Voltage divider with diode acting strange ? 2

[Chancy99]

I'm missing something fundamental here ... I have a typical voltage divider circuit, consisting of two resistors, a 39k and a 22k, plus a 10k pot to adjust things. The 39k is the top leg for input, the 22k and 10k pot are the bottom leg to GND. When adjusted for about 25k, it will divide 12V in at the top of the 39k to about 5V at the junction of the 39k/22k.

Works fine. Stick in 12V, get 5V out.

Now I add a 1N4625 5.1V zener, to make sure that the input voltage can't surge up and burn things out. As I understand it, the zener shouldn't conduct at all until 5.1V.

But it seems to be acting as another resistor to GND. WIthout the zener, I adjust it to show a clean exact 5.00V. Add the zener in, and the output drops to 4.61V. I can increase the input voltage to get 5.00V out of the divider - I have to go to 16.4V.

With 16.V input, if I remove the zener, the output jumps from 5.00V up to 6.2V.

I know this is going to be a DOH type slap the forehead deal ...

D.

 

[Madcow]

By clamping to Vcc with a diode the large amount of cap.
on the 5v rail should absorb most transients. You can also
add a zener or transorb across the 5v rail if you dont
have large capacitance. Adding the zener at the rails
should minimize the leakage on the measuring circuit.
I would use diodes on the +&- rails from the input of
the opamp. The negitive going clamp prevents the opamp
from lockup if the input recieves a negitive transient.
The output of the opamp probably doesn't need to be
clamped if it is running off the same supply as the micro.
Also why a full scale range of 12.75 are these measurements after a regulator or are they off the battery. I think my car charges to 13.5 on the battery.

Good luck

 

[Chancy99]

buzzp -

When you say the regulator should be able to handle this, we're referring to the clamp diode feeding into Vcc, *after* the regulator, right ? So the regulator should be able to handle having a large voltage on its' output leg.

The power input stage is pretty robust. +12V and Chassis come in via large inductors, feed into an RBO40 overvoltage/clamp, which then feeds into an AD7705 automotive regulator. 10uF and 0.1uF caps on input side, 10uF and 100uF on the output leg. Micro and opamps all have 0.1uF caps. The AREF pin is fed via a 10uH inductor and 0.1uF cap to an LM336-5.0.

Throwing all kinds of hash on the power pins doesn't upset it a lick. Nice and quiet. That is, here in the lab ...

It's the signal inputs I want to protect as well. From sloppy connections, shorts, transients, etc. The input signal goes through the 39k/22k/10kPot divider, into the opamp voltage follower, then to the micro ADC inputs.

Where and how is the best spot to protect those signal inputs. The micro is the most important to protect, followed by the opamps. A 400V spike will produce around 10mA or so input to the opamps.

Madcow -

High voltage of 12.75 was chosen for simplicity. The 8bit ADC high count of 255 equates cleanly to 1275 with a conversion factor of 5. It also provides for a count resolution of 0.05V, which makes for a nice clean display increment. Going to 13.5V would need a conversion of 5.29 which gets to ugly slow math.

D.

 

[buzzp]

Chancy,
That is where I was talking about. The 5V regulator for your micro should handle this with no problems. I have never relied on it but know of several designs that do.

It is easy to get carried away when talking about protecting these things from overvoltage. If it were me, based on what I know, I would use a bi-directional TVS at the input (before the resistors), and bypass caps on all supply lines directly next to the part that is using the power, ie a bypass cap located directly on the op-amp rails right next to the op-amp on the board. Same thing for your micro supply lines. When laying out the board (assume your going to make a PCB), pay close attention to keeping supply lines for these IC's away from any 'switching' lines. If this is done, then putting clamping devices on anything beyond the input (the signal to be read) is probably not going to help much. Also, several micro manufacturers have internal diodes to protect from a high voltage on an input pin (breakover diodes). Board layout in such an application is by far the most important since if it is not laid out correctly or with any thought, all the clamping devices in the world will not help.