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different power supplys currents Why?

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DigiTan

New Member
You're forgetting that I = V/R. If you supply a fixed 9 Volts, and your circuit has some load R, you need only measure to current going in.

The current knob on your supply either does one of two things. (1) It either sets the current limit (compliance), so that when the current grows to large, it will shut off or lower the voltage to prevent damage. Or (2) it sets the output current to some fixed value by ramping up/down the voltage until that exact amount of current is entering the circuit. It's tough to say which role it has without a picture of it.
 

audioguru

Well-Known Member
Most Helpful Member
walters said:
then Slowly cranks up the variable current on the bench supply untill then circuit heats up and backs off and measure that current from where the designer started to see smoke?
A designer who does that is worried that he made a big mistake in his design.

I have designed hundreds or thousands (millions?) of circuits and I just apply the supply and start using the circuit.
Since I designed the circuit:
I know what its current will be.
I know what the circuit will do.
I know how well the circuit will work.
I know what the circuit won't do.
I know the cost of the circuit.
They are all part of the design.

None ever made smoke and the few circuits that didn't work were easy to fix (because I made a stupid mistake when building the circuit, not in the design).
 

evandude

New Member
walters said:
Yes bench supply have a variable current but does a designer power the unit up with the voltage like +9 volts and then Slowly cranks up the variable current on the bench supply untill then circuit heats up and backs off and measure that current from where the designer started to see smoke?

you don't seem to understand current and voltage.

assuming a simple circuit just for ease of explanation, a circuit will draw a certain amount of current when you power it with a particular voltage. the only way the power supply can change the amount of current the circuit is getting is to change the voltage it is supplying to it. the variable current on a power supply is generally just a current limit, where the supply will not supply more current than you set it to. and how does it limit the current to the device? by lowering the output voltage if the current drawn begins to exceed the limit.

the only way a power supply can force a circuit to draw more current is to increase the supply voltage. but if you are powering your circuit with a fixed supply voltage, it will draw as much current as it needs. There is such a thing as a power supply not being able to supply enough current to the circuit (current rating of the power supply too low) but there is no harm in connecting a device that draws 200mA to a supply capable of 10 amps, if the supply is set to a fixed output voltage.

Thinking of ohms law, where V=IR so for a given load and voltage, the current is fixed, is a good way to get a general understanding, however MOST circuits do NOT behave like resistors (voltage and current are still related, but not linearly) therefore you cannot just take a multimeter and measure the resistance of the device at the power terminals. you simply use it to measure the input current when you are powering it with a given voltage, as others have said, that's all there is to it.
 

walters

Banned
Thanks guys alot

What im confussed about is this i seen products and circuit use +9 volts but have different current rates examples

+9 volts and the current is 200ma
+9 volts and the current is 600ma
+9 volts and the current is 1250ma
+9 volts and the current is 2000ma

How can i have the same voltage +9 but the current draw is different?

Wouldn't the power supply have to have a different current rating?
because its the same voltage +9 but the current is different
 

Styx

Active Member
walters said:
Thanks guys alot

What im confussed about is this i seen products and circuit use +9 volts but have different current rates examples

+9 volts and the current is 200ma
+9 volts and the current is 600ma
+9 volts and the current is 1250ma
+9 volts and the current is 2000ma

How can i have the same voltage +9 but the current draw is different?

Wouldn't the power supply have to have a different current rating?
because its the same voltage +9 but the current is different

As I said it is ecinomics!!!!
IF a product is known from bench-tests to ONLY draw 200mA why waste money,size and time providing a supply that CAN (doesn't mean it WILL) 2000mA it makes no sense.


AS to a designer setting a supply at 9V and then winding up the current limit that is a NO-NO!!!!!!! best-case the cct never powers up and you start fault-finding a fault that is none-existant (TRACO DC:DC are very fussy at power-up) worst-case IF you have FPGA's on your board then they are now DEAD!!!


AS I said you get a feeling for how much current your design will draw from going over the datasheets and summing all the currents up

Say I have a TLE2021 on my board
https://www.electro-tech-online.com/custompdfs/2005/11/tle2021.pdf (page 8)

From the datasheet it says:
OUTPUT CURRENT: +/-20mA
TOTAL CURRENT INTO +VCC : 80mA
TOTAL CURRENT INTO -VCC : 80mA
INPUT CURRENT (EACH INPUT) : 1mA

The input current can be ignored since that is current from the prev chip.

So summing up the currents:

80+80+20 = 180mA from +/-15V so WORSE-CASE it needs 90mA limit on the +rail and 90mA on the -rail

So I set the current limit at 250mA, why? becuase there will be inrush (to charge caps). I could set the current limit at 2A BUT if their is a design-fault (a short somewhere or cross-wiring) then the Power-suppply will just feed that short and 2A will do alot of damage as opose to 250mA

SO I turn on the supply, current limit kicks in (and the voltage collapses) I quickly turn it off and go over the board.

