Driving 24v solenoid with boost.

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I've got an Orbit water valve. It appears to be a 24v device and needs to be driven bidirectionally. The goal is to drive it with 5V, or possibly with 3.7V, so I'll use a boost circuit. Here is the very rough circuit:

 

Now if I drive the circuit with a 24V power supply, instead of the MT3608, and omit the cap, and fire the solenoid for 20ms, everything works great. You can open and close the valve fine, and it gives a nice solid thunk when it does it. But, if I sub in the MT3608 (which generates 24V fine) and the cap for backup juice, it just doesn't get enough juice to fire the solenoid, even if I stretch out the pulse quite a bit. You an hear the valve twitch a little, but not enough juice to kick it over. Commercial Orbit timers do this off battery, and seem to only have a 2200 uF cap, so it must be possible. Any ideas on where I am going wrong? A 10000 uF cap would seem to have enough juice.

My Fluke says the peak current seems to be around 2.5A or so, but that peak is very very short lived.

 

Any ideas on where I am going wrong here, or possibly where I might go right? Note the solenoid needs to be driven in both directions.

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Please provide links to valve and power supply datasheets.

 

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To boost 3v to 24v, you'll need 8 or 12 times as much current from your low voltage supply as the valve needs.

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Those valves work on 24VAC not 24VDC. You will fry the solenoid...I have already done it. If you want to run them on DC then use 12VDC.. They solenoid will still get warm on 12VDC but it works.

As Torby has said...you need a high current low voltage source when stepping up vtage...nothing is free.

Jim

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There is no valve data sheet, it is a consumer product. One of these: 

 

https://www.amazon.com/Pack-5887...

 

It is, in fact, 24V DC, not AC. These are not the commercial sort that (for example) auto-close when power is shut off; they toggle between open and closed (and rely on water pressure for good seal) and only require a short pulse of power to do so (I can do it with a 20ms pulse from a 24v power supply, for example). This one is typically driven by something like this:

 

https://www.amazon.com/Orbit-589...

 

Which is battery powered. Clearly my boost circuit (https://www.amazon.ca/Robojax-MP...) can provide very little current, that is what the 10,000 uF cap is for, to provide the necessary surge. Also clearly 2AA batteries cannot provide a lot of current; teardown of a very similar timer (also orbit) showed a 2200 uF cap, almost certainly used for the same purpose.

 

I used this guys analysis for initial work, though his is slightly different.

 

https://rayshobby.net/wordpress/...

Last Edited: Thu. Jun 28, 2018 - 05:45 PM
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This is where you really need a 'scope to see what is actually happening.

 

Without knowing what is happening, you don't know what  needs fixing!

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To amplify a bit:

 

Boost converters are (approximately) constant power devices. That is, the input power is  the same as the output POWER (plus a bit). So, if it takes, as an example, 24V at 500mA (which is 12W), then that supply would require from a 5V source the same 12W. At 5V, 12W comes out to be 2.4 AMPS.

 

Yes, it may be for just a short time, but you still would have to deliver the power from your ultimate power source. Simply, there is no free lunch.

 

Now, the fact that the Orbit system works from AA cells suggests that the control solenoid inside the valve might not take so much power. And, the idea that it takes current in one direction to close it and the opposite to open it is no so far fetched. I have seen similar in valves used in greenhouses. However, you MAY charge a large value capacitor to 24V and discharge that into the solenoid (in the appropriate direction) to do the job.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:

 

Now, the fact that the Orbit system works from AA cells suggests that the control solenoid inside the valve might not take so much power. And, the idea that it takes current in one direction to close it and the opposite to open it is no so far fetched. I have seen similar in valves used in greenhouses. However, you MAY charge a large value capacitor to 24V and discharge that into the solenoid (in the appropriate direction) to do the job.

 

As shown in the schematic, I have a 10000 uF cap to supply surge power (charged to 24V). The 754410 H-driver provides the directionality. If I don't use the boost, it works fine. But with the boost, there is not enough juice in the 10,000 uF to kick the solenoid, yet the commercial device uses only 2200 uF,  so I'm trying to figure out where I'm going wrong.

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Perhaps the cap needs to be charged to a higher voltage then 24v, what is the voltage rating of the cap (2200uf) in the Orbit system?

 

Jim

 

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awneil wrote:

This is where you really need a 'scope to see what is actually happening.

