[SOLVED] how do I current limit 300vdc 3 amps

[ian123]

Hi I require to make a power supply for 300vdc 3a with current limit.

the problem I am having is if i use a resistor to sense the circuit it has to be 900 watt resistor has anybody got a suggetion as to how i should limit the current to 3 amps.

 

[FvM]

the problem I am having is if i use a resistor to sense the circuit it has to be 900 watt resistor

Are you talking about a current sense resistor? It surely won't be designed for 300 V voltage drop, a few 100 mV should be sufficient. You need however a linear regulator respectively electronic switch capable of handling 900 W. Depending on the intended operation mode, it doesn't need to continuously dissipate 900 W.

 

[zorro]

Hi Ian,

I guess you have a regulated power supply and you want to you use a resistor for current sensing and feedback. Then it will dissipate low power.
For example, a 0.1 ohm in series produces a voltage drop of 0.3 V for 3A and will dissipate 0.9 W. Right?
Or are you thinking in current limitation just by the serier resistor?
Regards

Z

 

[ian123]

I have drawn a circuit like the one below but I have no Idea as to how to work out the values the supply should be able to limit current
to 3Amps at roughly 300vdc.

 

[ian123]

ok so if R2 is 0.1 ohm it is still in the circuit where the load is drawn so if the load draws 3 amps continuosly surely this resistor will get hot. or is there something i am missing, please ignore the 4r7 resistor at the base i just quickly drew the circuit.

 

[FvM]

For a pure current limiter circuit, you would omit R3. A value of 4R7 is absurd anyway. Q1 has to handle 900 W, without additional means even continuously. The purpose of the circuit isn't completely clear. What's the nature of the input voltage source, doesn't it expose current limiting on it's own? Do you intend unconditional current limit operation, or some kind of overcurrent shut down?

 

[WimRFP]

As FVM said, please provide us additional info.

It will be hard or almost impossible to find components that will withstand 300V/3A for reasonable time and for a reasonable price. So very likely you will go for a switching approach rather then a linear approach as shown in your circuit.

 

[FvM]

It's well possible to find suitable parts, kW linear amplifiers and regulators are still common in some applications (involving respective heatsinks, thermal protection etc.), but I doubt that it's reasonable.

 

[ian123]

i was thinking of take 220vac and rectifiyng it want to use it for the high voltage side of an hbridge that drives two probes in a solution my problem is as the solution saturates so the current goes up

 

[FvM]

A professional setup would use a 300V/3A lab supply, usually involving switched mode operation. I understand the purpose of your circuit, but I think, you should use a more sophisticated control circuit with a time delayed shut down. The 100 uF filter capacitor is by the way far from the value needed for supplying a DC voltage at 3 A, respectively the low capacitance would result in activitaion of the current limiting during input peak voltage.

 

[ian123]

Hi FVM

could you please give me some direction as to what i should design as i have hit a blank with this.

regards

ian123

 

[Orson Cart]

Either buy a purpose built bench psu capable of supplying 300V at 3 A with a current limit (even a single o/p switchmode would have a useable current limit) or you need to design your simple linear current limiter to hadle 900W, i.e. 10 x transistors (or mosfets) each dissipating 90W max on a long large heatsink rated to dissipate the 900W. Regards, Orson Cart.

here's a 750 watt 300V for $37 on ebay:

**broken link removed**