# How to amplify a DC voltage

## Main Question or Discussion Point

Hi, a newb on eleng here.

Recently i've been working on my project. I need to get a high DC (1000V - 10000V ) output voltage, low current (10-30mA would be fine) from a DC input voltage (9-12V). I'm thinking to use a transformer but it only able to amplify an AC voltage right?

Thanks!

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meBigGuy
Gold Member
30ma @ 10,000 volts is pretty deadly. And, it's 300 watts to boot.

You need a switching power supply utilizing one heck of a transformer. One heck of a switcher, too.

For example, if 100% efficient, 30ma @10000 volts is the same power as 30 amps at 10 volts.

Simon Bridge
Homework Helper
10A 240V (rms) is the NZ domestic supply.
One way would be to build a PSU specifically for the project ... but the answer to the question is that transformers can "amplify" DC voltages too.

I agree with meBigGuy's concerns. The 20-30mA current sounds like a guesstimate. Considering the danger - calculate it. How small can you have the current? What are you trying to do?

Well, maybe 10kV is too high, i will lower the specs then. Infact, it's okay for the amp to go lower than 10mA.

How can we use this transformer to "amplify" the DC voltage?

Thank you very much sir! :)

berkeman
Mentor
Hi, a newb on eleng here.

Recently i've been working on my project. I need to get a high DC (1000V - 10000V ) output voltage, low current (10-30mA would be fine) from a DC input voltage (9-12V). I'm thinking to use a transformer but it only able to amplify an AC voltage right?

Thanks!
Well, maybe 10kV is too high, i will lower the specs then. Infact, it's okay for the amp to go lower than 10mA.

How can we use this transformer to "amplify" the DC voltage?
You say you are new to electronics. What exactly are you wanting to do with this high voltage? What do you know about creepage and clearance requirements when working with high voltages?

Simon Bridge
Homework Helper
You say you are new to electronics. What exactly are you wanting to do with this high voltage? What do you know about creepage and clearance requirements when working with high voltages?
I share these concerns.

Variable, gut-reaction specs and an unwillingness to say what you want this for generally raise concerns. If you let us in on it we can help you avoid some of the hazards involved - they are not always obvious. It is certainly best practice for us to at least attempt to make sure of your knowledge. Maybe it's all fine and we are worrying for nothing?

Actually it's nothing really, my teacher told us to do a project that amplify a low DC voltage to higher one. Well he didn't told us to make it into 10kV though, i was just wandering IF i want to be that high, what should i do.
anyway thanks for your concerns guys :)

Bobbywhy
Gold Member
Transformers do not amplify DC voltages. Have you tried a "Google" search for "DC Amplifier". There are thousands of them.

If you are serious about building a HIGH VOLTAGE supply, you can do so safely by learning the basics of "switchmode power supplies for HVDC". You can also purchase small, sealed packages that use low voltage inputs and give you kilovolts out. Just learn to use Google search to find what you want.

Simon Bridge
Homework Helper
Actually it's nothing really, my teacher told us to do a project that amplify a low DC voltage to higher one. Well he didn't told us to make it into 10kV though, i was just wandering IF i want to be that high, what should i do.
anyway thanks for your concerns guys :)
Well that's extremely open ended but it is almost certain that your teacher does not intend the kid of high-power application you are thinking of. In fact, it may even be against the law.

You want to end up with a box which takes a few volts in and puts a few more out or what?
You should get a clarification on that from your teacher.

Look up "DC amplifier".

If you want to have fun, there's always building a van-der-Graaf generator ... it's the sort of thing I did at High School level but things have changed since then and it may be against some sort of regs. Check.

It could just be a second PSU ...

You can use a transformer as part of a project that steps-up a DC voltage, but a transformer by itself won't do that.

You want to end up with a box which takes a few volts in and puts a few more out or what?
You should get a clarification on that from your teacher.
yes something like that and a low amps too

Simon Bridge
Homework Helper
Yep - so are you given an input and a target output?
Can you have an extra power lead into the box or does it have to be driven entirely off the input voltage?

It sounds like what you really need is help narrowing down a very open-ended assignment.
If I know the parameters I can do that really fast.

the input would be from a DC battery, 9v to be exact.
Sorry but what do you mean by that? if i'm not mistaken understanding your question, i think yes i can, the power output doesn't really matter

yes, it seems like that :)

Simon Bridge
Homework Helper
the input would be from a DC battery, 9v to be exact.
Sorry but what do you mean by that?
Have an extra power lead into the box?
I mean - connected to another voltage source. I mean: if your box could have another 9V battery in it, you can easily turn a 9V input to a 18V output right?

berkeman
Mentor
Actually it's nothing really, my teacher told us to do a project that amplify a low DC voltage to higher one. Well he didn't told us to make it into 10kV though, i was just wandering IF i want to be that high, what should i do.
anyway thanks for your concerns guys :)

Then don't think about large voltage multiplications right now, since you do not yet have the skills to understand how to handle high voltages safely.

meBigGuy
Gold Member
The question is whether you want an amplifier (take a low dc voltage and amplify it to a higher one, which requires a separate power supply at the higher voltage) or a DC-DC boost converter power supply, which takes a DC voltage and creates a higher DC voltage from it alone (without a separate higher voltage) (essentially a DC transformer, but complex).

