Impose a current through diodes

Hello to all,

I'm new on this forum and I permit myself to post a message on this forum because I have a small question in electronics. In fact, I would impose a digitally (SPI 3 wire) controllable current (ranging from 1mA to 300mA with a 2mA scale) through a circuit consisting of 20 diodes in series (Vf=1.3V so 1.3V * 20 = 26V total).

I would like that the power is idependant of my montage (ie: the current is the same as I connect 20 diodes or 25).

For this, I thought to make an "howland source" montage controlled by a voltage DAC. But in that case, the output of the amplifier must be able to deliver a 300mA current under 26V. Do you know an amplifier reference that is able to do this?

Do you think this is a good idea? or do you see other way to do?

Thank you very much for your help

HRicochet

For the intended unipolar voltage range, an OP controlled transistor current source (PNP or PMOSFET) seems appropriate,

FvM said:

For the intended unipolar voltage range, an OP controlled transistor current source (PNP or PMOSFET) seems appropriate,

Click to expand...

Thank you FvM for your quick answer, so you mean a montage like the following one (even if it is a NPN on the link) ? :



So to conclude, this montage just takes an OP, a transistor passes 300mA maximum and a VCC>26V ?

Indeed, it seem less difficult to design than the "howland source" and it will need less components. Thank you again.

HRicochet

If you can work with a ground connected source, this circuit is the best solution.

Thank you very much FvM for your precious help, have a nice week end!

HRicochet said:

.........................so you mean a montage like the following one (even if it is a NPN on the link) ? :



So to conclude, this montage just takes an OP, a transistor passes 300mA maximum and a VCC>26V ?

Indeed, it seem less difficult to design than the "howland source" and it will need less components. Thank you again.

Click to expand...

You can use an N-MOSFET in place of the NPN BJT if you like. The MOSFET gives a slightly more accurate current source since there is no base current flowing through the RSENSE resistor.

The main reason for using a Howland source is if the load has to be grounded. In your case the load can be floating so the referenced circuit will work.

thank you crutschow for these detailed explanations

Hi all!

I come to you because I have another question. I tested the assembly previously mentioned in this thread and I noticed a little feature.

Indeed, I wish to impose a current from 0mA to 50mA (voltage controlled) through a diode array. This assembly "OPAMP CURRENT SINK" works very well and has the advantage of being simple to implement. However, when the control voltage is 0V, there is a current which flows in the circuit spite of everything (about 5mA :-().

To prevent this, I thought of using an N-MOSFET between the resistor R3 and GND which will act as voltage controlled switch (low Ron << R3). Thus, when Vgs = 0, no current will flow through the diodes.

https://obrazki.elektroda.pl/8995606700_1373702382.png

I do not had the opportunity to test this structure in practice. Do you see any problems? and do you think it might do the trick?

Thanks a lot,

HRicochet

A better solution would be an OP with an input voltage range including the negative rail. Many recent OPs have this feature, but not LT1001.

Thank you FvM,

I will see in your direction. But for information, do you think the N-MOSFET would do the job ?

Reason why output current can't go to zero is output minimum negative voltage of OPA. At single +15V supply it is about +2.5V. Add a 3.3V zener diode between OPA and base of transistor and 1k resistor between base and ground.

Thank you Borber,

I could experiment with SPICE and it works . However, in my design, I really need to cut the load circuit and that's why I considered the N-MOSFET. Can you tell me if this solution can work with the OPAM CURRENT SINK as I show in my previous link? I know that I will loose precision a bit with Ron but it is not really important for my application.

Thank you all

HRicochet

I don't see any purpose of using two output transistors instead of one. There are various ways to force a single output transistor to full off state, without using a second transistor.

My assumption is, that you primarly intend a linear current control down to (nearly) zero. This absolutely demands for a respective OP input voltage range. OP outout range, as mentioned by Borber, can be an additional problem. But a usual rail-to-rail or single-supply OP would be able to turn the transistor off without additional means.

Even under this conditions, there can be a small residual output current due to OP offset voltage. If it's a problem, you can superimpose a small offset to the circuit, or add a switch input.