 

 

I have a scope, though even a 1 ohm resistor in series with the solenoid keeps it from working with 24V, so I'll need to rummage up a smaller resistor if I want to measure current through the selenoid. 

 

I mean, I kinda know where I am going wrong ... not enough current. But I'm trying to figure out why 10,000 uF isn't giving the required kick; it can kick pretty hard. Maybe I'll have to visit the local thieves and get a smaller resistor and scope the current.

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ki0bk wrote:

Perhaps the cap needs to be charged to a higher voltage then 24v, what is the voltage rating of the cap (2200uf) in the Orbit system?

 

25V. Because they are battery powered, I'd guess they don't keep it charged; it takes a couple of seconds to open or close the valve, probably to charge the cap, but at least the cap sees little usage.

Last Edited: Thu. Jun 28, 2018 - 06:13 PM
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I'd just look at voltages first ...

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awneil wrote:

I'd just look at voltages first ...

Hmmm... that gives me an idea. I wonder if the +5V supply is sagging enough on the surge to make the 754410 cut out? I'll scope that out!

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Mr.Paul wrote:

...It is, in fact, 24V DC, not AC. ...

 

 

Not being conversant, I turned to Google.  First impression was "24VAC".  But then I found e.g.

https://tony-osp.dreamwidth.org/...

...Essentially the valve is operated by discharging 2200uF capacitor, charged to 24V. Positive pulse opens the valve, negative pulse closes the valve. Ray (in the article linked above) wanted to control this valve using Arduino MCU running off a battery, and he put together a special circuit for it with step-up voltage converter to get 24V while running on battery, H-bridge etc. ...

But perhaps Mr.'s valve isn't the same as the one in that discussion and linked blog/schematic.

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exactly.

 

look at all the voltages.

 

are they doing what you expect?

 

any that are not - investigate further.

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theusch wrote:
perhaps Mr.'s valve isn't the same

again - scope all the voltages to see what's happening in the unit that is working!

 

 

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awneil wrote:

exactly.

 

look at all the voltages.

 

are they doing what you expect?

 

any that are not - investigate further.

 

 

If they were doing what I expected, the valve would work, because I know what makes the valve work (24V for 20ms), and can easily make it work with a benchtop PS using this circuit. I perhaps have not make it clear enough: I can make this circuit work without the boost circuit. I know what that scopes out like. I know that with the boost, it does not do that. I'm trying to come up with some areas to investigate, hence the question. So, the stability of the 5V supply for the H-bridge is one area to investigate, I'll try it out tomorrow when I get another booster ... that particular unit on amazon dies if you look at it wrong (not just my experience, I should have read reviews...)

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It would be interesting to see the charging curve of that cheap 2 A boost converter charging 10,000 uF.

This module features the MT3608 2 Amp step up (boost) converter, which can take input voltages as low as 2V and step up the output to as high as 28V. The MT3608 features automatic shifting to pulse frequency modulation mode at light loads. It includes under-voltage lockout, current limiting, and thermal overload protection.

 

Last Edited: Thu. Jun 28, 2018 - 08:21 PM
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A previous thread on sprinkler valve control -

 

Need approach: 250mS Current dump into 4 ohm inductor

https://www.avrfreaks.net/forum/need-approach-250ms-current-dump-4-ohm-inductor?skey=sprinkler

 

Note - some urls were messed up in the September 2014 reincarnation, just make an obvious edit if a link fails.

 

Stan

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I wonder if the +5V supply is sagging enough on the surge to make the 754410 cut out? I'll scope that out

Is the battery pack also supplying the logic 5V? 

Use separate logic supplies for now & test that.

 

You can feed your logic through a schottky diode & to perhaps 100uF (or even 1000uF) cap to keep the logic flowing when the battery voltage dips/spike downward.  Of course if the batt sags for too long..son your logic will too. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

I wonder if the +5V supply is sagging enough on the surge to make the 754410 cut out? I'll scope that out

Is the battery pack also supplying the logic 5V? 

Well, worse; it is a USB power supply, which is nice current limited and will probably drop like a rock over a certain amount instead of fading off nicely.

 

Quote:

Use separate logic supplies for now & test that.

 

You can feed your logic through a schottky diode & to perhaps 100uF (or even 1000uF) cap to keep the logic flowing when the battery voltage dips/spike downward.  Of course if the batt sags for too long..son your logic will too. 