Google or wikipedia "boost converter" to see if that is what you want.

Have an extra power lead into the box?
I mean - connected to another voltage source. I mean: if your box could have another 9V battery in it, you can easily turn a 9V input to a 18V output right?
No i can't put another battery. Only 1 of them.

I've read about Boost Converter, but i don't really understand how it works. Can someone give me a simple schematic or an example on how it's work?

meBigGuy
Gold Member
Read the wikipedia artilcle on "boost converter". In the "operating principle" section, read (a) and (b) over and over until you understand what they are saying. If you can't get it. look at a few more examples. If you are still stuck, post your questions.

The single inductor boost converter described above is 1 approach to DC to DC conversion. You can also use a transformer based architecture. You essentially switch the DC (essentially creating AC), go through the transformer, and rectify back to DC. http://www.electroniq.net/power-supply/dc-dc-converters/low-power-variable-voltage-converter.html is one simple example.

EDIT: This is a better tutorial:
http://www.maximintegrated.com/app-notes/index.mvp/id/2031
Here is a more advanced tutorial. http://www.maximintegrated.com/app-notes/index.mvp/id/1109

1 person
Simon Bridge
Homework Helper
No i can't put another battery. Only 1 of them.
Good. That's a restriction you didn't mention :)
Any others now you know what sort of thing to look for?

You seem to be settling on a boost-converter though. Enjoy.
If you are having trouble understanding it, that may be because you are expected to use information from your classes to date. (Which would be another restriction.)

If you have covered electric motors, electric generation, and transformers, then you have a likely candidate that fits the bill.

I've seen the article on the first link. So according to it, a Boost Converter work depend on the time it switched on and off? and will the Vout comes in a constant? and the capacitor is only used to prevent Vout goes to the GND?

I've done a simulation on a software, when i close the switch, there won't be any V at the load (i put resistor to it) and the current at inductor increase linearly. but when i open the switch, the V out suddenly goes very high (like 10kV) but then steadily decreasing. meanwhile the inductor current dropped very low (like from 1kA to 1A, i don't know why it goes that high) and it even make a weird signal (like a very sharp sinusoidal wave) then it settle down at some value.

Is this to be expected? btw, i use 1mH, 10uF, 1k (as the load) and diode fwd voltage @1A, 1,24.

meBigGuy
Gold Member
You need to figure out for your self what is happening when you close the switch and what happens when you open it. Again, make sure you understand sections a and b that I referenced above. It is also explained very well in the first tutorial. Experiment with smaller inductor values and higher switching rates until you understand the relationships.

You will eventually need a boost converter controller to regulate the output voltage to the desired level. It will change the on-off ratio as needed to achieve the proper output voltage. Also explained in the maxim tutorial.

Make sure you use a schottky diode in your sims.

When the current in the inductor ramps down to 0 the circuit will ring. You can read about smps snubbers.

1 person
Okay thanks for the info for the Boost converter. I have another question now.

If i want to use a transformer, than i will need to convert it to AC right? How can i convert it? (i don't want to use power inverter).

I've once read about transforming it using a transistor, how does that work? (and if possible, please show me the circuit example)

Simon Bridge
Homework Helper
Important: You are best advised to complete the project within what you have been taught in class.

If you want to go beyond the class material, it will be best to do much of the discovery yourself.
i.e. google for "sine wave transistor circuit" you will get lots of examples - and some video tutorials.

But, technically, any circuit that turns DC into AC is called a "power inverter", so you cannot get away from using one.

meBigGuy
Gold Member
Okay thanks for the info for the Boost converter. I have another question now.

If i want to use a transformer, than i will need to convert it to AC right? How can i convert it? (i don't want to use power inverter).

I've once read about transforming it using a transistor, how does that work? (and if possible, please show me the circuit example)
One can use switching transistors to switch the DC forward and reverse through the transformer to make it appear as AC. Here is a good one with explanation. http://sound.westhost.com/project89.htm It turns 12V from a car battery into +35 and -35 for a car stereo amplifier. The switching transistors (FETS) are configured as an H-bridge to put the 12V across the transformer first 1 way and then the other.