# need constant current load, aprox. 1mA

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#### bogdanfirst

##### New Member
i need to build a constant current load, to demonstrate that if a constant current passes trough a capacitor, then the voltage across it increases or decreases liniary.
the problem is that i don't know how to build this.
the caps will be charged to max 16V, and if i can get a constat load over an interval of about 5V then it is very good.
but how can i build such a circuit?
i know that if it connect a LED trough a BF256 transistor, and connect the gate with the source, then the current maintained trough the led is between 10 and 15mA for voltage from 5 to 30V.
but how can i make a circuit that draws 1mA??
any help is much apreciated.

#### Nigel Goodwin

##### Super Moderator
bogdanfirst said:
i need to build a constant current load, to demonstrate that if a constant current passes trough a capacitor, then the voltage across it increases or decreases liniary.
the problem is that i don't know how to build this.
the caps will be charged to max 16V, and if i can get a constat load over an interval of about 5V then it is very good.
but how can i build such a circuit?
i know that if it connect a LED trough a BF256 transistor, and connect the gate with the source, then the current maintained trough the led is between 10 and 15mA for voltage from 5 to 30V.
but how can i make a circuit that draws 1mA??
any help is much apreciated.

Here's a simple constant current capacitor charging circuit, it charges C1 at a constant current set by the value of R1. The base of the transistor is held at a fixed voltage by the two diodes, this means that the emitter is held at a fixed voltage as well - about 0.7V, the forward voltage drop across the two diodes, minus the Vbe drop of the transistor. As the voltage is fixed, and the value of R1 is fixed, the current has to remain the same. You can calculate the value of R1 from ohms law - R1=0.7/I - which is 700 ohms for 1mA. R2 simply provides enough current through the diodes to swamp the base current, in this case 1mA would be more than enough - 15K should be fine.

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#### bogdanfirst

##### New Member
do you also know how to make constant current load, so i cand discharge the cap at constant current?

#### Nigel Goodwin

##### Super Moderator
bogdanfirst said:
do you also know how to make constant current load, so i cand discharge the cap at constant current?

Turn it upside down (except for the capacitor) - obviously change the PNP transistor for an NPN one, and reverse the diodes.

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