How to Choose an Under-Cabinet Lighting Transformer

What is an under-cabinet lighting transformer?

It is the LED driver that converts mains AC into the low-voltage DC that cabinet LED strips need. People often call it a transformer, but LED strips need a constant-voltage DC supply, not the AC output of an old halogen transformer, so look for a product described as an LED driver or LED power supply.

Our HM slim series is an example: it converts 220V AC to 12V DC, works at 50/60 Hz and is listed in six ratings from 18W to 72W in a slim aluminium shell.

What output voltage does my cabinet lighting need?

The driver output must equal the strip input voltage. A 12V strip such as the TSP-2835-60-8 or TSP-2835-120-8 needs a 12V driver. If you use a 24V strip, you need a 24V driver instead; a 12V driver will leave it dim or dark, and a 24V driver on a 12V strip will overload it.

Some short cabinet kits avoid a driver altogether. The DC5V-320 COB strip runs at 5V DC from a USB plug, so it can be powered from a USB port or adaptor rather than a mains LED driver.

How many watts should the LED driver be?

Work out the total strip load and choose a driver rated above it. Strip load is watts per metre multiplied by length: for example, 3 m of TSP-2835-120-8 at 9.6W/m is 9.6W/m × 3 m = 28.8W. On a 12V system that is 28.8W ÷ 12V = 2.4A.

The HM ratings convert to current the same way. HM-18W-12V-1.5A gives 18W ÷ 12V = 1.5A; the 24W model gives 24W ÷ 12V = 2A; 36W gives 3A; 48W gives 4A; 60W gives 5A; and HM-72W-12V-6A gives 72W ÷ 12V = 6A.

For the 28.8W example, an 18W or 24W driver is too small. A 36W driver is the smallest rating above the load, and a 48W driver would leave room for an extra strip later.

How much headroom should I leave on the driver?

Leave a margin rather than loading the driver to its full rating, and follow the driver's datasheet for its recommended continuous load. A driver running at its limit runs hotter, and in a closed cabinet void that heat has nowhere to go.

Headroom also covers strip lengths that change on site. If the installer adds another shelf or extends a run, a driver chosen with spare capacity can still carry it. If the total load is close to the largest rating you can fit, split the circuit across two drivers.

Should I use one large driver or several small ones?

Use one driver per cabinet run when the runs are far apart, and one central driver when the strips are close together and the total load fits comfortably. Low-voltage DC loses voltage over long cables, so a central driver feeding distant cabinets needs heavier cable or parallel feeds to keep the strips evenly bright.

Several smaller drivers also limit the effect of a fault: one failed driver turns off one run rather than the whole kitchen. The trade-off is more mains connection points, each of which must be accessible and wired by an electrician.

Can I dim under-cabinet lights with this kind of driver?

Yes, dim on the 12V side with a low-voltage dimmer or switch placed between the driver and the strip. The HM listing does not state mains dimming, so do not connect it to a wall dimmer on the AC side.

Low-voltage touch and sensor dimmers are rated by load, so check the switch rating against the strip current you calculated. The TSP-2835-60-8 listing, for example, notes that it works with touch, sensor or remote mirror switches.

What should I specify when ordering LED drivers for a cabinet project?

Specify the mains input voltage, the DC output voltage, the wattage and the quantity, plus how the driver will be mounted and connected. The input must match the local supply; the HM series, for example, is listed for 220V AC input at 50/60 Hz, so confirm the input before ordering for a market with a different mains voltage.

Also state the total strip load per driver so the rating can be checked, whether the driver will be visible (the HM housing has a silvery white finish that suits visible installations) or hidden, the output lead length and connector you want on the DC side, and whether you need a dimming solution. If the strips will be supplied with the drivers, list the strip model and length for each cabinet run so the whole kit can be matched in one quote.

Giving the full picture at the start avoids receiving drivers that are correct on paper but wrong for the cabinet: too large to fit, the wrong connector, or a rating that only just covers the load.

Where should the driver be installed?

Install it in a dry, ventilated and accessible space, such as above a wall cabinet or behind a removable plinth, not sealed inside insulation or a closed box. The HM series is rated IP20, which means indoor use in a dry location, so keep it away from the sink area and from steam.

Keep the low-voltage leads short and tidy, label which driver feeds which run, and make sure the driver can be reached later for inspection or replacement without removing fixed cabinetry.

Safety: the AC side of any LED power supply runs at mains voltage (110-240V). Switch off and isolate the circuit before working on it, and have the mains connection made by a qualified electrician. Only the DC output side should be handled during the low-voltage wiring described here.

Figures are from our product listings or worked out from them. Check the specific model before installation.