DMM incertitude - Page 1
Forum section: electronics / beginners

author: alecto
author_rank: Contributor
timestamp: May 20, 2017, 12:53:04 pm
content: hello everybodyI am sorry but I will ask some stupid questions...the situation:I have found a power supply that outputs 12v 1A .I build a pcb with a 7805 and an LM317 to get outputs 3.3v 5V and 12vwhen I connect a raspberry pi, the 7805 gets real hot(burned my finger)so I tried to measure the current draw...and this is the stupid question...first I moved the red cable of my multimeter into the 10A connector , set the select range on 200mA DC where 10A is mentionned(I have a bk precision 2704cI put the probes in serie and on the screen I get a max of "04.3"Is it correct to assume 430mA ?anyway, based on the assumption above, I tried to understand ...and here are my conclusions...am I right?7805Vin=12vVout=5vdrop=7vpi current=0.43Aso the power is 7v * 0.43A=3.01W wich I guess is too much...?again...stupid...what would be a solution for this problem?

author: stj
author_rank: Super Contributor
timestamp: May 20, 2017, 01:18:29 pm
content: how big was the heatsink on your 7805?? those will get very hot even with a lot of aluminium bolted to them.

author: alecto
author_rank: Contributor
timestamp: May 20, 2017, 01:36:16 pm
content: not that big... I dremelled a bit of aluminium of about 2.5cm² bolted and soldered to the heatsink of the 7805(TO-220)

author: stj
author_rank: Super Contributor
timestamp: May 20, 2017, 01:45:37 pm
content: so you want 5v from 12v with minimal losses?user a switching regulator.something like these - just remember to set the output voltage BEFORE you connect your valued stuff!http://www.ebay.co.uk/itm/1-2-5-10-pcs-MP1584-Adjustable-3A-DC-DC-Converter-Step-Down-Voltage-Regulator/152155497920

author: Benta
author_rank: Super Contributor
timestamp: May 20, 2017, 01:57:03 pm
content: 3 W on a TO-220 part can be VERY hot. Give it a proper heatsink and you'll be OK.

author: alecto
author_rank: Contributor
timestamp: May 20, 2017, 02:11:44 pm
content: Quote from: stj on May 20, 2017, 01:45:37 pmOK thanks for the solution...actually I have one of those things...never tested tough...and I know now that I shouldand yeah calculations BEFORE are better...I realize that now tooQuote from: Benta on May 20, 2017, 01:57:03 pmbigger sink won't be possible...So I measured right the Amps with my DMM? and my calculations were correct then?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 20, 2017, 02:34:16 pm
content: QuoteFor me it looks like the 10A range is shared with the 20mA range.

author: amspire
author_rank: Super Contributor
timestamp: May 20, 2017, 02:42:33 pm
content: The proper way to use your multimeter for greater then 200mA is to put the leads in the 10A and COM sockets, and switch to the 20m/10A range. You shouldn't be on the 200m range. So your 430mA reading may not be quite accurate.The Pi current may vary considerably depending on what type of code it is currently running so even if you find a heatsink that just copes for now, it might not at another time. You will find life much easier if you do buy some very cheap 5V (or variable) buck converters on ebay or Aliexpress. You can get converter boards that can manage 2A for a dollar or two and they will run cool.

author: alecto
author_rank: Contributor
timestamp: May 20, 2017, 02:53:24 pm
content: Quote from: amspire on May 20, 2017, 02:42:33 pmI just tested with 20m/10A and I got 0.43...so it seems accurate...I might have been lucky here...and you are right it was more obvious to use 20m/10A instead of 200m...bad labelling on my BK...and wrong instinct for me...and lesson learned...I'll buy some more of those convertersnot a day without learning !