[dtbaker]

I got it all together, but am popping a fuse when I turn it on.... How many amps SHOULD a typical ceramic core heater pull at 96v?

Is there a way I can test with a separate 12v battery or multimeter with the heater in place to rule out some internal short? I have to pull it all apart if I can avoid it!

should the heater leads show 'some' resistance, or 'no' resistance? I am wondering what is typical for these things?

[Woodsmith]

Using V=IR and P=VI you should be able to work out the resistance of the element based on its rated power and voltage.

You can then change the voltage to your pack voltage and knowing the resistance be able to work out the new power output and the current.

An example based on my workings:
For a given 1000W element at 240V
P=VI
I=P/V

I=1000/240
I=4.17amps

V=IR
R=V/I
R=240/4.17
R=57.55ohms

If we use 144V
V=IR
I=V/R
I=144/57.55
I=2.5amps

P=VI
P=144x2.5
P=360Watts

[dtbaker]

Originally Posted by Woodsmith

Using V=IR and P=VI you should be able to work out the resistance of the element based on its rated power and voltage.

ok, so following these formulas:
i=1500/120v = 12.5 amps at rated v
r=120/12.5 = 9.6 ohm

i=96/9.6 = 10a at my pack voltage
so.... 20a fuse SHOULD be plenty, right?!

when I look at my multi-meter, the ohm scale is rx1kohm, so I dunno if I can see just 10 ohm versus 0 (short). Do I need a different meter? Is there a different way I can test the unit in place without smoking anything?

Sounds funny, but even after building an EV, I really am not prepared to de-bug anything electronic. The rest of the car I just followed schematics, and everything worked!

d

[Woodsmith]

You could measure the voltage drop across it.

It's been a while since I have thought about stuff like this so maybe I am going to be wrong but if you get a similar sized resistor, say 20 ohms and put it in series with a 12 volt supply you should be able to measure the voltage across the heater and across the resistor giving about 4 volts and 8 volts respectively.

The voltage will be proprtional to the resisitances so with a degree of accuracy you can determine the unknown resistance.


However, it is a ceramic heater and they are supposed to be self regulating, the hotter they get the higher the resistance so they don't get too hot.

Perhaps when cold they have a very low resistance and won't reach a measurable resistance until it is hot. Once at full temperature you might then find it is drawing 9.6 amps but until then it might draw much more due to a low resistance.

[dtbaker]

the problem is that both times I run the 96v thru, it popped a 20a fuse. So my question was first what the theoretical draw is, and it looks like the 20a fuse should be fine.... unless the issue is that the amps are much higher in the beginning when the element is cold?

It sounds like I should NOT hook up a 12v battery, because that would try to pull LOTS more amps, right?

If there is an internal short, what would happen if I hook up my full 96v pack bypassing the relay and fuse? Would the smallest wire melt (basically like a fatter fuse?)

What is a way to figure out if the core has a short, or why it is popping the 20a fuse?

would it be relatively safe to bypass the relay and fuses and manually close the circuit with a small gauge (like 22g) wire as a fat fuse to see what happens?

[Woodsmith]

Heat up the element in a hot oven or with a hot air gun and then measure the resistance?

I'm not really sure of the answer in this case, my electrical training is so old we didn't have materials that responded in the way ceramic heaters do.

I've just been watching Jack Rickard's video on fitting a heater to his Mini EV. It may be worth a look as he dismisses the ceramic heater as not a good idea.

[tomofreno]

It sounds like I should NOT hook up a 12v battery, because that would try to pull LOTS more amps, right?

If there is an internal short, what would happen if I hook up my full 96v pack bypassing the relay and fuse? Would the smallest wire melt (basically like a fatter fuse?)

What is a way to figure out if the core has a short, or why it is popping the 20a fuse?

would it be relatively safe to bypass the relay and fuses and manually close the circuit with a small gauge (like 22g) wire as a fat fuse to see what happens?

The heater is a given resistance, so should draw LESS current with 12V applied: I = V/R. You should be able to measure the heater resistance by touching your meter probes to the output terminals of the P&B relay when 12V is not applied to the relay coil (relay open). I would not try to bypass the fuse with a wire connected to the pack. I don't think a photo will show much detail of the connections but I'll take one. Ensure the diodes are connected as shown in the schematic that came with the kit, with the arrow in the correct direction. If the large one is reversed, it will short the output terminals together. The arrow and bar on the diode should be oriented as: neg terminal >| pos terminal. Also ensure the negative terminal is connected to the negative terminals of the heater, and positive to positive terminals. No part of the large diode should be contacting any conductive surface other than the output terminals of the relay.

Tom