Switching on and off a Step Up Voltage Regulator

Discussion in 'General Electronics Chat' started by arosso, May 13, 2016.

  1. arosso

    Thread Starter New Member

    Apr 30, 2016
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    Hi,

    I'm just starting out getting into various Arduino and electronics projects and am trying to start simple as I learn. In this particular case I was trying to make a battery powered setup that could control one of my garden watering valves. The particular valve is a latching solenoid and requires a pulse of +24v to open and -24v to close. I got everything working fine running from 3 AA, a 3.3v Arduino, 2 Voltage Booster boards (one to do 24V and the other 5V) and a board with 2 5V relays. I'm trying to take it slow and use pre-made components/boards as I learn and get the various parts and equipment needed over time and then optimize.

    Anyway, the voltage regulators I got are these:

    http://www.amazon.com/dp/B00C9179WK

    But they seem to draw 10-15mA when doing nothing which isn't great for battery consumption. They are only really needed when the valve needs to be switched on or off and otherwise aren't needed. So I tried putting in PN2222 NPN transistors on the ground leg of the booster board to switch them on and off. I believe the current requirements don't exceed the PN2222 specs. This works but not very reliably. If it's been off for a long time and I power the system the 5V relay system sometimes doesn't work (and therefore the 24V system isn't used). Other times it will but it seems to happen more when off for a long time.

    I haven't found a good answer but from what I gather a voltage regulator/booster is not really meant to be continually switched on and off completely. Is this correct? I think some of the problem is that there is a big inrush current upon start up of the booster circuit to charge it's capacitors and I think this destabilizes the system somehow. Also since I'm only keeping the booster on for the duration of sending out the pulse (< 0.5s) I think it sometimes doesn't have enough time to stabilize.

    I don't have a good diagram to upload yet but I was wondering if someone had some thoughts on switching on and off these kinds of circuits. Maybe I should try to use a PNP on the positive leg, or maybe I should be looking at something completely different. There are a lot of voltage boosters out there that can be put together (some have some shutdown/standby inputs) but I didn't find something I could just plug in as easily. Eventually I want to dive into it and put one together in a v2 version of the circuit (and then a v3 with no relays but mosfets) but I would still have some sort of voltage booster in the system which would ideally be off when not in use.

    Any thoughts or direction?

    Thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Gin up a connection diagram and post it.
    It's worth a thousand words.
     
  3. arosso

    Thread Starter New Member

    Apr 30, 2016
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    Here is a quick shot at drawing the 5V part of the circuit. Hopefully it makes sense.

    The input is 3 Eneloop providing around 4V. The regulator mentioned boosts this to +5V. The relay board (http://www.amazon.com/SunFounder-Shield-Module-Arduino-Channel/dp/B00E0NTPP4) takes 5V to power. The D3, D4, D5 pins are from the Arduino which provides 3.3V. D4 and D5 need to go low to activate the relay.

    regulator.png

    My basic question is whether using the NPN when not in use to shut off the regulator is a bad idea?

    Thanks.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
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    That would probably be correct if you were trying to switch the regulator on and off many times per second, but surely you don't want to do that? I see no problem with switching it infrequently. The NPN should work ok.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Are you OK with the relays going to their default state regardless of the inputs when you turn the regulator off?

    You might want to be aware that removing the GND from a regulator might cause the current to find an unusual path to GND possibly causing a malfunction. It would be better if your regulator had an ENABLE or SHUTDOWN* input.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There has to be a short time delay between D3 and either D4 or D5. The boost converter has a startup delay. 1-2 seconds should be enough.

    ak
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Good point. So a high-side switch (PNP) might be better. The linked reg doesn't have an obvious shutdown pin.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    We took specific steps to prevent this with power available on a network cable to prevent the data lines from becoming a ground return.
     
  9. arosso

    Thread Starter New Member

    Apr 30, 2016
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    This thing will only switch on once a day if that. My plan is to try to have it switch on the solenoid valve on some sort of schedule and based on temperature inputs. Which is why I really would like to see it not draw any power at all the majority of the time.

    I just power the relays for < 0.5s to send a pulse to the valve. So even if they were to go to default state when the regulator switches off that should be fine. I assume what you mean by default state is the Normally Closed when there is no power.

    So if I understood correctly, if you cut off the low side of the circuit, especially a more complex circuit, the current could try to find a way to ground and cause issues. Interesting to know. So it would seem like a better idea to always switch downstream circuits using PNPs on the high side?

    Yes, I realize now that many regulators have some sort of shutdown pin used exactly for this. I think some other pre-made boards have this option but I didn't realize the importance at the time. Still the shutdown seems to still draw some current and in this situation it would make more sense to not even power them if possible.

    I'm also seeing what I think is an inrush current upon switching on the regulator. When I have my multimeter inline which limits current to 500mA the arduino resets when I try to power up the circuit even if I'm not using the power outputs of the arduino for the regulator. When I take out the multimeter and use batteries things work fine. I'm not sure if this big draw on startup is causing some issues or something I need to be concerned with (it does prevent me from look at the current in mA used by the circuit though). Maybe this is caused by something else but I'm not sure what.

    Anyway, I'll give the PNPs a try and see how things change. Seems like I also might need to wait longer after powering the regulator to let it stabilize. Thanks for all the replies.
     
  10. RichardO

    Well-Known Member

    May 4, 2013
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    The problem with shutting off a boost regulator using a "shutdown" pin is that the coil is _not_ turned off. The coil connects the input voltage to the output voltage through the rectifier diode.

    Note that there are boost regulators that have a synchronous rectifier that can turn off during shutdown. These regulators are typically low voltage, low current devices. An example would be an LTC3400.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    So removing the ground lead from the input side would also have less than desirable consequences?
     
  12. RichardO

    Well-Known Member

    May 4, 2013
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    Good catch. Since disconnecting the ground from the regulator does not disconnect the inductor the load is still connected through the inductor and rectifier diode. :(
     
  13. arosso

    Thread Starter New Member

    Apr 30, 2016
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    Hi,

    I've now setup my circuit to switch on the high side of the regulator. Had to do an NPN (PN2222) and PNP combination (8055) since the signal from the arduino is 3.3v and the battery voltage is 3 AA which could go up over 4.5v... From what I understood a PNP needs somewhere around the same voltage it's controlling to be able to shutoff.

    I still get 'mis-fires' when I continually try to trigger this but not as much as before. It overall seems better so I'll continue to check my circuit connections, etc.

    One doubt I have now is the ground. The negative terminal of the output of the regulator is not connected back to the ground on the input side. Is this good or bad? Doesn't this mean I have 2 different ground. I'm not sure if the regulator would somehow isolate the two from each other. Since the arduino is on a separate ground and driving the relay is that going to be a problem since the reference points might be different?

    Thanks.
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    I don't know. Is there a reason why you think isolation is required between the processor side and the relay side?
     
  15. arosso

    Thread Starter New Member

    Apr 30, 2016
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    The board has +IN, -IN, and +OUT, -OUT rather than +IN, +OUT and GND so I was assuming that there is a reason for that and didn't connect the -IN and -OUT on the same GND. I had read some posts about possible impacts of sharing GND if you have any noisy components but I don't know if this applies here. I'm driving a solenoid and the relays so not sure if these could cause any issue with GND.

    I can connect the two grounds and see if it makes any difference.
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    Can you determine if the grounds are connected internally already? Maybe the datasheet for the converter has words to the effect that the output is isolated from the input.
     
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