max safe power supply voltage please

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VA7RLX
 

Posts: 9
Joined: 20 Oct 2022, 21:36

max safe power supply voltage please

Post by VA7RLX »

Hi: I am looking for the max safe power supply voltage for a (tr) uSDX. I have two, one with a low bands RF board (BS170 FET) and one with a classic band RF board (FDT86256 FET). The classic band is a kit I have to build. it will be complex kit for me - beginner.

I am using a power supply with 8 1.5V AA size batteries, exactly 12V.

I also have four 14500 rechargeable lithium batters with are 3.6V each, giving 10.8V for three of them and 14.4V for four in series. I would like to use the rechargeables.

Is 14.4V too high and should I use a resistor bridge to bring it down to below 13V for the SDX?

Thanks in advance - ritchie va7rlx
DL2MAN
 

Posts: 706
Joined: 30 Dec 2021, 19:18
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Re: max safe power supply voltage please

Post by DL2MAN »

I would try not to deviate too much from 13,8V MAX.
Yes, it is rated for max 16V and I determined that by the loss involved from the 7805, but more Voltage always means less room for error (bad SWR).
I run my (tr)uSDX on 3 cells, which is usually more than the nominal 11,1V and I would advise you to do the same.

73 manuel; DL2MAN
VA7RLX
 

Posts: 9
Joined: 20 Oct 2022, 21:36

Re: max safe power supply voltage please

Post by VA7RLX »

Manuel

Thanks. I was thinking that 4 x 3.7 V was dodgy as well, you have confirmed that. Tnx for the quick reply.

73 Ritchie va7rlx
SP9TKW
 

Posts: 57
Joined: 03 Jan 2022, 21:11
Location: Krakow

Re: max safe power supply voltage please

Post by SP9TKW »

As I use other QRP rigs which handle 16V, I tend to use 4x18650 LiIon batteries (4S1P) which fully charged give 14.4V. I find that after 5-10min of operation (transmit) voltage drops to 13.6-13.9V.

I use truSDX during my SOTA activations so in order to use this battery pack with truSDX I made a lead which has two diodes in series and little switch to short them. Initially I work with switch open and current flows through diodes, after 10min switch is closed and battery powers rig directly.

Antenna wise, I use EFHW which has SWR below 1.5:1 on bands I am using. So far I done close to 500 QSOs and did not have any problems. I hope this helps you with dilemma and possible practical way to operate.

73 Marek
DL2MAN
 

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Re: max safe power supply voltage please

Post by DL2MAN »

Hi Marek,

I totally agree.
But from my remote perspective I cannot check everybodys equipment and skills, so my recommandation tends towards safety. I do not want a thread like " I burnt my FETs because of Manuels advise" ;)

73 Manuel; DL2MAN
VA7RLX
 

Posts: 9
Joined: 20 Oct 2022, 21:36

Re: max safe power supply voltage please

Post by VA7RLX »

I think both of you guys are correct! I've enjoyed sailboat racing and still actively downhill (alpine) ski here in BC, Canada. In both of those sports, the more experience and expertise you have the more risks you can take with a reasonable safety margin. I guess it is the same in amateur radio!!!

I'm a newly licensed amateur ( just over 2 yrs) learning as I go. I will have a ton of learning to get the toroids wound and aligned on my new classic bands rf board kit. I'm already running a ready-made (tr) u SDX and being cautious suits me just fine :)

Again, thanks for your comments.
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