Low Cost, High Accuracy & Low Noise Switch-Mode Constant Current Sink

Harish Singh

Other Parts Discussed in Thread: LM317

Hi there!

I need help in building a low cost, high accuracy and low noise constant current sink(250mA) based on the switch-mode design. This circuit will be used for the constant current excitation of electromagnetic flow meter.

Although I have tested and found proper, a tradition constant current sink with the help of LM317 but this suffers from a significant power loss across the LM317 that also rises the temperature of the pcb and creates problem for other ICs on-board. Also, it takes a lot of area for the heat sink for LM317 as well on the board.

L1 & L2 are the inductor coils wounded in series on the body of flow meter. Since the minimum and maximum resistance of the coil are 66.5 Ohms and 73.5 Ohms. So, power dissipated across LM317 will be 1.53 Watts (max) & 1.1 Watts (min).

Please suggest IC such as LED Drivers or DC-DC Converters that operates on switch-mode technique, which can overcome the above problems.

best regards,

Harish KS

over 6 years ago

0 JohnTucker over 6 years ago

TI__Guru**** 191445 points

Harish,
It looks like you have 7.375 V at U1-3 for the case where the coil resistance 66.5 ohm (worst case). So you will have to dissipate 7.375*.25=1.844 W one way or the other whether you use a switching regulator or a linear regulator. I don't see a particular advantage to it.

0 Harish Singh over 6 years ago in reply to JohnTucker

Intellectual 995 points

Hi John,

Thanks for the reply.
And so sorry for my late reply.

That's why I am thinking to change the circuit. Can you please suggest me one?

Thanks & Regards,
Harish KS

0 JohnTucker over 6 years ago in reply to Harish Singh

TI__Guru**** 191445 points

Harish,

I think the solution you already have is as good as any. You will have to dissipate 7.375*.25=1.844 W in what ever circuit you use. The LM317 in th TO-220 package can be attached to a suitable heatsink for heat dissipation.

0 JohnTucker over 6 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Harish,

Just a thought, is it possible to lower your 24 V input rail? Can you down convert to a lower voltage such as 18 V? Let me know.

0 Harish Singh over 6 years ago in reply to JohnTucker

Intellectual 995 points

John,

Since, the product is in development phase so I am open for every aspect.
The unit is ac powered based on SMPS. So I have to redesign the SMPS (or the transformer) to lower the DC output voltage.
Any suggestion/option without making any change in the smps would be appreciated.

Regards,
Harish KS

0 JohnTucker over 6 years ago in reply to Harish Singh

TI__Guru**** 191445 points

Harish,

Then go ahead and lower 24 V input to your circuit using a buck converter 24 V input to 18 V output. Since you don't require much current take a look at LM2674-ADJ: www.ti.com/.../LM2674

Power management

Power management forum

Low Cost, High Accuracy & Low Noise Switch-Mode Constant Current Sink