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Jack, Technician
Category: Consumer Electronics
Satisfied Customers: 9785
Experience:  35 years experience as audio/video repair and installation tech.
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If I want to build a constant 800mA current source, and the

Customer Question

If I want to build a constant 800mA current source, and the load is about 250 Ohm, what should I do?
Submitted: 1 year ago.
Category: Consumer Electronics
Expert:  Michael replied 1 year ago.

Hi I can help you do this simply but I need some details If you would like I can call you, I will send you an offer, so we can work on this until its fixed, If you don't want that just disregard the offer.

and we can do this here in text.

Thank you

Tech Michael

Customer: replied 1 year ago.
My request is simple: constant current source at 800mA. The load will vary around 250 Ohm, so the voltage will vary around 200 Volt. I need this current source be plugged into the normal AC source (110V~220V globally) and start work immediately. If you know how to realize such need, please tell me a brief idea. Thanks.
Expert:  Michael replied 1 year ago.

Hi clear this up for me. I see you want to suply with 110 vac, what is the voltage you need to achieve? Also would you be ok with something that supplies more then 250 MA, you said up top 250 ohm? Wast that a typo as I suspect?

Customer: replied 1 year ago.
Because the load (the thing that I want to apply the 800 mA current to) is about 250 Ohm. And I need a constant 800 mA current, so according to the Ohm's law, the total voltage is something about 200V.There is no typo in my description. I want this source to be used globally, so if I take it to the countries that uses 220VAC, it can also work.
Expert:  Jack replied 1 year ago.

Hi I`m Jack ,different tech. here

I believe you got your ohms law backwards here .

Your source is 120 volts and that's not variable so while it is correct that a 250 ohms load under 200 volts will give you approx. 800 milliamp (.8) , your source is not variable unless you where to use step up transformers.

Now if you use a constant voltage source like 120 volts and then a step down transfo 240/120 when oversea then you have a constant voltage source .

Now usually device will draw a specific amount of currents depending on the devices and the object of a power source is to be able to supply (in voltage and current) the device that loads that power supply .

For example if your device draws .8 amp then you want a power supply capable of delivering above that under a constant voltage not a constant current , that does not exist , the load determines the current draw not the power supply .

For example what you would need is a power supply that can deliver 1 amp and above if your device needs .8 amp

May be easier if you just said what it is that you want to power up and what voltage it works under , is it AC voltage or DC that you need ?

Customer: replied 1 year ago.
For a voltage source, yes the load determines the current draw not the power supply. But for a current source, the power supply will adjust the voltage so that the current keeps constant. This is a fundamental sense, isn't it? and I kept saying that I need a current source, not a voltage one. And I never said I don't need a step-up transformer. And also to be clear, that room supply of 110AC in the US and 220 VAC overseas, they are AC. What I want is a ~200 VDC. So I guess I need an AC-DC transformer for sure.Let me explain a bit more. Right now I'm using Keithley 2600 source meter as the current source. I can set the current output as 800 mA. When applying the power to a ~250 Ohm load, the voltage shows at ~200V (the current is at 800 mA of course). This worked fine for a long time. Now I want to get rid of the Keithley meter and use a customized current meter to realize the same functionality. That's how I started the question here.Jack and Michael, I'm tired of explaining my question over and over again. I'm a PhD and I know the basics of electronics. I'm not a kid who wants to do after school projects. I'm serious. And it is because this website says that it can get me real experts that I payed the money and expected to get the professional answer. If this kind of low-level communication keeps on, I will request a refund.
Expert:  Jack replied 1 year ago.

You never said what this was to be used for , I`m a practical guy and I like to know what I`m working with and right now thank you for the electronic course but we`ll call it a day shall we .

I will repost your question.

Good luck