Voltage reduction using Diodes

I have this circuit which needs to be powered by batteries and I am looking at a 6V SLA.

The uC datasheet indicates a voltage range of 2.0V to 5.5V and it will be driving a small DC motor that requires 1.5V to 4.5V. If possible I am trying to avoid using a regulator.
I was thinking of using a diode (D4) in series with the supply to drop the voltage to the PIC to 5.3V and another diode (D3) to further drop the voltage to the motor down to 4.6V.

Does this makes sens or do I have other alternatives?


Thanks,

Mike

ACtually, a fully charged 6V SLA has 6.3V across the terminals, and the voltage can go up to 7.2 during charging, and downto 5.8V when fully discharged. I would use two diodes in series to power the micro, and then a separate two diodes from battery to the motor. This way the current going through the motor will have lesser impact on the supply voltage for the micro.

Kubeek has a good point. Not only should you use separate diodes for the motor, but you should have separate wiring for the motor, all the way back to the battery.

I have added one diode and moved the other one so (D2 and D5) are now together to isolate them from the Motor.
As for the motor, it will be driven from the PIC so I don't see how I can wire it separately.
Since it will be driven with PWM I will reduce the Duty Cycle so it should be ok?

Here is my schematic updated.

Note: Parts names have changed since my first version as I have sorted them out.

How much current does the motor take? Are D3 and D4 big enough to handle that current?

Also, the IRFZ46 is not a logic level part. Have you looked to see that you will have enough gate voltage to turn it on enough to meet your needs?

I am using This motor (the one that ends with 2295)
I am presently testing the IRFZ46 with a LED while waiting for the motor and technically it does the job (with the LED)

Mike

lilimike said:

As for the motor, it will be driven from the PIC so I don't see how I can wire it separately.

Click to expand...

The separation should mean that you have a single wiring going battery+ ->D3 -> motor -> Q1 -> battery-, and then you have a separate set of wires that connect D5 to the battery+ and the ground from the pic and isp connector to the battery- terminal. This way the large current running through the motor doesn´t impact the supply for the micro much.

Kubeek,

I have read your post over and over but am afraid I don't understand or perhaps I don't understand the theory behind this. Doesn't it come up to the same thing if the wires meet on the battery posts?
It may make no difference but the parts within the "Programming components" section will be removed in the final product, if I remove these components and add a DIP socket for the PIC to pull it out for programming, I will save board space and assembly time and a few pennies.

The operation of this device is to run cycles in a loop, a cycle lasts 15 seconds and the jumpers will serve as configuration setting to make the motor run from 1 to 8 seconds and be OFF for the remaining time of each cycle.
The motor is expected to run at or near its slowest speed.

Mike

Can someone explain the following...

I connect the positive probe from my scope on test point 1) and I can observe the PWM duty cycle going from 0% to 100% as expected.
I wanted to test the difference after the mosfet so I connected the negative probe to test point 2) and I get only DC at almost full voltage even if I reduce the duty cycle to 0%.
Where I get confused is if I connect the LED+Resistor as shown in my drawing, it behaves as expected going from 0% to 100% intensity.
If I connect the negative probe from my scope while the LED is connected it jumps to full intensity.

I am not an expert with my scope but I am thinking it is not properly configured?
When I connect a probe I normally press the AUTO button and it gives me the right setup, and it did with the probe on test point 1).

??

Mike

Is R6 really 100K? That is an awfully high value for an LED. I'm sure it must be a lower value, or you wouldn't get any visible light at all from the LED.

Generally, you want to ground the scope to circuit ground. It sounds to me like the scope may already be at ground, since you say that when you connect the probe negative to the cathode of the LED it goes to full brightness. If so, you are just shorting across the mosfet.

No R6 is actually 180Ω, I just copied from R7 for the purpose of posting the image and the LED goes from 0% to close to 100%
When I connected the scope's positive probe at 1 and its negative to the ground.
When I connected the scope's ground at 2 and the positive probe to +5V just as the LED or motor would be connected.

The fact that the scope is not behaving like the LED connected at 2 is confusing knowing that I get proper readings when connected at 1.

Mike