 

Yah, that is my first order of business when my new boost shows up hopefully tomorrow. Funny you should mention a 1000 uF cap .... the commercial version uses one (in addition to the 2200 uF) and I'm willing to bet for exactly this reason. Unfortunately I don't have one, I just go by FCC pics.

 

Thanks to all, I have some things to pursue. 

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Do you have caps on you chips power pins?

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It would also be helpful to see an O'scope pic of the 24V pulse provided by the step up circuit, to validate that it has enough energy to power the device for 20 mSec.

 

And an additional O'scope image of the 5V rail to the H-bridge chip, although simply using separate power supplies for testing would be a good idea.

 

JC

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Kuch wrote:
Do you have caps on you chips power pins?

You mean decoupling caps? Yes. It went badly without them :)

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DocJC wrote:

It would also be helpful to see an O'scope pic of the 24V pulse provided by the step up circuit, to validate that it has enough energy to power the device for 20 mSec.

 

I'm not going to deny the scope pic might tell something, but for sure it does not have enough energy, otherwise the device would operate. If I supply lots via bench PS, it does operate.

 

Quote:

And an additional O'scope image of the 5V rail to the H-bridge chip, although simply using separate power supplies for testing would be a good idea.

 

 

Aye.

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You should have looked at the voltages with your scope to see what was happening,  before you came here.  That would save a lot of speculation.

 

Well, worse; it is a USB power supply, which is nice current limited and will probably drop like a rock over a certain amount instead of fading off nicely.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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When you overload the USB,
Drop it like it’s hot
Drop it like it’s hot

Or something like that according to Snoop Dog

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Mr.Paul wrote:
 for sure (sic) it does not have enough energy,

No, that's just a hypothesis.

 

It's probably quite a good one, but still just a hypothesis.

 

You need to test your hypothesis.

 

Then you can say, "for sure".

 

 

 

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Dual Darlingtons?  Bleh....You can get rid of that crappy driver chip & use something decent that loses a ton less voltage.

 

Instead of losing 4000mV (That's 4V!!!) driving 1 amp, the other chip would lose about 70mV, typical---that's a 98% lower loss.

 

You can even get it on an eval board, ready to use for about $4.50

https://www.digikey.com/product-detail/en/EV-VNH7070AS/497-17633-ND/7691036?utm_campaign=buynow&WT.z_cid=ref_octopart_dkc_buynow&utm_medium=aggregator&curr=usd&site=us&utm_source=octopart

 

https://www.st.com/content/ccc/resource/technical/document/data_brief/group0/fb/4f/a6/33/6a/e0/46/94/DM00338335/files/DM00338335.pdf/jcr:content/translations/en.DM00338335.pdf

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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awneil wrote:

Mr.Paul wrote:
 for sure (sic) it does not have enough energy,

No, that's just a hypothesis.

 

It's probably quite a good one, but still just a hypothesis.

 

You need to test your hypothesis.

 

I have tested it. When hooked up to lots of input power, it works. When used with the boost, it does not. Nothing else changes between these two. Both use the same voltage (at idle....) Now energy is both voltage, current *and* time, and I might have missed the time dropping due other factors, like power sag on the low voltage rail. I hope it is that, because that is easy to fix :) 

 

avrcandies wrote:

You should have looked at the voltages with your scope to see what was happening,  before you came here.  That would save a lot of speculation.

 

 

Well, I came here specifically for the speculation :) Alas, everyone speculated about what kind of solenoid it was (I already knew), what it took to drive it (I already knew).  That's fair, the person with the problem (me) has often made some bad assumptions, but in this case I tested that part pretty throughly. The discussion still gave me some ideas. And yah, I could have scoped everything, but I'm not fond of random troubleshooting, and it didn't occur to me to scope the low voltage rail.... clearly I'm rusty. 

 

That driver looks pretty interesting, but this is a personal project and it is make-it-with-what-you-have as much as is sensible (and the 754410 does get the job done, if not as efficiently as is possible). When I run out of of the current H-bridge, I'm definitely going to look at trying those instead, in part because it has a feature I really would love to have: 3V compatible inputs, so at least on the low voltage side I could do with only a 3.3V rail in some instances, which would be very nice. Thanks for the pointer.

Last Edited: Fri. Jun 29, 2018 - 12:55 PM
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Mr.Paul wrote:
I have tested it.