Find a bad wiring and correct. Power it up again and this time it draws 30mA great!!!!! So i design a PSU to deliver 50mA or the nearest amp level that is easy on the maths since I am lazy
 

Nigel Goodwin

Super Moderator
Most Helpful Member
walters said:
Thanks guys alot

What im confussed about is this i seen products and circuit use +9 volts but have different current rates examples

+9 volts and the current is 200ma
+9 volts and the current is 600ma
+9 volts and the current is 1250ma
+9 volts and the current is 2000ma

How can i have the same voltage +9 but the current draw is different?

Wouldn't the power supply have to have a different current rating?
because its the same voltage +9 but the current is different

A circuit only takes the current it needs, as long as the power supply can supply at least that much, that's all the matters - a larger capacity supply makes no difference (except for crappy unregulated wallwart type supplies!, where the voltage will be too high).

Think of the supplies as cars!, you have a choice of three cars:

Ford - maximum 80mph
Volvo - maximum 120mph
Ferrari - maximum 200mph

The speed limit down your street is 30mph - so you obviously can't drive any of those cars! - or can you?, and why?.
 

walters

Banned
But if i have the right voltage +9 volts and if i supply the current how do i know if its to High of current rating? i need to know the maxium current rating so it doesn't cause smoke or damage how would a designer know the maxium current draw ? before it starts to smoke or cause damage?
 

2PAC Mafia

Member
Hello,

But if i have the right voltage +9 volts and if i supply the current how do i know if its to High of current rating? i need to know the maxium current rating so it doesn't cause smoke or damage how would a designer know the maxium current draw ? before it starts to smoke or cause damage?

Just a few maths, experience and datasheets...

You have to know on each part of the circuit which current you will have through and then select the proper component which can support that amount of current or, let´s say watts.

An easy example is if you use an standard 7805 regulator which I think has a maximum supplying of 1A and then you connect a load which demands 1A, then you have the regulator working at its critical maximum behavieur and even if you didn´t put a heatsink on it the chip itself is getting so hot that you would see smoke and also the component would be damaged.

Always you have to achieve a proper tolerance on the design and select components with best properties comparing at the maximum current you will have through each point.
 

audioguru

Well-Known Member
Most Helpful Member
Hi Styx,
How do you get 80mA from the supply into a flea-power opamp that current-limits beginning at only 7mA? Its worst-case max shorted output current is only 20mA.
I think its 80mA max supply rating applies to the quad opamp, when all 4 of them have their outputs shorted.

Hi 2pac,
A 7805 regulator never smokes. If you short its output and it begins to heat too much, it automatically shuts-down.
 

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2PAC Mafia

Member
Hello Audioguru,

Hi 2pac,
A 7805 regulator never smokes. If you short its output and it begins to heat too much, it automatically shuts-down.

OK, it was an example for Walters, let´s say the same using an IC not short or thermal protected, or for example a 1/8 resistence where too much current is passing through...
 

2PAC Mafia

Member
Hello,

audioguru wrote:
Hi 2pac,
A 7805 regulator never smokes. If you short its output and it begins to heat too much, it automatically shuts-down.


In theory perhaps - but I change plenty of faulty 7805's

let´s say it can make smoke if internally fails the overtemperature or short protection... :D No problem :wink:
 

audioguru

Well-Known Member
Most Helpful Member
Nigel Goodwin said:
I change plenty of faulty 7805's
American and European regulators are all 100% tested for shut-down.
Chinese ones?????

BTW, one of the Chinese compact florescent bulbs that were imported by Osram/Sylvania failed already after lasting a whopping couple of weeks.
I'll tell their engineer and he'll say, "Yeah, we know".
I expect their German replacements will outlast me! :lol:
 

Styx

Active Member
audioguru said:
Hi Styx,
How do you get 80mA from the supply into a flea-power opamp that current-limits beginning at only 7mA? Its worst-case max shorted output current is only 20mA.
I think its 80mA max supply rating applies to the quad opamp, when all 4 of them have their outputs shorted.

you would think...
Since I dint actually state whether I was using all 4 or not (hypothetical cct to point out that you should check the datasheets to give an idea what a component will draw).

You state that the 80mA might be max drawn WHEN the output shorted. However, in my experience it is not always the case. I have a RS422 driver chip and it states power into the supply pin and also max output power. I sized the 5V reg based upon the max supply current.

It then turned out to be under-rated ALOT (taking a redesign to go from a DPAK to a D2PAK reg). it turns out that total current inclused output current as well.

Yes bad wording in the datasheet, but I got burnt, now I summ up all the currents. Doesn't really matter since the load gov'n the amount of current drawn, if I over-estimate it, it just wont draw.

Better to over-estimate then to set a current-limit to low
 

audioguru

Well-Known Member
Most Helpful Member
Nigel Goodwin said:
So, you think American and European components NEVER fail?, perhaps you should try living in the REAL world :lol:
Yeah, in the real world, even the American and European stuff is made "over there".

I had tens of thousands of audio equalizers built by a local company and each one was tested using a very critical tester of mine. The circuit used TI's TL074 quad low-noise opamp. Not a single failure! None were ever returned by dealers! Seven or eight years later it is still made and sold.

New Japan Radio copied TI's TL072 dual opamp but made it "better". They were mostly recalled and replaced with a 2nd generation "ordinary" one. Their "better" ones all oscillated!
The warning and appology page has been removed from their website. I couldn't stop laughing about it. :lol: :lol:
 
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