 1 lets you program a current limit without the intervention of microcontrollers or data converters. It consists of two miniature SOT-5 devices and a few external components. The sense-resistor value determines the maximum current limit.

Figure 1: This circuit limits the RLOAD current to a value determined mainly by R1.

(Click on image to enlarge)

The circuit consists of a charge-pump voltage doubler (U1, MAX1682), a current-sense amplifier (U2, MAX4376), and two n-channel MOSFETs. U1 doubles the supply voltage to provide gate drive for Q2, and U2 amplifies the voltage across the sense resistor (RSENSE) with a gain of 20. This U2 output drives the Q1 gate via the R2/R3 divider, which modulates the current through R1, which in turn sets the Q1 drain voltage and Q2's gate-drive voltage.

U2's output voltage is 20 times the voltage between RS+ and RS-, but has a full-scale limit of 2 V. Figure 2 shows the response of this circuit when you suddenly impose a heavy (low resistance) load. It allows an initial current surge to 10 A, and then (after 20 μs) settles to the desired limit of 7.25 A.

Figure 2: With VSUPPLY and VIN (at U1) equal to 5 V, the Figure 1 circuit limits load current to 7.25 V.

(Click on image to enlarge)

The maximum allowed in-rush current is set by the sense resistor. A 10 mΩ value, for instance, allows a maximum in-rush current of 100 mV/10 mΩ = 10 A. This steady-state limit is determined by the operating characteristics of Q1 and Q2, together with the values chosen for resistors R1-R3.

Figure 3 shows the ILIMIT values obtained by varying R1 alone. To set other values of ILIMIT , you can vary the ratio R2/R3 while holding the R1 value fixed.

Figure 3: With VSUPPLY and VIN (at U1) equal to 5 V, the Figure 1 circuit's steady-state current limit varies with R1 as shown.

(Click on image to enlarge)

Perwez Nawabi is an applications engineer at Maxim Integrated Products, Sunnyvale, CA. He graduated from San Jose State University (CA) with a BSEE in 2005. He then joined Maxim as a test engineer, and moved to his current position as an applications engineer two years later.

Budge Ing is an applications manager at Maxim Integrated Products, Sunnyvale, CA.. He received his MSEE from San Jose State University.

## 1 comment on “Simple current limiter is programmable via resistor”

1. 1o9sjf3
July 29, 2015

You know what that means: A new crop of real estate agents is trickling onto the scene. Cities like Detroit and Jacksonville are already seeing an influx of new real estate agents as the market rebounds and income rises.

Let's give those folks some pointers, shall we?

Tip #1: Be Kind

Agent Kris Wales: “People first. Always.”
Real estate is a service industry. Agents are constantly in service to their buyers, sellers and fellow agents. That means agents must possess the following:

Good listening skills
Ability to work well with others
Dedication
Solid work ethic
The takeaway? Inconsiderate, antisocial mavericks need not apply. If you're not prepared to put the needs of others first on frequent occasion (which could mean answering the phone at 2 a.m., helping your clients move or counseling sellers through a tough time), real estate may not be the field for you.

New real estate agents need a few pointers before embarking on their careers.
Tip #2: Thrive on Self-Motivation

Agent C.J. Johnson: “Write and follow a business plan. Run your desk like a business. You and only you control your success.”

Teamwork is an important part of life as a real estate agent. Agents are in constant contact with clients, other agents, lenders, title agents and a litany of other professionals.

And while it's essential to work well on a team, agents must prepare for one harsh reality: The “team” isn't going to pay your bills.

Agents typically work as independent contractors and are paid by commission. To succeed in this field, agents must be self-motivated.

You can't expect business to fall in your lap simply because you have a license. Will you make cold calls without being prompted? Are you willing to learn a new skill without it being a requirement? Are you constantly prospecting / networking / researching?

If so, expect your “pipeline” to be properly “stuffed.” Which brings us to Tip #3:

Tip #3: Stuff the Pipeline

Agent Stacy Kelly: “Keep your pipeline stuffed so that when a deal falls through, you haven't put all your eggs into that basket for the month!”
Say good-bye to a regular salary. Watch those regular office hours fade into the horizon, and get ready for the unpredictability of life as a real estate agent. Seemingly-perfect deals do fall apart on closing day, and commission checks will slip just out of grasp.

Don't ever assume a deal is going to go through. Don't make financial decisions based on an anticipated closing. Assume the worst, hope for the best, and don't forget tip #4.

Tip #4: Save for the Lean Months

Agent Lori White: “Have 6 months of expenses budgeted, in savings.”

Yet another financial reality of real estate is the lack of benefits.

According to the NAR, only 4 percent of REALTOR® members receive health insurance through their brokerages, and 83 percent are independent contractors. So don't forget to build up savings to pay for:

Payroll taxes
Health/dental/vision insurance (if necessary)
Brokerage / licensure fees
…and perhaps most importantly, save.

Fiscal crises have a way of popping up when you're least prepared to handle them. Pocket at least 6 months of living expenditures to get through the lean months and any financial emergencies.

Tip #5: Rest

Angie Rogers Nishnick: “Make sure you take a real day off and pick a time to call it a day. It's easy to get wrapped into working 7 days a week. Your clients will understand because they don't work 7 days a week either. The business will be there, but if you burn out you won't be useful to your clients.”

Showings. Open houses. Botched deals. Real estate is a stressful (and busy) industry.

So don't forget to recharge your batteries regularly. It's easy for real estate to become a 24/7 gig, especially if you don't set boundaries. So follow Angie Rogers Nishnick's advice:

A stressed, harried, exhausted, barely-there real estate agent is of no use to anyone. Not to a client, not to a family member, not to a friend.

So make yourself useful. Go get some rest, would you?

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