16. April 2026

Why LED Bulbs Don’t Work in 12V Downlights

Why LED Bulbs Don’t Work in Older 12V Downlights (Transformer Problem)

Upgraded your downlights to LED but now they don’t work properly?

This is something I come across quite often when carrying out electrical work around Banbury.

The bulbs get changed to LED, but the original transformer is left in place — and that’s where the problem starts.

What’s the Problem?

Older downlights (especially MR16 types) were designed for 12V halogen bulbs.

These use a transformer to step the voltage down from 230V to 12V.

The issue is:

• Halogen bulbs use a higher load
• LED bulbs use a much lower load
• Old transformers often need a minimum load to operate correctly

When that load isn’t met, the transformer can’t run as it should.

Common Symptoms

If your downlights have been switched to LED but the transformer hasn’t been changed, you might notice:

• Lights not turning on at all
• Flickering or flashing
• Lights coming on intermittently
• Only some lights working on the circuit

It’s easy to assume the bulb is faulty — but often it isn’t.

Why It Happens

Most older transformers were designed for halogen lighting and aren’t fully compatible with modern LED lamps.

In some cases, LED bulbs may appear to work at first — but this isn’t always reliable.

That’s because:

• Old transformers often need a minimum load to operate correctly
• LED bulbs use much less power than halogen
• The transformer may struggle to stay stable at low load
• This can place extra strain on the internal components of the transformer

Even if the lights seem to be working, the transformer may be running outside of its intended operating range.

Over time, this can lead to intermittent faults or complete failure.

Similar problems can also happen with dimmer switch compatibility when using LED lighting — explained here:
Lights Flickering When Using a Dimmer Blog

Important to Know

Sometimes LED bulbs will work on an older transformer — but you may still experience problems later on.

This can include:

• Occasional flickering
• Lights dropping out
• Shortened lifespan of the bulb
• Faults that come and go

I often see cases where the lights seem fine after a bulb change, only for issues to return later.

The Fix

In most cases, the solution is straightforward:

• Remove the old transformer
• Install a suitable LED driver
• Match the driver correctly to the load
• Replace the lamp holder if needed, as older ones can become worn or heat-damaged from years of halogen use

Once this is done, the lights usually work properly with no flicker or issues.

Real Example from a Recent Job

On a recent job, I was called out to look at bathroom downlights that had stopped working after being changed to LED.

The fittings were older 12V MR16 types, still running off the original transformers.

Result:

• Lights not working properly
• Intermittent operation

I replaced the old transformers with LED drivers, and the lights came back on straight away with no issues.

When to Get It Checked

If your downlights started playing up after switching to LED, it’s worth getting it looked at properly.

In some cases, it may also be worth upgrading the fittings altogether — especially if they’re older or showing signs of wear.

Need Help with Downlights in Banbury?

If your lights aren’t working properly or you’re unsure what’s causing the issue, it’s always best to get it checked.

All work is carried out by Richard, so you’ll always get clear advice and a consistent standard of work from start to finish.

Call Richard today to get it checked and sorted
RW Currion Electrical
01295 533378
07740 100378
www.rwcurrion-electrical.co.uk

Covering Banbury, Adderbury and surrounding areas.