different power supplys currents Why?

[walters]

Why do some products use Center Positive VS center Negative ?

Why would a designer choose Center Positive over Center Negative?

What makes the power supply current? because some products use 200ma or 500ma or 1250ma why does each piece of equipment or product use different currents?

How would a designer know how much "current" the power supply needs to be for the type of design or circuit?

 

[Oznog]

1. I prefer to see negative on the outside, since it is more likely to short to ground and almost everything out there is negative ground which would not be harmed by this happening.
2. The actual current used is determined by the load, not the supply. The supply's rating is a maximum rating.
3. That's what designers do.

 

[walters]

Thanks

The actual current used is determined by the load, not the supply.

So you measure the Load of the power supply and thats how u get the current rating on the outside of the product is from the load?

But the Load is going to measure in ohms so how do u get the Current rating from that?

 

[walters]

If i don't know what the voltage is going to be for the circuit or product how would i measure the load and get the current rating for the circuit to operate

Its like making a power supply voltage and current made for the circuit but how do designer know which voltages and current rates are for the circuit?

 

[DigiTan]

Well, generally the designer already knows what voltage the circuit needs to run at. Once he knows that, he has to calculate how much current the circuit will draw when supplied that voltage.

For example, when I built the circuit to power my calculator from a wall wart, I knew the whole system needed to operate on 6 Volts because that's what the calculator used. I also knew (from using my ammeter on the calc once) that the calculator could draw a max current of 125mA, so that told me the wall wart would need to handle at least 125mA. So I used a power supply rated 750mA. It was a little overkill, but it worked. To summarize that procedure:

(1) Knew the circuit needed to run off 6 Volts
(2) Supplied the circuit with 6 Volts and measured the current draw
(3) Choose a supply that could supply all the current I needed.

 

[walters]

Thanks alot

So what u do is give the circuit the voltage once u have the voltage just put a ammeter to see the current draw when measuring and thats how u get the current rating?

The thing im worried about is when doing the measuring what if the current is to high it can damage the circuit

To measure current draw would mean putting the ammeter "before" the power supply or after the power supply by the load?

If i use my test bench power supply and put it to 6volts do i leave the current knob to ZERo so no current to being sent to the circuit and put the ammeter between the test bench power supply and the current to measure the current draw?

 

[2PAC Mafia]

Hello,

To measure current draw would mean putting the ammeter "before" the power supply or after the power supply by the load?

Usually you should measure at the final load, at the circuit could be different points where you read other values not so interesting for you.

If i use my test bench power supply and put it to 6volts do i leave the current knob to ZERo so no current to being sent to the circuit and put the ammeter between the test bench power supply and the current to measure the current draw?

If you put the knob to zero then you limit the current supplying and you don´t have current through any load you connect to the power supply, what do you want to measure then there? It will be the same you don´t give any voltage to the circuit...

 

[walters]

thanks

So what do i do then? supply it with how much current to really know how the how much the current is drawing ?

 

[mstechca]

The thing im worried about is when doing the measuring what if the current is to high it can damage the circuit?

Yes, because each component of a curcuit has maximum current ratings. If any one of the components has too much current, it will heat up, and possibly damage the entire circuit (probably over one component).

 

[walters]

Then how do i test to know the maximum current ratings without it heating up or damaging the circuit?

If i designed a circuit without a power supply but i know the voltage rated lets say +9 voltages my problem is knowing how much "Current" the power supply should be how do i find out how much current the circuit needs?

1.) Built the circuit without a power supply
2.) just know the voltage is +9 volts
3.) I do not know how to get the max. current draw or rating
4.) If i supply the circuit with some current and put a ammeter in series
it still won't tell me the max. current draw or rating because i just supply it with some current

 

[audioguru]

When you design a circuit you select how much current each part uses by the design.
You can design a circuit to use extremely low current if you want. But it won't operate at high frequencies.

 

[walters]

I still don't get how the transistors,resistors,capacitors current ratings can tell me how much the power supply current should be outputing like 400ma or 1250ma how would i know this ?

