Solutions for using low current rating slide switch for high current load - Page 1
Forum section: electronics / projects

author: kian0079
author_rank: Contributor
timestamp: October 23, 2023, 11:55:57 am
content: Hi all,The circuit I am designing requires a small SMT slide switch that has a current rating of 100mA. I am constrained by space hence I can only use a small SMT slide switch. I need a switch to cut of the battery supply to turn on or turn off the rest of the circuit (consisting of a DC-DC boost converter, MCU, sensors, dc motors and a WS2812 LED strip) which can draw up to a maximum of 1.5A current when everything is on at the same time.I was initially thinking of using a MOSFET slide switch circuit using this as reference:https://www.pololu.com/product/2811, and implement a design like this block diagram below:But I realized that the DC-DC boost converter has an EN pin, hence I am thinking if it is even possible to not use the MOSFET slide switch circuit and just connect the slide switch directly to the EN pin of the DC-DC converter. So the new block diagram would like something like this:The DC-DC converter I am using is the RT4812 (https://www.lcsc.com/product-detail/DC-DC-Converters_Richtek-Tech-RT4812GJ8F_C250395.htmland I am thinking of having a connection like this:Would you foresee any problems with the second solution? I am more inclined to use this since I can do away with the MOSFET switch circuit.

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2023, 12:09:17 pm
content: You might want a week pulldown on the En pin so it's not floating while the switch moves between each state. Like 1meg.Just in case the regulator might do something odd, like go unstable, if the En pin gets noise on it while floating.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2023, 12:44:53 pm
content: Well, check the datasheet:QuoteCool, it's got controlled substrate voltage on the PMOS so it fully disables the output (compare block diagram). Yep, that'll do.EN pin says typical/max 0.1, 1 uA sink current, which doesn't sound like an internal pulldown resistor, and they do show an external one in the example circuit. I would add one.You might even use a battery monitor to force-disable the regulator when it reaches critical low charge; in that case the switch wouldn't be wired to +V, but to the "battery good" signal from the BMS. Assuming of course you have a separate charge circuit (which seems to be the case).Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: October 23, 2023, 04:09:07 pm
content: Quote from: Psi on October 23, 2023, 12:09:17 pmMight as well make it 909k if there's already one of those on the board (in the FB divider).Or 100k if the load on the battery (although only 30-40uA) is not a concern.One other point: make sure you're using a break-before-make slide switch, and not a make-before-break one. If the latter, you will briefly short the battery to ground while sliding the switch!Speaking of the slide switch, will contact bounce not be a concern here? Might be a good idea to put an RC pair on the EN line alongside the pull-down, although I suppose that would only be okay if the EN input is schmitt trigger, which the datasheet doesn't state.

author: PlainName
author_rank: Super Contributor
timestamp: October 23, 2023, 05:06:30 pm
content: QuoteThat's a good point, but wouldn't it be better to skip the ground connection on that pin and just have a pulldown to ground on EN? That way the battery won't be shorted (problem with make before break) and the EN pin will never float (problem with break before make). Bonus is it doesn't need to be a 2-pole switch (but can be for aesthetic or other reasons).The small leak from battery when turned on (via the pulldown) should be insignificant compared to what else is going on.

author: kian0079
author_rank: Contributor
timestamp: October 24, 2023, 06:36:13 am
content: Thanks everyone for suggesting the pull down resistor on the EN pin. I totally missed that out. I think a 909K resistor should work. I will try it. Should I skip the ground connection on the slide switch and just have the Vbat voltage connected as suggested by @PlainName? In this case, I don't have to worry about short-circuiting the battery.I am actually monitoring the battery voltage using a simple voltage divider and reading it using the ADC on my microcontroller. I will be nice to be able to disable the DCDC boost converter when the battery level gets too low. I presume the MCU will have an output pin to drive the EN pin on the DCDC boost converter. But if that is the case, how should I connect the slide switch? The slide switch should still be there to turn on or turn off the device manually.Quote from: HwAoRrDk on October 23, 2023, 04:09:07 pm

author: PlainName
author_rank: Super Contributor
timestamp: October 24, 2023, 07:46:02 am
content: QuoteJust seen this on some kit and it's a bit offputting! Not the auto-shutdown but the slide switch being overridden.In my situation the device is a boroscope with meter-style display. Slide switch turns it on, no problem. Slide the switch to turn it off and it doesn't turn one like a switch should have done but goes through a shutdown sequence. If it'd been a pushbutton it wouldn't have stood out at all, but the slide switch says "this is really mechanically off" and it isn't, so triggers a WTF reflex.Just mentioning that because I think you'd be better off with a pushbutton. That way the MCU can control things as well, but 'off' can still be 'really off'. It's a common option with lots of examples, including the use of battery and charging.Speaking of charging, you may want to not have the load powered by the battery when you have external power available (that is, the battery is charging). The reason is that the charger will typically be expecting a simple battery and could get confused by a complex load.Both the above (the push on/off and not powering from the charger) can be sorted with reasonably simple MOSFET circuits.

author: kian0079
author_rank: Contributor
timestamp: October 24, 2023, 12:57:25 pm
content: Just to side track a bit, I found some push button controllers from Torrex which has somewhat a similar function except that it uses a push switch instead of a slide switch. These are quite good because it allows the power supply to be cut of by both the push switch or a signal from the MCU which is useful if I want to shutdown then the MCU measures a low voltage on the battery.https://product.torexsemi.com/en/category/push-button-controllersI still prefer a slide switch for my application but I can't seem to figure out how I can cut off the power supply by either using the slide switch or with a signal from the MCU.