# Increase sourcing current capability?

#### jpborunda

Joined Apr 9, 2014
55
Hello everyone.
So I am trying to implement an op amp based, constant current source, which needs to sink OR source around 0-100mA (Max), for a varying R_LOAD of about little to 2kO(Max).
Attached is the basic circuit I have so far. In the simulation, the current is maintained constant the way I need it to, following the relation I= Vin/Rsense = 1V/10O= 100mA.
The current can be maintained as long as my "Vcc" source is greater than the voltage at the R_LOAD. I've found a miniature, "unregulated DC-DC converter" which provides 200Vout, @ 6.25mA,and it is powered by 0-5V DC. Its all great except that the DC converter does not source the current that I need, so I need to find a way to increase the current it can source to about 100mA (maybe a little more I guess?)while also delivering the original 200 V to the load. The original idea was to power the circuit with batteries, since the control signal Vin was coming from a microcontroller. Im trying to make this as "portable" as possible, however this is NOT an absolute requirement. To sum up, what kind of arrangement would I need to add in order to increase the DC Converter source current capability?
Thank you guys, I'd appreciate any information!

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#### ronv

Joined Nov 12, 2008
3,770
You need a bigger power supply.

#### AnalogKid

Joined Aug 1, 2013
8,537
Three things.

1. You state that you need a circuit to source OR sink current, but your schematic can not do that. It is a constant current sink only.

2. You power supply of 200 V @ 6.25 mA is only 1.25 watts, while your requirement is for 200 V @ 100 mA, or 20 watts. There is no way to draw 16 times more power out of a supply than it is capale of producing.

3. At your design current, the FET will be dissipating over 12 watts, something to watch out for.

ak

#### jpborunda

Joined Apr 9, 2014
55
Hi, thanks for the responses, @AnalogKid: When saying sink or source current, I meant one or the other, but thanks for pointing it out, that circuit can only sink current.
Regarding the current capabilities, I was thinking of something like a buffer, then again, the current would be limited to its Vsupply capabilities. Is there some other configuration I might be able to implement, so that more current can be drawn, say indirectly, (by means of a transistor or something like that) but maintaning the 200V?
I'll think about it, thanks guys.

#### Alec_t

Joined Sep 17, 2013
11,527
Is there some other configuration I might be able to implement, so that more current can be drawn, say indirectly, (by means of a transistor or something like that) but maintaning the 200V?
The average output power (Volts x Amps) can never be more than the average input power, whatever configuration you use.

#### FuzzballJack

Joined Apr 12, 2014
13
I don't want to be a pain but I think the Alec and AK are right. You need to invest in a bigger DC DC converter. There is no way what so ever you will do this with the converter you have now.
BUT(!) if you do ... hush hush ... contact me and we will conquer the world with the circuit that generates more energy than you put in.

Sorry about that ... I didn't mean for it to be rude, I hope the smiley at the end sums up my intent.

#### wayneh

Joined Sep 9, 2010
16,399
... but I think the Alec and AK are right.
You cannot be more right than when you are quoting the first law of thermodynamics.

It's turned out to be one of the more enduring and useful laws of nature.

It has a major flaw, though, which I'll share with anyone that's interested.