I think this has to do with Power and Wattage formulas maybe to find out the power supply current

 

[Styx]

I still don't get how the transistors,resistors,capacitors current ratings can tell me how much the power supply current should be outputing like 400ma or 1250ma how would i know this ?

I think this has to do with Power and Wattage formulas maybe to find out the power supply current

Well this is how I do it (other might not).
I design the cct to work (using say generic OPAMP and such).

Once I get past this say concept design I move onto detailed design where I start specifying actual components.

ie I need a Schmitt inverter so I us a 74HC14 from Philips, I need an OPAMP and due to certain requirements (temp, slew,...) I choose a TLE2021.
I get to the resistors and check what voltage/current they will be seeing and make sure their footprint is sized for their power.

Once a detailed design is done I start by going over all datasheets and summing up all the MAXIMUM current (in worst-cases ie high temp).

I then have a rough current-level for the design.



Vero-board the design up and attach to a bench power-supply to get a more realistic current-drawn.


Now I do have a current-level being drawn I design/spec the DCC converter. Say I had a design that took 100mA from 28V on the bench. I could design it for 1A at 28V but the isolation trasformer and FET's would be bigger then they need to be and thus useless, so I spec and size everything for 150mA (or higher depending on the inrush but that is part of the detail testing)

That way my PSU is only as big as it needs to be

REmember a load will draw as much current as it needs. IF the PSU is capable of supplying more, doesn't mean the load will take it (it will in a fault condition, another reason for keeping PSU to their minimum current for protection)

 

[upand_at_them]

walters,

As was already mentioned the load determines the current, not the power supply. If you have no load connected to the power supply is uses zero current.

Once you connect a load circuit it draws as much current as it needs and the power supply provides it. When the supplied current exceeds the maximum it can it'll either blow a fuse or start to burn up.

Mike

 

[walters]

thanks alot guys for the help and information


So if the Load measure 100K and the voltage is 9+ volts
i just use ohms law to find the current?

By load use mean Circuit Load? like measure the circuits resistance from ground to Positive+?

The hard part for me is where and how to find the load circuit resistance value?

 

[Styx]

in most cases it is pretty hard to calculate the cct input impedance on the suppy rails since this changes as a "load" is being driven

Take a logic chip, a HC-logic family has a Quiecent current of a few uA, but when a gate is being used this can go upto a few mA.

Just putting a Ohm-meter onto the supply rail isn't going to show you the cct impedance, it will show some impedance (leckage through caps, any dividers...) but no real useful infomation for IC (since they will not be powered).


Unfortunetly it is a case of measure it or read the datasheets to get a rough idea of how much current would be drawn

 

[walters]

So whats the way to really meausre current draw when and during designing a circuit the Datasheets are just going to tell how much current/voltage it needs to fire up the transistor or the Wattages for the componentts if your circuit have muliple components with all different currents on the datasheets this gets complex because looking at a schematic and datasheets for each components its not going to be the Total current draw so how would i measure the Total current draw then?

 

[DigiTan]

The hard part for me is where and how to find the load circuit resistance value?

For a circuit that only dissipates power, you can treat it like a single resistor once you know its operating voltage and current draw characteristics.

On my example earlier, you could say the resistive load during peak current draw was 480hms, from R = V/I. (48 0hms = 6V/125mA) Likewise, since the average current draw was 25mA, you would say the average resistive load was 240 0hms. In other words, a power-hungry load resembles a small resistance and vice versa.

 

[Styx]

So whats the way to really meausre current draw when and during designing a circuit the Datasheets are just going to tell how much current/voltage it needs to fire up the transistor or the Wattages for the componentts if your circuit have muliple components with all different currents on the datasheets this gets complex because looking at a schematic and datasheets for each components its not going to be the Total current draw so how would i measure the Total current draw then?

EXACTLY!!!
Looking at teh datasheet will give you a ballpark figure, but for a complex cct (one with logic and OPAMPS...) you need to power it up from a bench supply

We have a nice bench supply at work that has a variable voltage supply (0->32V) and a variable current-limit (0-3A)

 

[walters]

Thanks guys for the information and replys

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?