Need Help Making This Circuit Idea Work With As Minimum Components as Possible

[rs14smith]

1. General Information:
I have a **broken link removed** that contains a DC motor (could not find much info on the type of motor it is) and 3x 12v batteries hooked up in series to produce 36v.

I took apart the handle bar that starts/stop the motor and on the switch it stated 25A max, so I assume the current in this lawn mower does not exceed 25A.

To charge this lawn mower you use an AC adapter which was included that converts the AC to DC. Additionally, you should not charge the lawn mower with the adapter while the lawn mower is running based on the instructions manual.

2. Additional Information:
I have a separate DC 24v power system (2x 12v batteries in series), and a basic 300W AC to DC inverter. Note that this system is not connected to anything on the lawn mower.

3. What I Want To Accomplish Automatically:
I will have the lawn mower's AC adapter plugged into the inverter, and the inverter will be hooked up to the 24v system, and I will thus be charging the lawn mower when the mower is "not" running.

When I start the lawn mower, some how "automatically" the inverter will be disconnected from the 24v system, thus the AC adapter will no longer be attempting to charge to the lawn mower.

When the lawn mower is turned back off, the inverter is some how automatically connected, and the lawn mower AC adapter thus can start charging the lawn mower again.

*I would like to implement this concept with as few components as possible.
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Hopefully this is enough info, but if not, let me know and I'll do some more digging for you.

I have drawn up a small sketch that I think may be suitable for this which involves using relays, however, I'd like to hear back from some of you all first.

Thanks

 

[ronv]

Your relay method is probably as good as any. Pick the relay with the same switch that starts the mower. When the relay is energized it can break the circuit to the inverter thru it's contacts.

 

[rs14smith]

Your relay method is probably as good as any. Pick the relay with the same switch that starts the mower. When the relay is energized it can break the circuit to the inverter thru it's contacts.

Well the switch that starts the motor is just a basic manual switch or I like to think of it as a push button. You have to hold the handle down to keep the mower running. If you let go of the handle, the mower stops running.

The problem for me with relays are, finding the correct relay for this application. Going by your solution, I would need one that could accept 36v input and I'm not sure what I'd use to limit the current to it...maybe resistors...am I going in the right direction here?

 

[carbonzit]

The problem for me with relays are, finding the correct relay for this application. Going by your solution, I would need one that could accept 36v input and I'm not sure what I'd use to limit the current to it...maybe resistors...am I going in the right direction here?

I'll eliminate one little part of your problem. There's no need to limit the current to a relay (I ASS-U-ME you're talking here about the relay coil, not the contacts). Just let it draw whatever it wants to draw.

I agree that a 36-volt relay may not be easy to find. You might be able to do it in two stages, say using a solid-state relay for the 36-volt side, then switching a more readily available relay at a different voltage to actually control the load.

 

[rs14smith]

I agree that a 36-volt relay may not be easy to find. You might be able to do it in two stages, say using a solid-state relay for the 36-volt side, then switching a more readily available relay at a different voltage to actually control the load.

Hmm so like a SSR relay that controls a 24v automotive relay for example?

 

[rs14smith]

I found this from a Google search however, I'm not quite sure if it functions the same as a standard relay, or if it's triggered off of 36v, or is it stating it allows 36v to pass through it's output side as well as up to 50A: https://www.monsterscooterparts.com...roductid=36-volt-dc-contactor&channelid=FROOG

Here's another which is cheaper, but not sure how many Amps it can handle, and it also states it's a solenoid relay which again I'm not sure if it works the same: https://www.amazon.com/Heavy-Terminal-Continuous-Solenoid-Relay/dp/B004VEPNZ8

 

[crutschow]

You are just disconnecting the charger, correct? Thus you just have to switch the charging current? How much is that?

The amazon solenoid relay looks like it would handle 50A. Are you concerned about the power required to energize the coil continuously while it is charging?

