Selection & sizing

The reality of triac dimmer minimum load

The minimum load of a leading-edge triac dimmer is not a driver capability - it is the dimmer's holding-current requirement. This guide gives the current magnitudes, four reasons published figures fail in the field, a three-step on-site test, and how to size a dummy load.

Last updated：2026-10-31

"This driver has a 10 W minimum load" is misused about nine times out of ten. Minimum load is not a driver parameter. It is the smallest current the dimmer needs to stay conducting. Change the dimmer and the same driver can go from dimming to 5% to dropping out at 30%.

The one-sentence answer

Fix the order of investigation: check the dimmer's holding current first, then the driver's bleeder and bleeder capacity, and only then the driver's published dimming range. Reverse the order and you will keep swapping drivers while the problem sits in the wall.

The mechanism: holding current is the real threshold

A triac is a semi-controlled device. Once triggered it stays on only while load current remains above its holding current (I_H). Drop below that and it switches off part-way through the half cycle, then re-triggers on the next one - which shows up as low-end flicker, stepping, or the light simply going out.

Common wallbox leading-edge dimmers sit in the 20-70 mA range. Professional dimmer racks demand more; the figure quoted in theatre engineering discussions is around 100 mA. A single 12 W LED driver on 230 V at PF 0.9 draws roughly:

I = P / (U x PF) = 12 / (230 x 0.9) = 0.058 A, about 58 mA

So one 12 W driver already sits in the critical band of some dimmers. That is why a driver advertised with a "10 W minimum load" still flickers on certain wall units.

Why the published figure collapses on site

Three-step on-site test

No special instruments needed beyond a clamp meter and the dimmer actually specified for the job.

Step 1 - sweep one unit

Connect a single driver to the real dimmer and ramp down slowly from 100%. Record three points:

• where flicker starts (for example 22%)
• where stepping or jumps appear (for example 15%)
• where the light drops out (for example 8%)

Flicker above 20% points to holding current or curve mismatch, not a faulty driver.

Step 2 - add a dummy load

Put a resistive dummy load across the circuit, then repeat step 1. If the flicker point moves clearly down or disappears, the cause is confirmed: the circuit current is not enough to hold conduction.

Size the load from the current you need to add:

R = U squared / P      e.g. 230 V, 5 W  ->  R about 10.6 kohm
I = P / U              e.g. 230 V, 5 W  ->  I about 22 mA

In practice 5-10 W of resistive load is the usual starting point; theatre installations have long used a 40 W tubular heater as a dummy load, mounted where it cannot spill light. More power works better, but it costs energy whenever the circuit is live - a 5 W load left on all year is roughly 44 kWh, and the client should hear that number up front.

Step 3 - swap to a trailing-edge (ELV) dimmer

Repeat step 1 with the same driver on a trailing-edge dimmer. If trailing edge ramps smoothly below 5% while leading edge cannot, the verdict is clear: it is a topology mismatch, and changing drivers will not fix it.

Common mistakes

• Accepting the whole circuit instead of each unit - hides weak units until extra fixtures are added later
• Reading minimum watts without counting fixtures - eight 12 W units and one 96 W unit look completely different to a dimmer
• Expecting 0-10 V behaviour from a triac - 0-10 V is a signal and has nothing to do with holding current; it will reach 1% cleanly. A triac cannot be equally linear
• Blaming every flicker on the driver - run the three steps first
• Mounting the dummy load inside a sealed luminaire - a 5 W resistor adds heat, raises ta and shortens driver life. Keep it outside or in free air

Acceptance checklist

1. Record the dimmer model and its stated holding current; if it is not published, design conservatively for 20-70 mA
2. Work out the minimum circuit current: I = total P / (U x PF), compare with holding current and keep a 1.5 factor
3. Sweep each unit individually and record flicker, step and dropout points in the acceptance sheet
4. If flicker starts above 20%, retest with a dummy load before deciding to change the dimmer
5. Run the trailing-edge comparison to settle topology versus driver
6. Re-run step 3 whenever fixtures are added to the circuit

Deciding circuit count and per-unit wattage at design stage is far cheaper than adding dummy loads later. H01 and H19 in the H series and K01 in the K series are all specified for triac dimming, but compatibility is not a promise of identical dimming depth on every dimmer - confirm the dimmer model at the same time. For the differences between dimming methods see the dimming comparison table, and for other causes of low-end flicker see the LED flicker troubleshooting guide.

© 2026 YFX. Figures are for selection purposes. Always follow the datasheet and local wiring regulations for final installation.