Most power supply failures in lighting projects are not component failures. They are selection failures: a constant-voltage adapter was chosen where the load needed a constant-current driver, or the reverse. The two look nearly identical on a spec sheet — both take 100–240V in, both put out low-voltage DC, both come in similar housings — and they behave completely differently once a real load is attached.
This guide is written for buyers and project engineers sourcing power for LED lighting, signage, camera systems and small equipment. It explains what the two topologies actually do, which load types each one serves, where the common mistakes happen, and which standards apply so that the certificate you receive matches the product you are installing.
What “Constant Voltage” Actually Means
A constant-voltage (CV) supply holds its output voltage steady and lets the current float according to what the load draws. A 12V 5A adapter produces 12V whether the load takes 0.2A or 5A; the 5A figure is a ceiling, not a setpoint. If the load asks for more than the ceiling, the supply either current-limits or shuts down.
That behaviour suits three things especially well:
- LED strips. A strip is built as repeated segments, each segment carrying its own series resistors. Feed it a stable 12V or 24V and each segment regulates its own current internally. This is why strips are specified by voltage, and why the lengths are cut at marked points — cutting between segments breaks the resistor arrangement.
- Electronic devices. Routers, cameras, controllers, speakers and the usual run of small equipment expect a defined input voltage, not a defined current.
- Distribution over parallel branches. When several strips or devices run in parallel from one supply, each branch needs the same voltage, which is exactly what CV provides.
The consequence buyers notice first: voltage drop over the cable becomes the limiting factor on long runs. A 24V system tolerates roughly twice the voltage drop of a 12V system before the same percentage loss shows up as dimming at the far end — which is why long strip installations tend to move to 24V even when 12V products are cheaper per metre.
What “Constant Current” Actually Means
A constant-current (CC) driver holds the current steady and varies its output voltage within a stated window — typically written as something like 30–42V at 700mA. The load must present an impedance that falls inside that window; if it does not, the driver either clamps at its maximum voltage or refuses to start.
CC drivers exist because high-power LEDs are current-driven devices. Their brightness is proportional to current, and their forward voltage varies from part to part and drifts with temperature. Feed such an LED from a fixed voltage and the current becomes unpredictable — small voltage changes produce large current changes, and the junction can be destroyed. Holding the current constant inverts the problem and makes brightness repeatable across units.
CC loads are typically:
- Single high-power LEDs and COB arrays on a metal-core board.
- LED modules and panels designed for a driver, where the datasheet states a rated current.
- Architectural and commercial fixtures where lumen output must be consistent and dimming is controlled at the driver.
Which One the Project Needs: A Decision Table
| Load | Correct supply | Why |
|---|---|---|
| 12V / 24V LED strip with cut segments | CV | Each segment carries its own resistors; only voltage needs to be fixed. |
| COB or single high-power LED | CC | Datasheet specifies drive current; forward voltage varies part to part. |
| Store signage with parallel modules | CV | Parallel branches each need identical voltage. |
| Commercial downlight with dimming | CC (usually) | Fixture is designed around a driver and a dimming protocol. |
| Router, NVR, camera, small controller | CV | Device expects a defined input rail. |
| Mixed strip + device on one supply | CV, sized for the sum | Everything in parallel needs one voltage; total current is additive. |
If the fixture datasheet states a current (mA) and a voltage range, it needs CC. If it states a single voltage and a wattage per metre, it needs CV. There is no third option worth improvising.
Where Projects Go Wrong
A CV adapter on a high-power LED
The LED lights up, which is the trap: it appears to work on the bench. But the current is set by the LED’s forward-voltage curve rather than by design, so it varies with temperature and between units. Some run dim, some run over-current, and the ones that run over-current fail weeks later. Because the failure is thermal and delayed, it usually surfaces after installation — the most expensive possible time.
A CC driver on a strip
Strips are not designed for a defined current; they are designed for a defined voltage. A CC driver will often refuse to start, or will drive the strip at whatever current the strip’s impedance allows until the driver hits its voltage ceiling. Either way the result is unpredictable brightness, and often a driver that goes into protection the moment a strip is cut shorter.
Ignoring the current ceiling on a CV supply
A 12V 5A supply feeding four strips that draw 1.5A each sits at 6A — over the ceiling. A decent supply current-limits, which drops the voltage, which dims everything; a poor one simply overheats. Always add headroom: 20% is a reasonable minimum, and more where the supply sits in an enclosure with poor airflow, because heat derates the available output.
