Dropping 12V to 5V using voltage regulator(s)

BabyGeezer: Okay, looks like i'll be going with that option - how about the current, those converters will pass the input supply current through with no losses ?

It doesn't work like that. The short answer is don't worry, it will do what you need.

A DC DC converter swaps current for voltage. If you feed 12V in and take 5V out at 1A then the input current at 12V will be a lot less. If there were no losses then the input current at 12V for 5V 1A out would be 5/12 * 1 = 0.42A (This is where some kind person tells me I got the maths wrong!) However, they are not perfect, they do waste some power so I would assume that for 1A out at 5V then 0.5A in at 12V would be needed.

Just connect it up, it will be OK.

PerryBebbington: A DC DC converter swaps current for voltage. If you feed 12V in and take 5V out at 1A then the input current at 12V will be a lot less. If there were no losses then the input current at 12V for 5V 1A out would be 5/12 * 1 = 0.42A (This is where some kind person tells me I got the maths wrong!)

No.. you got the math right !

Deva_Rishi: ...the 7805's will tell you if it's okay, they have automatic shut down for over-current and over-heat.

i see, good to know ! Thanks, that puts my mind at ease :)

PerryBebbington: It doesn't work like that. The short answer is don't worry, it will do what you need.

A DC DC converter swaps current for voltage.

i see - i kinda figured that the power (wattage) is the limiting factor.

conversely, with a boost converter, we can then expect a drop in the current ? (eg. from a 5V@1A supply, we won't get 1A if we boost it to 9V)

PerryBebbington: ... Just connect it up, it will be OK.

okay - i shall proceed with confidence !

Just to be clear a 7805 is not a converter like Perry is suggesting but a regulator, which drops voltage leaving current the same and dissipating the extra energy as heat, not very energy-efficient of course.

You may use a resistor to drop the excess voltage. Or a few diodes. Use the linear regulator as the last step.

BabyGeezer: i see, good to know ! Thanks, that puts my mind at ease :)

I see - I kinda figured that the power (wattage) is the limiting factor.

Conversely, with a boost converter, we can then expect a drop in the current ? (eg. from a 5V@1A supply, we won't get 1A if we boost it to 9V)

Okay - i shall proceed with confidence !

Power is a limiting factor, read the data sheet for the DC DC converter. However, for what you have described, I don't think you will have a problem as long as the converter you are using is rated to supply at least 1A. If a DC DC converter were 100% efficient (obviously they are not) then the input power would equal the output power, so Vin * Iin would equal Vout * Iout.

Yes, if you use a boost converter then the output current will be lower than the input current.

(I'd better hide in case it goes BANG!)

BabyGeezer:
i see, looks like i still can’t do the test yet then, no heatsinks yet.

just to be sure though, from the datasheet;
it says Maximum Power Dissipation is PD=(TJ(max)- TA) / *0JA (*0 is actually theta)
that becomes PD=(150-33)/19 = 6.16W
so; it won’t handle 7W, right ?

That’s using a PCB as heatsink or nothing as heatsink, using a block of copper that 19 becomes 3, and the power handling is 30W for
a 90 deg rise above ambient. Personally I’d not recommend trying to more than 20W out of a TO220 package
as you have to allow for the heatsink itself and any thermal pad or grease.

Perhaps cut up a couple of aluminium cans, open them out and arrange three or four layers together with a hole through them, use a washer to hold them together where you screw them to the tab of the 7805. The separate layers can fan out as fins on the heatsink.

Stop fiddling with linear regulators, and buy a switching (buck) converter. Don't you have a car cigarette lighter style USB charger (same thing). Leo..

Deva_Rishi: Just to be clear a 7805 is not a converter like Perry is suggesting but a regulator, which drops voltage leaving current the same and dissipating the extra energy as heat, not very energy-efficient of course.

yes, thanks for confirming this - it's what i had in mind with my rudimentary knowledge of electronics.

Smajdalf: You may use a resistor to drop the excess voltage. Or a few diodes. Use the linear regulator as the last step.

ahh, good tips - thanks !! obviously they should be properly rated, my stock of resistors are just the 1/4W ones though, but i do have a few rectifier diodes to line up in series.

PerryBebbington: ... Yes, if you use a boost converter then the output current will be lower than the input current.

ok, good - thanks !!

