Single voltage op amp

Other Hardware General Electronics
1

Dear forum,
I need help salvaging a project. I have converted pwm output to analogue voltage using a resistor and a capacitor. I now wish to buffer the output before using it to control some high power LEDs with buckpucks. I designed the circuit with an 074 quad op amp in voltage follower mode (I have 4 pwm-converted outputs).

However, in a moment of clarity, i realise that my power supply is single ended (0-19 v) so an 074 won't work single ended (correct?). Can anyone tell me if there is a pin compatible quad op amp available that operates at this voltage. I have already built the circuit and there is not much room available for major changes.

Thanks,
Nick

2

LM324, or do you need rail-to-rail? How you buffered after OPA?

3

I think an LM 324 will be fine. It goes up to 32volts with a single supply. I wish to connect vcc- to zero and vcc+ to 19 so I think this is NOT rail to rail.

The output of the op amp drives a control voltage on the buckpuck so I think additional buffering is not required.

Thanks,
Nick

4

I believe the LM324 output voltage only goes to (Vcc - 1.5V) max, but that's
probably better with Vcc = 19V than 5V.

5

Vcc and GND -1.25 + 1.55V.. No not quite, the chip can source 20 mA typ but sink only 8 mA the load it would be driving could only be +/- 8 mA max and still be symetrical.
I started reading the nitty's on the data sheet and although Von Hi was clearly stated and the source current but different terms were used to describe the output sink current, So another closer look was required and sure enough the last DC entry was the asymmetrical output drive.. It bit me once many years ago.

Doc

LM324.pdf (312 KB)

6

Thanks for the answers so far.
I found a different op amp LM3900N. It is not pin compatible with an 074 but the only thing I could get hold of today that will work from a single power supply. I have read the data sheet but don't really know what to look for.

Anyway, I tried a voltage follower circuit and according to various notes i found on the web, a single power supply op amp can work in voltage follower mode where the output is connected to Vin - and the input goes to Vin +.

No surprise (since i am posting again) is that it doesn't seem to work. I am not sure if I have destroyed the chip or it simply won't work.

Can some one tell me if I am likely to be able to use this op amp to buffer the input (0-5 volts supplied from an arduino after converting PWM to analogue voltage) when using a 19V power supply to supply power to the op amp. Someone mentioned rail to rail in an earlier reply which I think I now understand. Is my problem that I can't get down to zero volts with this chip.

Thanks,
Nick

7

lm3900 is so called norton amp and it works differently than the normal op-amp. Its output is related to the difference in the input currents, afaik.
Tell us exactly what is your goal.. Do you want to buffer a 0-5V signal from your simple RC pwm filter to 0-19Volt range?

8

Hello,
I have a four 0-5 V signals from the arduino obtained by converting PWM to voltage with an RC circuit. This works fine.
I want to buffer these signal because they feed into the control input of 4 buckpuck. The required 0-5 volt signals that control the buckpucks draw more current than the arduino can provide.

The buckpucks require a large voltage and current to run some heavy duty LEDs. I am therefore using 19V, 5A power supply. Therefore, the only voltage available to me is 0-19 V.

So all I want is to buffer the arduino output. If the LM3900N is no good as you suggest, then should I go for the LM324? Will it do what I need?

Thanks,
Nick

9

Again, tell us exactly what you want:

  1. to buffer 0-5V from your pwm to 0-5V with higher current capability, or,
  2. to buffer 0-5V from your pwm to 0-19V (with 4x amplification)
    You have got 19V power supply available.
10

Option 1.
No voltage amplification required, just higher current capability.

11

Then do use unity gain buffer amplifier (voltage follower), an LM324/358 or, better, a modern rail-to-rail one (for 30V Vcc).

Figure n.26

Mind the simple RC is a poor filter for pwm, and the voltages around 0V are not easy to achieve even with rail2rail op-apms.

12

Thank you,
Can you suggest a quad op amp that is rail to rail and ideally pin compatible with an 074. If not, will an LM324 go all the way to zero? I have plenty of "room" on the high voltage end if I run it off 19V
Nick

13

g...le "rail to rail op-amp"
there are zillion of r2r opamps available, everybody is producing them. The main issue I see the majority of them today are for 5V VCC max. (So you need a voltage regulator).
The modern rail to rail (or "beyond the rail" as well) op-amps go to ~5mV when not loaded too much (ie 100k load), or 50-200mV when loaded.

14

NickH:
Thank you,
Can you suggest a quad op amp that is rail to rail and ideally pin compatible with an 074. If not, will an LM324 go all the way to zero? I have plenty of "room" on the high voltage end if I run it off 19V
Nick

An LM324 output will go all the way to ground if you ask it only to source current, not to sink current - in other words, when driving a resistive load between its output and ground.