bittware
Full Member level 4
high side current sense circuit
Hello experts,
I'm developing a high side current sense circuit. First, I tried to use classic opamp+NPN transistor scheme. See scheme 1 in attached figure. But after I got the prototype board, I found when light loaded, V1=24V, V2=4.1V and V3=3.5V. Although, I've thought of op amp output offest, I never expected this much. What happened to this circuit? Or which mode was the NPN transistor working in? Moreover, if I switched off S1(which is ON by default), the op amp turned extremly hot. Why? The op amp is LM7301 rail-to-rail output swing(0.275~28.65V under 30 power supply) and input offset voltage Vos-max=8mV.
For the next step, I want to try another scheme (scheme 2 in the attached figure) which is proposed by Bob Pease in his EDN column.
See **broken link removed**
I am wondering if scheme 2 will work better than scheme 1.
Thanks for any hints.
Best regards,
bittware
Hello experts,
I'm developing a high side current sense circuit. First, I tried to use classic opamp+NPN transistor scheme. See scheme 1 in attached figure. But after I got the prototype board, I found when light loaded, V1=24V, V2=4.1V and V3=3.5V. Although, I've thought of op amp output offest, I never expected this much. What happened to this circuit? Or which mode was the NPN transistor working in? Moreover, if I switched off S1(which is ON by default), the op amp turned extremly hot. Why? The op amp is LM7301 rail-to-rail output swing(0.275~28.65V under 30 power supply) and input offset voltage Vos-max=8mV.
For the next step, I want to try another scheme (scheme 2 in the attached figure) which is proposed by Bob Pease in his EDN column.
See **broken link removed**
I am wondering if scheme 2 will work better than scheme 1.
Thanks for any hints.
Best regards,
bittware