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28v Power required

I am planning on taking my equipment for higher bands out to some SOTA summits when the nights get lighter. I would normally operate my radio from a 4s LiPo with a bridge rectifier configured as the dropper.
For the higher bands I also need approx. 28v for the amplifier and TX/RX relay.
(the amp works from 12-32 v, but the relay is 28v ).

Should I get another pack the same as my current one (Turnigy Graphene Professional 10000mAh 4S) and put them in series to achieve the required higher voltage, tapping from one to get the 12v as usual, or would I be better sourcing an 8s LiPo and having separate supplies for the radio and amp?

Any other suggestions for a better, but not complicated, solution?


There’s an item in February 2017 RadCom on a “24V” supply. It’s using a 555 to drive a doubler circuit. You could probably improve on that with more modern options but perhaps it would provide some inspiration…


I have used a 555 timer chip running at about 400 Hz and a voltage doubler to drive relays - about 100 mA is obtainable. I’ve also used a step-up regulator to do the same. A bit of an overkill as I could source 2 A.

If you don’t want to build a circuit you can buy suitable populated PCB’s from China, or get another battery.


Just go and buy a modern boost converter for pence from your favourite ChineseCrud vendor. This is the best solution if you need to 28v for TX.

Yes, that’s much better. 555s are really quite nasty (disliked then 30 years ago - better options around now), but at least it gets you thinking. Easy to extend to much higher V. A friend ran a GM tube with one.

When I was 13 I made a GM tube supply from a pair of BFY51s in an astable configuration driving a 9v transformer backwards. I got enough volts and amps to make the GM tube work. My late father’s watch was impressively hot (Radium & Zinc Sulphide luminous markings). I have a control from a WWII USAF remote radio panel that is really, really hot that is kept well packaged. It may only be alpha but you’d be stupid not to be careful.

If you really only need 28v for the relay then you don’t even need a boost converter.

First use a variable psu to check that the relay, once pulled in, will hold at 12 volts (I’d be very surprised if it doesn’t). Then use the “Relay Speedup” circuit from


What the circuit actually does is provide a pulse at double the input voltage, which is enough to pull the relay in. I’ve used the circuit for a 28v coaxial relay antenna change over for years.

73, Colin G8TMV

You get an extra advantage from this solution too, because the “holding” current is much less than the “operating” current, so you reduce battery drain - always nice :o)


Whatever happened to the KISS principle and why is everyone ignoring the voltage for the amp? Surely having 2 LiPOs in series is the best solution as it provides more volts for the amplifier. For a given amount of watts drawn, the current is obviously half when the voltage is doubled. It also maybe that the amp is more efficient at a higher voltage.

As for the availability of an 8S LiPO and a suitable charger, that might be an issue. A matching pair of 4S LiPOs makes best sense on many fronts… IMHO. Just check the relay actually goes over on less than 28V.

73, Gerald G4OIG

Thanks for the suggestions. The relay will switch at around 20v and is a latching relay, thus holding current is reduced, but a bit of additional circuitry in the form of a hupRF relay board.
(Cheaper than a 12v failsafe relay)
Gerald is right in that the extra voltage is most advantageous in getting the most out of the PA.
My current LiPo charger issuitable for 3S to 8S batteries.

Well I wouldn’t be charging an 8S 10AH LiPO in my overnight accommodation when away from home! I often do charge 3S 4AH batteries at a low rate (necessary during an extended stay), but the bigger they get, the more cautious I become. There is something to be said for QRP…

The easy way is to use a MC34063 converter. It will do the relay with no bother and be a lot lighter than any battery. You can build it on a bit of strip board.


73 Andrew

I use following circuit in my 3 cm transvertor. Transistor KC508 - BC508 npn.
There is an