Actually not

 

When hooked up to lots of input power, it works. When used with the boost, it does not.

You are assuming that it is because the boost lacks power - but it could be some other artefact.

 

scoping the voltage would help to confirm whether or not the assumption is correct.

 

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awneil wrote:

Mr.Paul wrote:
I have tested it.

Actually not

 

When hooked up to lots of input power, it works. When used with the boost, it does not.

You are assuming that it is because the boost lacks power - but it could be some other artefact.

 

No, I'm not assuming that at all. I know there is not enough energy is being delivered to the solenoid (that is not assumption, that is fact), and I know that lots of power in does work, and when I switch to a boost+large cap, it doesn't. That is what I know. The actual question originally was, and is, why. I'm not assuming the booster is at fault, but I know introducing it causes the problem; I know full well there are artefacts, this is why I was asking in the first place. The question never was why doesn't a booster supply enough power. It was why doesn't this circuit work. Note the circuit includes a large cap to provide surge energy, because clearly the peak current by the booster is insufficient, by its own spec sheet, and its input power (the USB power supply) doesn't have enough even if the booster could handle it.

 

However, you assumed I assumed you assumed is way more debate then electronics, so I'll just try some new stuff this aft (separate rails for H-bridge and booster) and get back on its success! One unfortunate is that I switched boosters (as that one fails like crazy) and the new one might work out of the box, which would be good for success but bad for learning.

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Mr.Paul wrote:
I know there is not enough energy is being delivered to the solenoid

but how do you know that?

 

 The actual question originally was, and is, why

could be because the above assumption is wrong

 

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Since voltage, capacitance, current and time are all knowns then it is trivial to check that the capacitor alone stores enough energy to move the solenoid.

You could even try charging the capacitor from your bench psu and then disconnecting the psu before energising the solenoid.

Another random thought.... Put a diode between the booster and the capacitor. Or even a resistor sized to charge the capacitor but not to supply any load current.

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Testing involves more than just saying "its working" or "its not working", especially when that's plain to see.  This includes using the equipment you have at hand to measure what is going on when things are off in the ditch, as well as when everything appears to be perfectly fine.  Actual measurements are always the order of business.  Rather than claim you already tested it out, its more accurate to say you arrived, looking for advice.

 

Well, I came here specifically for the speculation

 That's fair, the person with the problem (me) has often made some bad assumptions, but in this case I tested that part pretty throughly

It doesn't sound like many thorough tests have been run (waveforms?, data?)...you are at the beginning, not the end. We are just trying to help you get there!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Are you just using the mt3608 with just a cap??? Do you have the mt3608 connected up as per the datasheet??

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Can you measure the DC resistance of the solenoid, since you are having trouble measuring the current?

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avrcandies wrote:

Testing involves more than just saying "its working" or "its not working", especially when that's plain to see.  This includes using the equipment you have at hand to measure what is going on when things are off in the ditch, as well as when everything appears to be perfectly fine.  Actual measurements are always the order of business.  Rather than claim you already tested it out, its more accurate to say you arrived, looking for advice.

 

I did actual measurements, including waveforms. I built up everything and got it all fully working (including firmware for the tiny, and a test routine that checks operation, etc) with a bench PS and that involved a fair bit of mucking around to get the right params. Then I introduced the boost circuit. I didn't realize this was a test where I had to prove I tested. I asked for some ideas on what might be wrong, that is all. I don't want to assemble a 20 page white paper proving my methodology. I do this for fun!

 

Quote:

It doesn't sound like many thorough tests have been run (waveforms?, data?)...you are at the beginning, not the end. We are just trying to help you get there!

 

I didn't post them. I felt asserting that the circuit worked with a power supply of the same voltage would be sufficient.

 

Kuch wrote:
Are you just using the mt3608 with just a cap??? Do you have the mt3608 connected up as per the datasheet??

 

No, it is the unit linked to up above. It has the requisite inductor, etc. The booster works (well, worked...), the circuit it is in does not. Note it does move the solenoid some (in circuit), just not enough.

 

El Tangas wrote:

Can you measure the DC resistance of the solenoid, since you are having trouble measuring the current?

 

4.3 ohm Tried two different peak-reading multi-meters (Fluke 87V and Mooshimeter (sampling at 8kHz)), both sometimes caught about ~2.7A surge on actuation, but clearly the surge was very very short as they only catch that once in a while. When the solenoid is wet it takes more to move it, that could affect some of the readings as well.