A solenoid relay performs the same function as any other relay. It just is constructed with a solenoid plunger that directly carries the contacts rather than the typical pivoted contacts of most relays.

 

[rs14smith]

The amazon solenoid relay looks like it would handle 50A. Are you concerned about the power required to energize the coil continuously while it is charging?

I suppose, I mean, once the mower starts, part of the power from the 36v battery should trigger the relay and start the mower. When it triggers the 36v relay on the input side, that some how should disable the inverter/AC charger.

The concept seems simple, its just finding all the right parts to do it.

I attached a quick sketch I made for this circuit, hopefully it gives a slight idea of what I originally had in mind. Since I have a microcontroller I was going to use that, but if it's easier to do without it, I'll use another approach.

 

[carbonzit]

You definitely do not need a microprocessor to accomplish this. That would be like using dynamite to swat a fly.

 

[rs14smith]

You definitely do not need a microprocessor to accomplish this. That would be like using dynamite to swat a fly.

HAHA! I didn't think I did after some here suggested some ideas but I thought I'd post it anyway. Do you have a basic circuit in mind you can share?

 

[carbonzit]

I guess I'm a little confused about the configuration of this thing.

You say you want the charging system disconnected from the mower when it's not being used, which makes sense. But don't you have to physically disconnect the charger anyhow when you use the mower? Somehow I can't see you dragging the cord around with you as you mow your grass.

It sounds like what you want is just a changeover switch. In one position, the mower operates normally, and the charger circuit is disconnected. In the other position, the mower motor is disconnected and the charger is connected to the batteries.

Actually, I think all you need is this:

**broken link removed**

All you really need to do is to disconnect the motor when you're charging the batteries, correct? You have to disconnect the charger anyhow to use the mower, as stated above.

Unless I'm missing something here ...

After thinking about this a little more, you could also use this circuit in reverse, if you want to leave the charger permanently connected: just connect the left side (labeled "to charger") to the mower motor, and wire the relay contacts to the charger to remove it from the circuit when the motor is on.

 

[rs14smith]

Actually, I think all you need is this:

**broken link removed**

Is this a particular device or is this something I would have to build?

All you really need to do is to disconnect the motor when you're charging the batteries, correct? You have to disconnect the charger anyhow to use the mower, as stated above.

Unless I'm missing something here ...

I don't have to, but the manual said it's recommended. I'm guessing it has something to do with the load of the motor that may damage the charger...who knows, I just don't want to take a chance lol. The way it was originally made was, the handle had a special port that allowed for a "key" to enter it, or the charger to charge the mower. If the key is in, you cannot put the charger in and thus you can run the lawn mower. If you take the "key" out, you can plug the charger in that same port. So basically they made it so that you cannot have both going at the same time, so I'm trying to keep that functionally.

After thinking about this a little more, you could also use this circuit in reverse, if you want to leave the charger permanently connected: just connect the left side (labeled "to charger") to the mower motor, and wire the relay contacts to the charger to remove it from the circuit when the motor is on.

Which circuit are you referring to here? The one you have above or the rough sketch I made, as I don't see how that would work if referring to my sketch.

 

[carbonzit]

Obviously you'd have to put this together; you can't go in a store and buy this. But it's so simple: just a single relay wired to the mower parts.

So your charger can stay on the mower while you use it? I guess you did say it was operated from batteries and an inverter. But isn't that a lot of stuff to drag around your lawn while you mow it?

I'm referring to my relay circuit. That's really all you need here; you need to disconnect either the motor or the charger, depending on how you rig this.

 

[rs14smith]

So your charger can stay on the mower while you use it?

No, the circuit or mower should not even know about a charger when the mower starts. So the charger needs to be disconnected...basically one wire on the charger needs to be disabled/disconnected...break the charger's circuit I thought by just disconnecting one of the wires on the inverter would work the same...either way the charger needs to be off once that mower starts.