PerryBebbington: (I'd better hide in case it goes BANG!)

i think... :-P

MarkT: That's using a PCB as heatsink or nothing as heatsink, using a block of copper that 19 becomes 3, and the power handling is 30W for a 90 deg rise above ambient. ...

ahh, i see - that's very handy to know - the datasheet wasn't very informative on that, it didn't even mention heatsinks at all which was strange - although i can't say what's normal, i think i've seen a datasheet which gave specs with heatsink usage advice but i don't know if that's the norm.

Paul__B: Perhaps cut up a couple of aluminium cans, open them out and arrange three or four layers together with a hole through them, use a washer to hold them together where you screw them to the tab of the 7805. The separate layers can fan out as fins on the heatsink.

Thank you, very handy tip !!

Wawa: Stop fiddling with linear regulators, and buy a switching (buck) converter.

yes sir - just purchased and on the way !

Wawa: Don't you have a car cigarette lighter style USB charger (same thing). Leo..

oh - you mean like phone chargers that plug into the mains AC ? yeah - those supply 5V don't they (usually at 1A) - now, there's a thought...

Thanks again everyone for such informative tips.

BabyGeezer: oh - you mean like phone chargers that plug into the mains AC ?

No, I meant a USB phone charger that plugs into the cigarette lighter hole in the dashboard of a car. You just asked for a 12>5volt/1A regulator. A 5volt/1A phone/tablet USB supply is ofcourse an easy way to get 5volt/1A. Leo..

Wawa: No, I meant a USB phone charger that plugs into the cigarette lighter hole in the dashboard of a car. You just asked for a 12>5volt/1A regulator. ...

ahh yes, those are common enough indeed - but i've never had one.

might be worth getting if it had a good enough rating, Thanks !

I would never buy one new but ... Garage sales!

Paul__B: I would never buy one new but ... Garage sales!

better yet, old car in a junkyard ? :)

BabyGeezer: obviously they should be properly rated, my stock of resistors are just the 1/4W ones though

You may use several resistors in parallel to get more power dissipation. It won't be nice but it is usable to get it working ASAP.

Collect wall worts. Any electronic equipment being thrown away keep the wall wort. Ask friends and family to save them for you too. That way you will always have a suitable power supply for pretty much anything.

Smajdalf: You may use several resistors in parallel to get more power dissipation. It won't be nice but it is usable to get it working ASAP.

i see - so, to calculate Ohm's Law - i'd then divide the effective current (for the wattage) by the number of resistors in parallel ? (not forgetting that resistors in parallel also drop the total resistance)

PerryBebbington: Collect wall worts. Any electronic equipment being thrown away keep the wall wort. ...

yep, that's what i've been doing - 5V, 9V but just the one 12V. and they are all under 1000mA! (i have 500mA, 600mA and 900mA !!)

i should repurpose an old USB cable - i have a phone charger (1000mA) that has a USB socket.

BabyGeezer: Yep, that's what i've been doing - 5V, 9V but just the one 12V. and they are all under 1000mA! (i have 500mA, 600mA and 900mA !!)

I should re purpose an old USB cable - I have a phone charger (1000mA) that has a USB socket.

OK, good!

Well, one of the delights of electronics as a hobby is finding new uses for old things, or getting things to work in ways the manufacturer never intended.

BabyGeezer: i should repurpose an old USB cable - i have a phone charger (1000mA) that has a USB socket.

Which is a bit old. Most are now rated at 2.1 A (to match the iPad).

Going price should not exceed $5. Purchased locally (as eBay stuff is not trustworthy).

I have a 52l plastic storage bin full of power supplies (but not all are in that bin and not all are lightweight switchmode). Trouble is, getting to it in the stack ... :astonished:

Paul__B: Which is a bit old. Most are now rated at 2.1 A (to match the iPad).

i'm nowhere in the circle of iProducts - the phone charger is actually new, for a chinese brand and that is 1A only.

it's the USB cable that is old, still with a type A plug at the other end.

Paul__B: I have a 52l plastic storage bin full of power supplies (but not all are in that bin and not all are lightweight switchmode). Trouble is, getting to it in the stack ... :astonished:

i haven't quite reached those hoarding levels yet :) but i do find the older adaptors that i've "stocked" are quite useless when it comes to the crunch, including a cheapo one supposedly rated at 1000mA but doesn't seem to be able to provide it. (i have an INA219 module otw to start getting proper numbers on my "inventory")