SUCC device for lowering the voltage

Nearly all my activations are done with LiFePo4 batteries. But for my new build QMX I need a voltage not higher than 12.4 V (or so). It should be a SUCC (C) device (simple universal convenient cheap) :wink:, usable also for other sensitive devices up to 1 or 1.5 A current and a voltage drop of 0 … 2.1 V (switchable).

I build the well known solution with diodes in series - with three diodes, a rotary switch, a diode in reverse direction parallel to the input and a small voltage meter. (Antiparallel diode only if the battery contains a short protection). Here is the device.

All the parts except the cheap voltage meter and the housing are from the junk.

The housing is printed.

PLEASE TAKE CARE. The voltage should better not be switched to higher values if QMX is on. SM5EIE grilled his QMX with a (large) step from 6 V to 12 V. Maybe + 0.7 or 0.9 V are not dangerous but better …


For extra protection, could you add a zener diode to the circuit to ensure the voltage never exceeds a set value?


Check out the Anyvolt3 made by Dimension Engineering. I use it from PO4 batts and small solar panels as well.

John N0EVH


Hello Pete,

a z-diode with 12.2 V or so parallel to the output seems to be an additional protection. But … if the Voltage reaches this value a current flows through the z-diode. Depending from the source (accumulator) and the diodes in series this current could be high, with my LiFePo4 up to 10 or 12 A before switched off by the BMS. This would destroy the diodes in series and/or the z-diode. Ok, better the diodes than the TRX.

If the diodes in series are not destroyed. (with a current from 1 or 2 A or so) and the z-diode handles this current, you will get much heat from 12…24 W. I don’t know how to handle this in this small case.

So I don’t use a z-diode and hope to take care every time.

73, Ludwig

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Hello John,

thanks for the information, a really fine device.

It’s from a different league, much more universal and with a factor of 20 in price.

73, Ludwig

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