Schematic MAX712 charger + relay replacement

Thread Starter


Joined Aug 3, 2007
I have quite a few questions regarding the attached schematic that I designed.

General info:
I am making hand-held device, I will put everything in it. The device powers another 2 microcontrollers (besides the one in this schematic).

When powered by AC, it will charge the batteries (NiMh 2600mA), and REL1 will switch so that it will draw power from AC instead of the batteries while it's charging.

REL2 powers on the device. PB1(Power ON) button turns on the relay, once the microcontroller has power, it will turn RC4 ON, and this in turn will "hold" the relay ON, keeping itself ON even after button release. The microcontroller then turns OFF the power when needed.

The PIC monitors each battery cell, and if any cell reaches 1V, it will turn off the device.

1) First off, I don't know how to replace the replays, especially "REL2" because it draws power from the batteries to keep itself ON. I have an optocoupler FOD3180 that I am thinking to use instead of the relay, but I never used it before, and I can't find an example schematic on the net.
Will a MOSFET work also? I have IRLR7807, can I use that?

I used relays because I was worried about drop-off voltage.

2) I don't know if I setup the charger MAX712 correctly, I don't want to charge the batteries too fast, so I calculated for a charge of 4-5 hours (I don't mind if it's even double that), but the TIMEOUT minutes has a max of 264 minutes.

3) I don't have 2N6109 transistor, can I use the TIP32 instead ?

4) The "FASTCHG" output for MAX712, how can I connect it to a microcontroller input ?

5-a) The device will turn off if any of the batteries reach 1V, so this means that the duration of the device operation is until the first battery dies. How can I discharge the batteries equally so that all batteries reach 1V at the same time (or almost) ? (if that is even possible).

5-b) If discharging equally is not possible: If I add 2 more cells (6 in total), and monitor all 6 cells, how can I remove from the series the battery that discharges first... and then the second battery, and finally turn off the device if the third battery is discharged ?


Thread Starter


Joined Aug 3, 2007
Waw, you guys must have something against me, because I just don't understand how any of my questions cannot be answered...

Anyways... did some trial & error, and found that when using IRLR7807 I can replace the REL2, the only problem is that I have to put 7.6 volts on the gate so that there is no dropout voltage. So this means I will have to make a DC-DC step-up.

I think I will replace that relay with a Latching relay...


Joined Dec 28, 2011
A latching relay should help with your current draw issues with the regular relay or MOSFET.
As to the 3rd question you had. The TIP32 has a lower Ic rating than the 2N6109, as well as pretty much every other voltage and current rating. Do you know what the current through that transistor is? I don't think the voltage ratings will be an issue since the TIP32 is rated for 40Vce & Vcb and your input voltage is 12VDC.
Other than that I cannot be of too much help as I do not have much experience with Pics nor the MAX712.