Automatic series/parallel configuration - Page 1
Forum section: electronics / projects

author: Jr.Maxwell
author_rank: Contributor
timestamp: March 08, 2020, 09:34:52 pm
content: Hi Guys,I've been thinking about this issue for a while now and came here for your guided assistance haha.Situation:Ik have 6 heater mats spec'd at 12 volt. I need to power them with either a 12 or 24 volt battery. When powering them with a 12 volt battery I simply put all the heater mats in parallel.(see image: "knipsel1")When powering the heater mats with a 24 volt battery I configure it as 2x 3 mats in parallel. (see image: "knipsel2")The problem rises when I've physically placed the heater mats. I cannot change the placement nor connections of the heater mats on the PCB after physically placing them. However the input voltage can and will continuously change and be EITHER 12 or 24 volt.Question: How can I wire the heater mats so that it works with 12 and 24 batteries? Is there a way to automatically make the switch? maybe an IC?I came with this solution but I feel asif there are way smarter solutions: in this solution the cpu turns on/off mosfet's to create parallel and series configurations. (see image: "knipsel3")As always,Thanks amazing community!

author: Stray Electron
author_rank: Super Contributor
timestamp: March 08, 2020, 09:48:44 pm
content: Are you saying that the supply voltage will be either 12 v or 24v or that it can vary between the two voltages?It looks like your heaters are only spec'd for 2.5W each or a total of only 15 watts so that's not a lot of power. Can't you just regulate the power to a constant 12 volts and feed that to the heaters?

author: Jr.Maxwell
author_rank: Contributor
timestamp: March 08, 2020, 09:52:51 pm
content: I mean that the input voltage will either be 12 or 24, nothingin between. Thanks for the head's up, I've clarified it in the main post.I can use a 12 volt regulator, but that's my plan B. I'm looking to see if there's a way to solve this puzzle without a regulator (just to create more insight for myself).

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 08, 2020, 10:08:21 pm
content: Can you consider a good old fashioned relay?

author: Nusa
author_rank: Super Contributor
timestamp: March 08, 2020, 10:35:26 pm
content: Take knipsel2 and draw a center rail (let's call that B) connecting the midpoints of each resistor pair. Now disconnect the top rail (let's call that A) from V+. Configure a 2-pole break-before-make relay that connects Ground , V+, A and B in two different configurations. When not energized, it's in 24V configuration, connecting V+ to A and nothing to B. When energized, it's in 12V configuration, connecting V+ to B and Ground to A. I'll leave the rest of the details to you.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 08, 2020, 10:51:47 pm
content: I think I drew what Nusa described with fets. If cascading failures are a problem, sprinkle some diodes in where the resistor pair connects.

author: Jr.Maxwell
author_rank: Contributor
timestamp: March 09, 2020, 01:21:37 pm
content: That would work! I like it more then my design haha. Why the resistor on the lest top of the circuit?

author: Jr.Maxwell
author_rank: Contributor
timestamp: March 09, 2020, 01:22:23 pm
content: I'm going to give this also a try !

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 09, 2020, 05:13:23 pm
content: Quote from: Jr.Maxwell on March 09, 2020, 01:21:37 pmJust to call out that it's a PMOS and you'll have to pull it just about all the way to 12/24v to shut it off.Personally I think a mechanical relay has the advantage here: with a little care you might be able to exploit the pull-in voltage to get the thing working with little or no additional circuitry. Maybe a zener or something. Although you don't need to consider operation between 12v and 24v you get some hysteresis with it for free which would help if say you ended up with 18v on it for some reason like being connected to a dying battery.Flip side is fets are cheap and don't go "click".

author: Jr.Maxwell
author_rank: Contributor
timestamp: May 28, 2020, 07:26:52 pm
content: Hello again,I'm having trouble finding the values for the 3 mosfets. I came up with the following thought process but now I'm stuck.The Pmos should have a VGS of -24V, to open up and give a voltage drop of 24V over 2 resistors.The left most Nmos should have a Vgs of 24V. This so the current can flow to ground when 2 resistors are in series.The right most Nmos should actually be a Pmos with a Vgs at -24V to stop the current flow to ground.My problem is that If I lookup digikey I just cant find mosfet's with these parameters, where I'm I going wrong here?

author: PlasmaArcEng
author_rank: Newbie
timestamp: June 10, 2020, 11:35:20 pm
content: I have found that Mosfet gate voltages are usually a max of +-15V for the common varieties.I did a schematic for you of what Paul Moir suggested.