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R Pi 4 working voltage

Posted: Wed Aug 18, 2021 6:46 am
by G0SFJ
Hi

I hope to use the PSU mounted inside an old Amstrad STB for SKY.

I measured the voltage on the yellow line which drove a SATA hard drive and on my cheapo DVM it reads 5.57 volts.

Is this too high? The spec for the R Pi 4 chip MxL7704 says operating conditions has a maximum of 5.5v and Absolute maximum is 6v.

Looks OK? But I thought I would ask before burning out the Pi!!

Grateful for your comments, please.

73 de andy g0SFJ

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 8:17 am
by G8GKQ
Hi Andy

Too high. I would recommend operating at 5.2v, with a never exceed voltage of 5.3v. Remember that all the USB devices see this voltage as well.

Dave

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 9:22 am
by G0SFJ
Many thanks indeed for advice. You have saved my pi and Minitiouner!

I can see official R Pi PSUs at 5.1v, and as they terminate in a USB-C plug, I see a C female breakout board to take out "VBUS" and "GND". So i can do that?

The only 5.2 volt I can find has an EU plug, not UK.

I will also measure my selection of PSUs marked 5V but taken from old routers.

any further comments received with further gratitude.


73 de andy g0sfj

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 9:55 am
by radiogareth
Being someone who has (possibly foolishly but so far fine) never flinched from the metaphorical 'red line' on engines or electronics, I'd try the PSU on a similar load say 1.5 amps and see if it sags a bit. Or use a high current (so low voltage drop) Schottky diode in line and see what you get on load - you might be lucky.....
As my shack is also often taken out /P, I've gone for a universal 12-13.8V supply and then use eBay down-converters suitable over-specced and heat-sunk, with a bit of extra filtering.
Good luck.
Gareth

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 11:34 am
by G0SFJ
Thanks, I should have thought about dropping the voltage with a diode, my head was full of resistors.

I found a selection of big diodes in my junkbox.

With the S217D, the voltage reads 5.22v.

I am not sure what sort of diode it is as I can't find a datasheet (it's by Telefunken I think) but it seems to work as a voltage dropper and it's quite chunky in a current sort of way. (I have 2 or 3 new and have no idea where they came from originally).

Of course this is unloaded, but then I think i have been talking about unloaded values throughout, and that the adverts for the R pi PSU are speaking of unloaded.


any other comments?

73 de andy g0sfj

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 11:46 am
by g8vpg
Andy,
I have built several RPi based devices - Portsdown streamer and Ryde receivers - for repeater use where they see continuous duty for years at a time. I feed them with 5.25V straight onto the GPIO pins and I haven't lost one yet. Any less than this and you can get strange effects with broken up sound etc and the low voltage lightning symbol can light up. I use one of the generic voltage regulator boards with a fine adjustment pot for output voltage, fed from 13.8V or anything 2-3V above the output required. Its worth getting a self-adhesive heat sink set for the major chips on the Pi.
73 Shaun

Re: R Pi 4 working voltage

Posted: Wed Aug 18, 2021 3:01 pm
by G0SFJ
Heat sink

I have taken a 4cm * 4cm heatsink from a sky+ box and hacksaw-cut back two rows of vanes to get 4 *3 (and cut off ametal screw loop). And cleaned up the flat base, scraping off the dried excess thermal paste.

I have a syringe of thermal paste left over from fitting a clean heatsink onto a Dell Inspiron conincidentally running on Win7.

I know it's inexpensive to buy new, but I do like to recycle as much as I can!

73 de andy g0sfj