I guess you did say it was operated from batteries and an inverter. But isn't that a lot of stuff to drag around your lawn while you mow it?

Well you don't know the overall reason I'm doing this which isn't important, I just need this functionality to happen hehe

I'm referring to my relay circuit. That's really all you need here; you need to disconnect either the motor or the charger, depending on how you rig this.

Yeah I like your simplicity behind it, it's just that last part in your sentence I'm not too sure how to do. I know for sure, well I think I know for sure I need that 36v relay off Amazon I linked to earlier which stated in the image it can handle 36A. After that point I'm still a bit lost on where to go next

 

[rs14smith]

I kind of think it will be similar if I can find a 36v DC or greater manual on-off-on toggle switch that can handle 25A or more...? I've been searching the web for the past 30min and having no luck finding DC switches that have that feature. Most are made for AC that i've ran into. Does anyone else know of a place that sells them?

Only reason I'm kinda veering off the automatic idea is because of deadline concerns with this project. But if I have time I can try to come back and get this relay idea to work because as of now I'm still stuck with both finding all the parts for the circuit above, and two I'm not 100% understanding how it will successfully turn off the motor/adapter.

 

[carbonzit]

There's nothing special about a switch that makes it work on AC and not DC (or vice versa); the critical thing is how much current it can take with either voltage. If you find a switch that's rated at significantly higher than your current (25A) on AC, you can safely use it on your mower.

 

[rs14smith]

There's nothing special about a switch that makes it work on AC and not DC (or vice versa); the critical thing is how much current it can take with either voltage. If you find a switch that's rated at significantly higher than your current (25A) on AC, you can safely use it on your mower.

I thought the same myself as I've done it in the past, but since I was dealing with more current this time around I did a little digging and read that if you use an AC switch for a DC circuit, and you are pumping a lot of amps to your load, when you turn that switch off, more damage is done to the contacts of that switch because the voltage for DC is constant. So in other words, it would take longer for the DC voltage to get back to zero than it would for an AC circuit which of course has a sin wave. So what would result is a shorter lifespan if used with a DC circuit that uses a lot of amps.

However, I kept reading and saw that, if you do want to go with a AC switch and use it for a DC circuit, the voltage for the DC should only be 25% of what that AC switch is rated for. So in other words, if you find an AC switch that is rated for 100VAC, you should be okay using it with a 25v DC circuit.

This is stuff I read from a Yahoo Answers post someone made about the same question I had...but from what you know, does this sound somewhat right?
If so, basically since the largest voltage in this circuit is 36v, I need to find a AC switch that is rated for at least 140VAC or greater?

 

[rs14smith]

By the way, do you think this switch would work even though it's rated for 30VDC: **broken link removed**

I'm having the hardest time finding 36VDC or > on-off-on switches and even AC switches that allow 25A or more...this project is starting to be a pain in the butt big time!!! Never will work with 36v again!!

 

[carbonzit]

You should try using Digikey's parametric search facility. You can specify parameters like voltage and current to narrow your search.

So you might look at this switch. A little expensive ($38.75), but it'll handle 50A @ 125 VAC, so it'll definitely work for your mower.

 

[rs14smith]

You should try using Digikey's parametric search facility. You can specify parameters like voltage and current to narrow your search.

So you might look at this switch. A little expensive ($38.75), but it'll handle 50A @ 125 VAC, so it'll definitely work for your mower.

Yeah I used that search facility but only got back that switch I listed above. Note* I was looking for On-Off-On switches as I thought that Off feature would be nice to have too even though it's not needed, I'd be able to disable both if I ever needed to...but it's not required.

Regarding your 125VAC switch, using that 25% rule, that would be 31.3v at 50A, so about 53.3v at 40A...so basically this should work fine. From what I've been seeing online, they reduce the Amps by 10, and add 12v, and that tells you if you can use it or not for your DC system. Just wondering if I'm doing this right just for future reference?