How can I make this 600w Safer?
- Thread starter kenkoh
- Start date
From what I can see it works pretty nice, good job
NB: Ihold (in the datasheet) is not the current still passing after the PTC has tripped (what I assume you posted), it's the current the PTC will reliably hold without tripping. I.E. the PTC will trip with a current between the hold current and the trip current (depending mainly on the ambient temperature and tolerances).
It's guaranteed to not trip below the hold current and it is guaranteed to trip over the trip current.
The gap might seem large between the two but as you can see in the datasheet the hold current is given for a pretty high temp and the trip current for a pretty low temp, so at 20 °C ambient those values are a lot closer to each other.
Long story short when you power up an inverter for the first time (if it has been unplugged on the DC side) there's a massive current surge because of the capacitors inside of it. The precharge circuit limits this current to a safe level.
nosys70
Solar Enthusiast
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when you connect a battery to a device that has big capacitors, there are chances the capacitors are empty.
so the big spark you get when connecting battery cable to device. In fact this could happens with any device that has a big load (low resistance) or a big coil (motors). This will likely happens with battery because they are able to provide a huge amount of instant current. If you do that with AC, you would probably blow the fuse before.
This is not good, because you got some big current inrush (and usually a big noisy spark) , not good if you get some "smart" devices in the path , like a bms or other device with transistors.
So, to avoid that, you pre-charge the device (could be an inverter) by connecting the battery directly, so you avoid the possible surge through the bms. When capacitors are charged, further connections should be less hard.
Since bypassing the BMS requires modifying the wiring, the use of a resistor to slowly load the device is a nice feature.
so the big spark you get when connecting battery cable to device. In fact this could happens with any device that has a big load (low resistance) or a big coil (motors). This will likely happens with battery because they are able to provide a huge amount of instant current. If you do that with AC, you would probably blow the fuse before.
This is not good, because you got some big current inrush (and usually a big noisy spark) , not good if you get some "smart" devices in the path , like a bms or other device with transistors.
So, to avoid that, you pre-charge the device (could be an inverter) by connecting the battery directly, so you avoid the possible surge through the bms. When capacitors are charged, further connections should be less hard.
Since bypassing the BMS requires modifying the wiring, the use of a resistor to slowly load the device is a nice feature.
Last edited:
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
The PTC gets hot to the touch, but that is to be expected.
I'll play with it some and see if I can figure something out.
@BiduleOhm Yes, I noticed that too. I set up the circuit and actually measured the current before and after the trip. You are correct that it seems like it should not be holding at that low of a current. What may be happening is that since the voltage is less than Vmax, the current is also lower, but the power is still enough to keep at a high enough temp to stay triggered.From what I can see it works pretty nice, good job
NB: Ihold (in the datasheet) is not the current still passing after the PTC has tripped (what I assume you posted), it's the current the PTC will reliably hold without tripping. I.E. the PTC will trip with a current between the hold current and the trip current (depending mainly on the ambient temperature and tolerances).
It's guaranteed to not trip below the hold current and it is guaranteed to trip over the trip current.
The gap might seem large between the two but as you can see in the datasheet the hold current is given for a pretty high temp and the trip current for a pretty low temp, so at 20 °C ambient those values are a lot closer to each other.
Long story short when you power up an inverter for the first time (if it has been unplugged on the DC side) there's a massive current surge because of the capacitors inside of it. The precharge circuit limits this current to a safe level.
I'll play with it some and see if I can figure something out.
That's perfectly normal.
That's normal too, once tripped if the current stays high enough that the temp is high enough too, the PTC will not reset and hold at the Ihold but far lower instead (i.e. stay tripped). That's called hysteresis and it's pretty big on PTCs.
That's why I want to use a very low resistance PTC (0.x Ohm), that should allow for a higher steady state current after tripping. But what you have is not a problem at all, it's just a design choice. The main goal is to precharge the inverter safely and then to not let the resistor burn itself in case the switch is left in the wrong position; you've done both so that's ok
PTCs can be tricky (I've a love/hate relationship with them...), they are often used to replace a fuse or a breaker but they are not the same at all, even if they can sort of do what a breaker/fuse does they are much more blurry components. If you find the breakers/fuses tolerances and variances annoying, wait to see what PTCs can do... ?
So, yeah, be happy your first test was successful even if you don't find it perfect.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
I actually like the fact that it is holding at a lower current than expected. Even at the lower current, it stays pretty hot. Even though it is designed for the heat and even though in normal usage it should never even trip..... I don't like hot electronics.
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