Maintain Voltage while reducing amperage

[Jack Harkness]

Hello, I know that there have been similar questions but I am still a bit confused as to how this can be done.

The problem: I have a DC 12V 200mA power supply that I want to convert to a DC 12V 10mA supply.

The application: I want to use this supply to power 2 amplifiers (one instrumentation and one operational) that I want to used to regulate the output of a load cell (they output 3mv/V). So I have done some research and by using these amplifiers I can get the output of the load cell to be between 4-20ma (standard) but it had not occurred to me that the amperage of the power supply would be a problem (I have used op-amps before and never had a problem with amperage). So it all comes down to how can I reduce that 12V 200mA supply to 12V 10mA without frying any components (the end product will be left running for weeks at a time). So power dissipation is also a concern.

Any help would be appreciated. Also please let me know if any of what I have said is unclear.

 

[ramuna]

Hi Jack,
Welcome to ETO!
While I, reading your description above, cannot understand why you need to limit your power supply current to 10 mA, I can suggest a method of doing so. Refer to the attached image. Your load cell is represented by R5. Resistor R2, is a very low value resistance, say 1R. Suppose that the max allowable current of 10 mA passes through the load. The same current would pass through R2, and create a voltage of 10 mV across it. The non-inverting amplifier comprised of U2, R3 & R4, whose gain is 1 + (R3 / R4), amplifies this low voltage to, say 2 V (so R3 / R4 is 199 in this case...choosing R3 = 200k & R4 = 1k would achieve this). R6 and C1 are to reduce high frequency spikes produced in the load cell for whatever reason; suitable values are R6 = 10k & C1 = 10uF.

V_ref is the current control reference voltage. Set this to 2V in line with the component values selected so far. Op amps U1 & U2 are the two halves of a LM358 dual op-amp IC. R1 is around 2.2k and transistor Q1 can be any general purpose NPN, eg 2N3904, 2N2222, BC239, BC547 etc.

Circuit operation is straightforward. When the current thru' the load exceeds 10mA, the voltage at the inverting input of U1 will exceed the V_Ref of 2V. This will cause the base current to Q1 to fall, which in turn will cause the load-cell current to fall. Conversely if the load-cell current falls below 10mA, the voltage at the inverting input of U1 would be below the V_Ref. This would boost the NPN base current and thence boost the current through the load-cell.

 

[ronsimpson]

You don't have a problem.
You power supply is 12V and is capable of outputting any amount of current from 0 to 200mA. Note I said zero.
I think you have a amplifier that needs 5mA and another amp that needs 5mA and a load cell that needs some thing from 4 to 20mA. All that adds up to less than 200mA.

You power supply will try to make 12V. As far as current goes, It is very happy at 0mA, 10mA, 20mA......200mA. It probably will not be happy above 200mA.
It is too common for people to think the power supply will force 200mA down the wire. It won't. It will force 12V on the wire.

Just to give an example. Your car had a battery in it. On the side of the battery it says "12 volt, 650 cold cranking amps". The battery is capable of starting you car at 600A. But if you only have the radio on the battery is delivering only 2 amps. And with no key in the car it is doing 0 amps.

 

[Jack Harkness]

You don't have a problem.
You power supply is 12V and is capable of outputting any amount of current from 0 to 200mA. Note I said zero.
I think you have a amplifier that needs 5mA and another amp that needs 5mA and a load cell that needs some thing from 4 to 20mA. All that adds up to less than 200mA.

You power supply will try to make 12V. As far as current goes, It is very happy at 0mA, 10mA, 20mA......200mA. It probably will not be happy above 200mA.
It is too common for people to think the power supply will force 200mA down the wire. It won't. It will force 12V on the wire.

Just to give an example. Your car had a battery in it. On the side of the battery it says "12 volt, 650 cold cranking amps". The battery is capable of starting you car at 600A. But if you only have the radio on the battery is delivering only 2 amps. And with no key in the car it is doing 0 amps.

Wow, I did not know that at all. I guess I did have that misconception. So then if it will force 12V on the wire how is the amperage that it will push determined?
Also will there ever be a situation when it will try to increase the amperage?
My concern here is that the amplifiers have a maximum rated amperage they can take and exceeding that would blow them.

 

[ronsimpson]

The amperage is determined by the load. (As long as the supply can handle the load.) It sounds like you supply is 5 or 10 times bigger than the load. That's a good thing.

Example: You have a amplifier that is rated to pull 10mA, (probably less if it is not working hard), and maybe a little more if it is working 'fast'. That 10mA is probably worst case and could be 5mA average. In your case you need to use worse case numbers. If you had two of these amplifiers then they together could pull 20mA total, but more likely less because we are talking the worst part. Your 200mA supply can power 20 of these amplifiers all at the same time.

Because you are new to electronics I will give you a story.
I like to eat apples. I like to eat one apple a day. I will never eat more than two. Some days I eat 1/2 of a apple and some days no apples.
So the maximum consumption of apples is 2, the average is 1 and the minimum is 0.
The store gets the apples in a truck that can carry 100 apples. The supply is 100 maximum. They take from the truck how much they need to restock the store. (some number from 0 to 100)
I am not forced to eat 100 apples. I only eat what I want. (0 to 2)

 

[KeepItSimpleStupid]

Take a house with a 200 A service @ 240 VAC or 2 x 200 AMPS at 125 VAC. Now you plug in a 3 W night light.

 

[flatfootskier]

Or the water taps in your house.....
You've got a max capability of 1500 ml per second (i.e. current)
The pressure ( i.e. voltage ) on your supply is the same all the time.
All that stops gushing water is the resistance (i.e. resistance!) of the tap.
When you brush your teeth you might draw only 30ml per sec.

Electronics is *very* similar

 

[JimB]

Clean your teeth after eating all those apples, and make sure you turn the light on so you can see what you are doing.

(I am sorry but the devil made me say it. )

JimB

 

[tvtech]

Clean your teeth after eating all those apples, and make sure you turn the light on so you can see what you are doing.

(I am sorry but the devil made me say it. )

JimB

LOL....that was funny

At last a sense of humor prevails here again. I love the "not too serious approach"....sure facts and all are really important to guide and help those that ask questions...but a little humor helps to keep things interesting.

And keeps people coming back. Not like a Nazi forum....." Ve haf sed enuf. Obey and do as ve say, do not kweston"....

And we luckily have no grammar Nazi's here.......uhmm...spell checkers

ETO is blessed.

Regards,

zetvtech

 

[JimB]

LOL....that was funny

All valid analogies, I don't want to detract from them in any way, but something just clicked in my disturbed brain and the connection was made and I had to let the genie out of the bottle.

JimB

 

[tvtech]

All valid analogies, I don't want to detract from them in any way, but something just clicked in my disturbed brain and the connection was made and I had to let the genie out of the bottle.

JimB

Hello Mr JimB

I feel really flattened that you think that I am a Genius in a bottle

You make me so happy

I feel really proud now.........I am soo happy

Can I post more stuff tomorrow when the Sun comes up???

 

[Mikebits]

Haha I like the apple thing. To the OP, one thing you can do to protect your circuit is to put a fuse between the supply and your circuit. But as said you good to go.

 

[ronsimpson]

The problem: I have a DC 12V 200mA power supply that I want to convert to a DC 12V 10mA supply.

Where are you going to find a 10mA fuse?

But as said you good to go.

 

[Mikebits]

https://www.newark.com/littelfuse/0...125v-very-fast-acting/dp/39P9205?CMP=KNC-GPLA