A 905 nm laser rangefinder module is usually shortlisted on range and accuracy, then rejected late because the host rail cannot supply it. The published power figure is not a current rating: the same wattage draws a different current at 3.3 V and at 5 V, and a pulsed module can draw far more than its average for a few microseconds. This note turns the published numbers of four ERDI modules into a regulator and decoupling budget the integrator can check before wiring a sample.
Why a watt figure is not a current rating
Electrical power is the product of voltage and current, so a module specified as “average ≈ 0.45 W · max ≤ 0.75 W” needs 90 mA average and 150 mA maximum when the rail is 5 V, but 136 mA average and 227 mA maximum when the same module runs from 3.3 V. The module has not changed; the rail has. The NIST SI unit definitions cover the watt, volt and ampere relations used here, which is the arithmetic behind every row below. Use the module’s own published power, never a current copied from a different voltage rail.
This is the first place a power budget goes wrong: a 3.3 V design inherits the current that a 5 V prototype measured, and the regulator is sized about 30 percent too small before any margin is added.
Average, maximum and start-up are three different numbers
The SPD1200N2 record publishes an average of about 0.45 W and a maximum of 0.75 W, over a 3.3–5 V supply, with a start-up time of 200 ms or less. Those are three separate requirements. The average sets the thermal and source capacity. The maximum sets the regulator’s continuous current rating and the trace width. Start-up sets the bulk capacitance that keeps the rail from sagging while the module initialises. A module that meets the average on a bench supply can still reset on a battery when the first measurement pulse coincides with start-up.
A practical starting point is to add the module’s maximum current to the host’s worst-case load, then confirm the regulator’s datasheet can deliver that current at the lowest battery voltage and the highest ambient temperature. Average current is not a substitute for either check.
Measure at the module connector, not at the bench supply’s display. A 100 mm wire pair and a spring terminal can drop enough voltage during the measurement pulse to move a 3.3 V rail outside the module’s supply range, and the supply’s own meter will not show it. Clip the scope probe across the module’s supply pins and trigger on the falling edge of the measurement burst; that trace is the only evidence that the regulator and the decoupling network work together.
Worked budget: the same module on 3.3 V and 5 V
| Module and published power | At 3.3 V | At 5 V | What it constrains |
|---|---|---|---|
| SPD1200N2: average ≈ 0.45 W, max ≤ 0.75 W | ≈ 136 mA average, ≤ 227 mA max | ≈ 90 mA average, ≤ 150 mA max | Regulator rating, trace width, bulk capacitor |
| LR1000E2: short-range < 0.6 W, max ≤ 1 W | < 182 mA, ≤ 303 mA | < 120 mA, ≤ 200 mA | Same, with a wider short-range-to-maximum gap |
| LRF3000A1: ≤ 1.8 W at 25 °C, ≤ 2.1 W at 70 °C | ≤ 545 mA, ≤ 636 mA | ≤ 360 mA, ≤ 420 mA | Thermal headroom at the top of the temperature range |
The LR1000E2 has the largest ratio between its short-range and maximum figures, so a budget built only on the short-range number leaves the most headroom unaccounted for. The arithmetic is simple, but it has to be done per rail: a 5 V design that later moves to a 3.3 V bus needs a new regulator review, not a new module.
Heat is a second, separate limit
The LRF3000A1 publishes 1.8 W or less at 25 °C, 2.1 W or less at 70 °C and 1.6 W or less at −20 °C. The figure rises at the top of the operating range, which is exactly where the enclosure is least able to reject heat. A sealed IP67 housing that is comfortable at room temperature can exceed its component rating in summer sunlight. Budget the highest published power at the highest ambient the product will actually see, then check the module’s own temperature range and the enclosure’s thermal path. Do not average the three numbers.
Modules that publish current instead of power
Some ERDI modules publish current directly. The LRF1500H record gives 9–36 V DC supply, 55 mA average current and 100 mA peak current, with 1 kHz or 50 Hz operation selected automatically to match echo strength. Current-specified modules are easier to budget on a fixed rail, but the supply range is wide: if the stated 100 mA peak applies across the whole 9–36 V range, the input power at 36 V computes to 3.6 W, and the record does not publish a power figure to confirm that. Treat the top of a wide input range as a question for ERDI rather than a number to assume, especially in a sealed housing.
A regulator and decoupling checklist
- Convert every published power figure into current at the rail you will actually use, and record the division in the design file.
- Size the regulator for the sum of the module’s maximum current and the host’s worst-case load at minimum input voltage.
- Add bulk capacitance for the start-up interval and for the measurement pulse; verify the rail stays inside the module’s supply range with an oscilloscope, not a multimeter.
- Re-run the thermal calculation at the highest ambient temperature with the highest published power, not the average.
- For a wide-input module, ask ERDI to confirm the current at the top of the range in writing before choosing a 24 V or 36 V rail.
These checks do not require a laboratory. They require the module record, a calculator and a scope trace at the rail. The model pages for the SPD1200N2, LR1000E2, LRF3000A1 and LRF1500H carry the source conditions behind each number, and the datasheet fields guide explains which of them change with configuration.
FAQ
Can I power a 3.3–5 V module from a 5 V USB rail?
Only after checking the module’s maximum current at 5 V and the USB port’s available current. The supply range states compatibility, not the current the host must provide.
Is the maximum power the same as the peak pulse current?
No. The published maximum is a module-level figure over its operating modes; a pulse-by-pulse peak can be higher and shorter. If the host regulator is sensitive to microsecond transients, add bulk capacitance and measure the rail.
Why does the same module need a bigger regulator at 3.3 V?
Because the power is roughly fixed, so a lower voltage requires proportionally more current. The 3.3 V rail must supply about 1.5 times the current of a 5 V rail for the same wattage.
Does a higher supply voltage always reduce heat?
No. For a current-specified module the input power rises with voltage, and the module’s internal conversion losses are not published. Confirm the current at the intended rail before assuming a cooler design.
If the rail, the ambient and the measurement rate are known, send them with the target and range requirement through the contact form and ERDI will check the module record against the intended configuration.
Category pages: Laser rangefinder module
Models mentioned in this article
- SPD1200N2905 nm · up to 1.2 km · UART · TTL · ≤5 gFrom $70 · shipping includedSee price & buy
- LR1000E2905 nm · up to 1 km · UART · TTL · ≤10 gFrom $60 · shipping includedSee price & buy
- LRF3000A1905 nm · up to 3 km · UART · TTL · 12 ± 0.5 gFrom $180 · shipping includedSee price & buy
- LRF1500H905 nm · up to 1.5 km · UART · I2C · 20 ± 2 gFrom $250 · shipping includedSee price & buy