BATC Forum

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

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

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

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

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

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

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