 

Added: The surge is so short the bench top PS current limit (set at around 500mA) doesn't catch it. Mind you, it is a nice-ish one, but not top of the line or anything.

 

 

Last Edited: Fri. Jun 29, 2018 - 06:21 PM
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4.3 ohm at 24V, that's 5.6A current and 134W of power! I think this is a bit too much, isn't it? Must be a really big solenoid. I doubt the MT3608 can handle so much power. Even the bench power supply you are using, can it supply so much?

You must be having big voltage drops on the 24V rail.

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Mr.Paul wrote:
I did actual measurements, including waveforms.

But we cannot see any of that - can we?

 

So we cannot tell whether your conclusions are well-founded or not.

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awneil wrote:

So we cannot tell whether your conclusions are well-founded or not.

 

No. You could proceed in good faith, though, unless an error in my methodology was the only reasonable answer (and it is not the only reasonable answer). I know I'm a stranger here, but benefit of a doubt would be OK. 

 

El Tangas wrote:

4.3 ohm at 24V, that's 5.6A current and 134W of power! I think this is a bit too much, isn't it? Must be a really big solenoid. I doubt the MT3608 can handle so much power. Even the bench power supply you are using, can it supply so much?

You must be having big voltage drops on the 24V rail.

 

I'm sure the MT3608 can't, that is why the big cap is there. Mostly the boost is there to charge the cap. The bench top PS has no problem (without the cap; the PS is rated for 3A continuous at that voltage), but I've not looked at how much it drops. Remember, it is only for 20ms; the bench top probably has good sized output caps. I can dial down the voltage some and it works when there is no water pressure, but once you turn on the pressure it really likes a solid kick; i've not explored how low it could go. As awneil points out, the 754410 takes a good chunk out of that.

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El Tangas wrote:

You must be having big voltage drops on the 24V rail.

 

As a matter of interest. Supply set at 24V or so, current limit set to 1A. Pulse width 20ms. Solenoid activation. CH1 is the 24V rail. This is good activation of the solenoid. The PS recovery is interesting.

 

 

 

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Excellent.

That's what I wanted to see.

 

Now, in my mind, it would be good to see the same with the booster circuit.

 

And, if two channels are available, see the 5V rail simultaneously.

 

Also, Have you tested yet with the 5 V rail powered totally separately?

 

Obviously, things to consider:

H-bridge eats up to much voltage, and the solenoid is under driven, voltage wise.

5V rail droops and/or has poor regulation, which then prematurely shuts off the drive for the solenoid.

The cap doesn't have the value you think it has, (wide tolerance, damaged, mis-labeled, etc.).

The booster chip acts strangely when there is a sudden load on the output, (I've not looked at its output circuitry...).

 

I know you wish to use parts available, but I'd consider ordering a "better" part while you continue trouble shooting...

 

You might, for fun, wire up the booster circuit and energy storage cap to the solenoid with a simple NFet as a low side switch, and see if that works, (removing the H-bridge from the circuit, for the time being).

 

You would have to "reset" the solenoid using another configuration.

 

JC

 

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DocJC wrote:
it would be good to see the same with the booster circuit.

Essential, I'd say.

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Ok, I did some calculations, if the current is 1A at 23V for 20ms, this means about 0.5J of energy are needed to activate the solenoid. This works out to the 10mF capacitor dropping from 24V to 22V so it should be no problem, it must be something else.

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Well, good news / bad news.

The good news is that the new booster will actually drive it (with a cap). The new one is XL6009 base and is specifically this one:

 

https://www.amazon.ca/gp/product...

 

It has much nicer components then the previous one, and is cheaper to boot (at least if you buy 4 of them...). 

The bad news is now I'll never be sure what the problem was with the previous one. But, WTH, lets see some scope output since I said I would.

 

For your amusement: Booster set to 24V, driven off 5V shared rail (which is driven by USB PS). No extra caps. I don't expect this to work (not enough juice in that 5V supply) and it didn't. 20 ms pulse Here are the 5V rails and the 24V rail:

 

 

As you can see, the 24V rail crashes and burns. The 5V rail is hurting.

 

Here, I add a 4700 uF cap to the 24V rail. For the record, it is a Panasonic, sourced from Digikey, so probably not fake. This setup actually works, seems to solid turn on the valve, even with water pressure.

