need help finding a resistor

I bought a teco VFD and managed to fry the main surge resistor. I am having a tough time finding a new identical one. It is a black ceramic resistor with 20SP and 012 on it with two symbols. Any help would be greatly appreciated. Thanks, Ken

It looks just like the one pictured, but with the bottom numbers being 012.

Check out this data sheet. Looks like 20mm 12ohm NTC thermistor. Most likely used for inrush current limit.

www.uei.com.tw/psp.pdf

Try Radio Shack. Might have to order it.

Looks like an MOV. Mouser and Digikey have them, cheap.

Jeremy said:

Looks like an MOV. Mouser and Digikey have them, cheap.

Click to expand...

No, it's an NTC thermistor, 12 ohms @25°C, 5A steady state, 132 seconds TC, 23 mW/°C, 4 watts, and a temperature range of -40 to +200.

UEI is a Taiwanese company and I'm not sure who would have them in stock. I've had a quick look for possible substitutes and the closest I've found so far is 12 ohms and 4A.

One that is close will work just fine I am sure. I just couldn't find the exact one. I appreciate all the fast responses.

If the load is too high for one 4A device, you can adhere two of them tightly together and use them in parallel. The sensitive issue is Beta... resistance change over temperature... the NTC characteristic needs to be maintained, by bonding them tightly, they'll tend to share temperature close enough to yield a fairly close response. Last time I did something like this was two decades ago in some piece of transmitting equipment, I used a cementous 2-part epoxy, and wrapped them tightly in foil, and it worked fairly well... it's still in operation, 290 in the air, atop a grain tower...

By the way, that failure is likely the result of too aggressively attempting to brake a load without an external braking resistor, so if you replace it, add the resistor or set it for coast to stop so this doesn't happen again.

It happened because I had a dumb moment and hit it with 220

I will check out radio shack. I looked all over the internet for a data sheet and all I came up with was color break down sheets. thanks for the link.

These guys have some UEI resistors, but not the exact one. I emailed and asked if they could get me the one I need.

I still cannot find an exact replacement.

barrakudaman said:

I still cannot find an exact replacement.

Click to expand...

I did not look up, but if previous is correct: "No, it's an NTC thermistor, 12 ohms @25°C, 5A steady state, 132 seconds TC, 23 mW/°C, 4 watts, and a temperature range of -40 to +200."

then just buy it.

digikey, $ 1.90, in stock!

MF72-012D20 Cantherm | 317-1230-ND | DigiKey

barrakusuck

OK....

Yes, it is an inrush current limiter. NTC, its resistance goes down with temperature.

The very first question is whether this is the only part in-circuit, or if there is a relay that shorts it out after the voltage comes up to a set threshold. That is very common, and makes much more sense, but may not be how the designers did it. I would use the relay, myself.....

These parts are usually rated for capacitance.... the capacitance that can be connected at a given voltage, because that is really a measure of the energy the part will absorb each time the unit has power applied. Go too low on energy capability and the part will not last.

In reality, the part is fairly non-critical, they are usually fairly loose on their resistances, etc to begin with, so a similar part should work, so long as you don't have to maintain a warranty, etc (a non-stock part would void the warranty, obviously).

At Digikey, a 14 ohm SL22 14007 is rated for 4800 uF at 120V and 1200 uF at 240V. If the part is bypassed with a relay, it will almost certainly do the job, assuming your capacitance on the bus is not more than the rating at the particular voltage, which I assume is 120V, since 220V was too much.

http://www.digikey.com/product-search/en?x=13&y=18&lang=en&site=us&keywords=SL22+14007

If the part is not bypassed by a relay, then you may need to use the exact OEM part.

Mike....

The part you linked is non-stock, with a 300 pc minimum order.............. Not too useful to the OP....

Thank You. I am not wise on the electrical stuff. What kind of relay are you talking? I am a diesel truck guy and know how that giant relay works, but I assume this would be a much smaller relay.

OK...

The resistor (NTC) is in series with the rectifier, typically (but not always) on the DC side after the rectifier. What it does is to provide a resistance to reduce the large surge of current when the switch is closed. The surge can damage rectifiers and capacitors in the power supply. Rectifiers and capacitors have little series resistance inherently, so they form basically a short across the line until the capacitors charge up. Adding a resistor cuts the surge down to a non-damaging level.

The resistor gets a huge dissipation of power for a short time when the surge occurs, which is also reduced after charge-up.

But, once the power supply is charged up, the problem is the resistor is still there, and it will cut legitimate input current, reducing the output voltage, and dissipating power, which is a waste and reduces performance. It would limit the VFD output power if something is not done to reduce the resistance.

There are two ways to cut the waste.

One is to use an "NTC" type resistor, which reduces resistance when it gets hot, and so the power dissipation is less. As you draw current, the device is heated, so the resistance stays low. "NTC" means negative temperature coefficient, the resistance decreases with temperature. When current flows it heats the resistor, and an NTC type will reduce in resistance until a balance is reached where the resistance is just enough to keep it hot at that current. With that kind of operation, the "NTC" resistor stays in-circuit, and stays hot as long as the VFD unit is working.

The other better way is to close a relay contact to bypass the resistor, so it is totally out of circuit, and cannot limit power input to the VFD circuits. The relay is closed when the voltage reaches about 2/3 of normal.

In that case, the resistor may be an NTC type, OR a normal type with a good 'surge" capability. The NTC type are made for the purpose, though, and carry ratings that allow selecting a suitable one based on voltage and bus capacitance.

They tend to be very reliable if not abused. But if the relay does not close, the resistor can be overheated and fail, no matter which type it is..

The relay is usually physically small, but will have a good current rating. It is operated from an internal power supply voltage in the VFD, often 24VDC.

Post Deleted

Thought it said stock

So pick

http://www.digikey.com/product-sear...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Do we know the capacitance on teh bus?

That part is rated 2720 uF at 120, and 680uF at 240V. With a doubler, somewhere in between, depending.

If the capacitance is larger than max spec, the NTC *WILL* blow out, it's only a matter of time. been there, seen it, done that.

we also don't know how many are in parallel so just assuming certain value is not necessarily correct either. For $3, I would buy it and stick it in and expect to be good for many more years.

Mebbe....

We managed to put in an undersized NTC in a proto unit, maybe 25% to 30% undersized for capacitance rating, and it blew out in a week of working with the prototype. I don't know how many power-up cycles, but probably no more than 50.

So it's not always that easy....

What happened was a change to the bus capacitance for some reason in testing, and failing to go back and upgrade the NTC. Was supposed to be a temporary change due to availability.....