Assuming identical part numbers behave identically
Two LED batches from different production runs can have different forward voltages. A CC driver set to a fixed current absorbs that variation; a CV arrangement exposes the strip’s resistors to it. This is one reason module-based fixtures specify the driver rather than leaving it to the installer.
Ripple, Flicker and Dimming
Two electrical characteristics matter more in lighting than in most other applications.
Ripple. Output ripple is the residual AC riding on the DC output, usually expressed in millivolts peak-to-peak at a defined bandwidth and load. On camera systems and audio equipment it produces visible artefacts or audible hum; on lighting it can interact with dimming. For general-purpose adapters, keeping ripple under roughly 1% of the output voltage is a reasonable target, and it belongs in the purchase specification rather than in the brochure.
Flicker. Dimming a CV strip is usually done by pulse-width modulation on the DC side, or by a dimmable driver on the mains side. Low PWM frequencies produce visible flicker and, on camera, banding. IEEE 1789-2015 is a widely cited reference for flicker risk assessment; it is a recommended practice rather than a regulated limit, but it gives a defensible basis for stating a minimum dimming frequency in your specification. For CC fixtures, dimming normally happens inside the driver via 0–10V, DALI or phase control, and the driver datasheet should state which.
Safety and Certification: Which Standard Applies
This is where buyers most often receive a certificate that technically exists but does not cover the product in front of them. The right question is what the unit is classified as:
- External power supply sold as an adapter. Usually certified to IEC/EN 62368-1 (or IEC 61558-1 with 61558-2-16), with efficiency obligations under DoE Level VI (10 CFR Part 430, Subpart B, Appendix Z) and EU Regulation (EU) 2019/1782 when sold as an external power supply.
- LED controlgear — the driver itself. IEC 61347-1 gives the general requirements, and IEC 61347-2-13 covers DC or AC supplied electronic controlgear for LED modules. A 62368-1 certificate is not a substitute for the controlgear standard.
- The luminaire as a whole. IEC 60598-1 covers luminaires, and that is where the installer’s obligations usually live.
- EMC. Lighting equipment is assessed against EN 55015 for emissions and EN 61547 for immunity — different documents from the EN 55032/55035 pair used for generic ITE equipment. Asking for the wrong EMC report is a common and avoidable delay.
- Ingress protection. IP ratings follow IEC 60529, and an outdoor installation needs the rating claimed on the label to match the test report — not the marketing text.
One further point worth writing into the specification: whether the output is SELV. Safety extra-low voltage, as defined in IEC 61140 and reflected in the ES1 limits of 62368-1, is what allows an installer to run output wiring without the protective measures that mains wiring requires. A supply with the same 12V on the label is not automatically SELV inside — isolation design decides that, and it should be stated in the certificate scope.
A Buying Checklist for Lighting Power
- Match the topology to the load: voltage-specified load gets CV, current-specified load gets CC.
- Size current with at least 20% headroom, more where the supply is enclosed or the ambient is warm.
- Specify ripple in millivolts peak-to-peak at full load, with the measurement bandwidth.
- For CV strip runs, calculate voltage drop at the far end; move up to 24V when the run is long.
- Name the standard your market requires, and check the certificate covers this exact model and this exact classification.
- Confirm SELV status if output wiring will be accessible.
- For dimming, state the method and the minimum PWM frequency rather than accepting “dimmable”.
Two related guides cover the adjacent details: the output voltage reference for standard DC rails, and the DC connector guide for plug dimensions and polarity. The voltage-drop arithmetic behind long 12V runs follows Ohm’s law directly: loop resistance times load current, and it is worth calculating before specifying cable gauge rather than after.
Specifying a 12V or 24V Supply for a Project
Four figures decide the quotation: the output voltage, the total load current including headroom, the ambient temperature inside the enclosure, and the destination market that determines which certificates are needed. With those four, matching a constant-voltage unit from the 12V/24V range is straightforward — desktop supplies up to 120W for larger runs, wall-mount and detachable-cable formats for smaller ones, all hi-pot tested and aged before shipment.
What is worth flagging in the enquiry: if the load is a bare high-power LED rather than a strip or a device, a constant-current driver is the correct part, and a CV adapter should not be substituted for it. Saying that up front saves a return. Typical configurations are listed under the 120W desktop power supply and the 48W detachable adapter.
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