 

 

Note the time scale is different then the above one.

 

You can see that the cap discharges for ~22 ms (cursor probably isn't accurate), then starts charging again. The 5V rail suffers for this period. The other devices on the rail (2 micro controllers) are ok with a volt here or there, but in theory the low of the 5V rail is outside 754410 spec, but it seems to be OK. Once it charges up again, we are OK.

 

It would be interesting to completely disconnect the booster just before firing to stop the 5V sag. I think there is enough juice in the cap, or if I switch the 10,000 uF one there will be. Would require another FET or something. That would also be good if I wanted to run it off battery so I could charge the cap only when needed. Might be interesting to limit the charge rate of the cap but let it discharge full tilt; not sure the best way to go about that. Another 'feature' that would be good would be to have enough stored juice to run the 5V rail and the 24V rail long enough to turn off the valve on power failure; one of the big disadvantages of these types of valves is that they latch open and if you lose power, they don't close.... my micro closes it on power up, but it would be nice to detect power loss and kill the valve on the way down. Tricky though, the direct controller is outta pins :) And of course that 5V sag looks like power loss....

 

Anyway, many thanks.

Last Edited: Sat. Jun 30, 2018 - 02:01 AM
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Glad you now have it working.

 

You might put a Schottky diode, (with a low forward voltage drop), in series with your 5V supply to the 5V regulator, (or to the micro, H-bridge chip, etc.).

After the diode, which is forward biased, you put a cap, 100 uF, 22 uF, whatever...

Now, when the 5V rail sags, the diode prevents the cap from back feeding the source.

The cap also powers the uC at the original 5V, (or a bit less, really, because of the Vforward drop on the diode).

 

JC

 

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Ah, good idea. There will probably be a Pi ZeroW on the bus eventually (well, still up in the air vs XBee's.. I like my XBee, but one of them is more expensive then an entire Pi ZeroW, and both get the job done for me)., and it is pretty picky about power. Could drive it with something a bit "raw", schottky + cap + reg for the sensitive bits, raw directly to boost for the loose bits. Hmm. Or I could keep it simple with the XBee, (sure wish that 75441 was 3.3V friendly.... yah, I do need a better bridge) Definitely rusty :) 

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which is driven by USB PS

 

Do you mean a PC?  ...that is usually pretty current limited (500mA)...A powered hub  or wall wart can be much  beefier

 

The bottom line is that a usb does not have (or barely has) the power to supply the valve, so the most you can do is set up some cap storage & only supply for a transient time (which is all the valve needs).  You can see from your scope the voltages don't hold steady, so the caps are on the small side. If the scope shows the 24V rail, there is also additional 4V drop from the driver chip---that's quite a drive loss (& probably helps save the 5V somewhat).

 

You'll be somewhat better off adding the caps to the 24V & keeping the time short so as to not deplete them.  Stored energy is 1/2 CV^2   & 24^2 is 23 times more than 5^2.

 

I mentioned adding a diode way back in  #20---no more dilly dally, just put it in & at least keep your logic going.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. Jun 30, 2018 - 07:14 AM
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Mr.Paul wrote:
The new one is XL6009 base (...)

 

They are nice, aren't they? Previously these cheap converters used the LM2596 (step-down) or the LM2577 (step-up) but more recently the Chinese developed their own chip, XL6009 which can do both.

Anyway, I'm glad your problem is progressing to a solution.

 

So the 4700uF cap dropped from 24V to 15V, let's see how much energy was used:

start: 1/2 * 0.0047F * 24V * 24V = 1.35J

end: 1/2 * 0.0047F * 15V * 15V = 0.53J

 

So, 0.82J were used over 0.02 seconds, about 41W average power. Since the average voltage is 19.5V (actually it should probably be some kind of RMS, but whatever), the average current is 2.1A which is in the ballpark of the surges you were measuring.

 

If the 10000uF cap was used instead, assuming the same energy is needed:

start: 1/2 * 0.01F * 24V * 24V = 2.88J

end: 1/2 * 0.01 * x * x = 2.88 - 0.82

 

This works out to x = 20.3V so the drop will be a lot less.

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assuming the same energy is needed

Well, perhaps, but the energy supplied is related to the voltage strength (so keeping the voltage high means higher power & thus faster cap decay).  Since the voltage is changing a lot, probably so is the power.

At least your calcs shows more capacitance take us in that direction.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!