The Elektroblock internal fuse keeps blowing

Why a fuse blows at all

A fuse is a thermal device: it carries current up to its rating and opens when current exceeds that rating for long enough to heat the filament past its melting point. If the T 3.15 AT (3.15 A time-lag) fuse of the Elektroblock internal charger blows once, it may be a mains transient. If it blows repeatedly on replacement, something downstream is drawing abnormal current.

The T 3.15 AT sits in series with the AC input of the toroidal transformer. Anything that creates a near-short on the rectifier output, on the capacitor bank, or on the regulator output, pulls huge current through the transformer secondary and therefore through this fuse.

The three real culprits

1. Rectifier bridge (KBPC25) shorted. Most common. A shorted diode in the bridge means AC flows backwards and generates a near-short on every half-cycle. Fuse blows in under a second. Visual: black mark or crack on the KBPC25 case. Measurement: diode mode, between AC and DC terminals of the bridge. Expect ~0.5 V drop in one direction, open in the other. A reading of 0 Ω in both directions on any diode = replace.

2. Shorted electrolytic capacitor. Less common but possible, especially on EBLs over 15 years old. The capacitor has internally shorted and presents a near-zero resistance to the rectified DC. Visual: bulging top, brown residue at the base, electrolyte leakage. Measurement: with EBL unplugged and capacitors discharged, measure resistance across each cap. Expect several MΩ. A few ohms = short.

3. Shorted PWM MOSFET or regulator. The output stage has failed in a shorted state and pulls huge current from the capacitor bank. Diagnosis: measure resistance between drain and source of the output MOSFET in both directions. If both directions read 0 Ω, replace.

Why you shouldn’t keep replacing the fuse

Each time the fuse blows, the damaged component downstream pulls a huge surge through the transformer secondary. This stresses the transformer winding. A few cycles may be fine; many will damage the transformer, turning a €100 repair into a €180 one.

Workshop repair

Correct repair requires finding and replacing the actual culprit (rectifier, capacitor or MOSFET) and only then fitting a new fuse. All components available from standard distributors. Repair typically between €100 and €140.

Related guides

• Elektroblock not charging on 220 V — the full 5-stage charger diagnosis.
• Internal charger failure — stage-by-stage repair.
• Auxiliary battery not charging — if the fuse blows and nothing else charges.