Can PTC Thermistors Oscillate?

[Galgso]

Context:
I was looking at the datasheet for some PTC thermistors (Vishay PTCEL Series, see attached datasheet) that I want to use for a circuit I'm designing to discharge high voltage capacitors (588v Max). The datasheet provides a handy formula for determining the necessary amount of thermistors to absorb a particular amount of energy from a capacitor (or capacitor network) and I have determined that a single thermistor will work well for my application (capacitance < 320uF). I am looking to use the PTCEL17R102UxE404 type with a resistance of 1kΩ at 25 degrees centigrade.

If you think it's odd that I am in charge of designing a safety-critical circuit like this with my limited experience, it is because this is an extracurricular project I am undertaking at university. The circuit shall be inspected by experienced technicians and tested thoroughly in their presence.

Issue:
My circuit involves a discharge contactor and I am required to design a circuit that will not blow up if the discharge contactor (single normally-closed contact) fails to disengage or gets stuck in the closed position. Since this contactor is required to be normally closed for safety, if for whatever reason power fails to reach the contactor coil the contactor will remain closed with 588VDC across the discharge thermistor in series with the contactor. While the datasheet states that the thermistors are "Self protecting in case of overload with no risk of over-heating" (see features section with bullet points on page 1 of datasheet), it seems to me that if 588VDC are applied continuously across the thermistor, it shall first remain in the low resistance state for a while and then increase in resistance significantly. I fear that this will cause a form of oscillation where the thermistor cools down after it heats up initially due to the increased resistance and then overheat again.

Looking at the resistance vs temperature graphs on page 4 of the datasheet, it seems clear to me that it is unlikely the thermistor will reach an equilibrium temperature that it can maintain indefinitely since it seems to be deliberatley designed to avoid this. The graphs are incredibly steep and I don't see how it could actually be self-protecting as the datasheet states unless it can actually handle repeated cycles of heating and cooling continuously.

As for the calculations I made to arrive at the conclusion that the thermistor will overheat and oscillate, one need only look at the dissipation factor (19.5mW per Kelvin or less). With 588v across the thermistor and a resistance of around 1kΩ at 25 degrees centigrade, it is quite obvious that it will heat up pretty quickly.

Summary:
Can a PTC thermistor heat up, increase in resistance and then cool down again leading to a sort of astable oscillation condition that may destroy the thermistor if it persists?

Thanks in advance for any help

edit: specified that I wish to use the PTCEL17R102UxE404 type with a resistance of 1kΩ at 25 degrees centigrade.

 

[Galgso]

As I understand it, the requirement is to discharge the capacitor at switch off, for safety reasons, so the thyristor won't stay ON as the capacitor will be discharged.

Or you could turn the thyristor OFF once the capacitor has discharged enough?.

The requirement is that if power were applied permanently with the discharge circuit active it should not be damaged.
That's why I want to use a PTC thermistor.

 

[Nigel Goodwin]

The requirement is that if power were applied permanently with the discharge circuit active it should not be damaged.
That's why I want to use a PTC thermistor.

Get one and try it, see what happens - although as it will be permanently really hot, how fast will it cool down to discharge the capacitor? - it's purely a thermal effect. You might also try adding a second one in parallel (keep them spaced apart), and see what difference that makes - they will both run at the same high temperature, but should half the discharge time.

 

[Diver300]

I don't think that there is a limit for cool-down time, just that it won't be permanently damaged if it is run continually.

The time constant for cooling in still air is 130 to 155 s. If the discharge circuit is run continually, it will need a minute or two to cool down before it meets the 5 second discharge requirement. I think that is allowed.

 

[Diver300]

As I understand it, the requirement is to discharge the capacitor at switch off, for safety reasons, so the thyristor won't stay ON as the capacitor will be discharged.

Or you could turn the thyristor OFF once the capacitor has discharged enough?.

I thought that turning off a thyristor is difficult.

 

[Nigel Goodwin]

I don't think that there is a limit for cool-down time, just that it won't be permanently damaged if it is run continually.

The time constant for cooling in still air is 130 to 155 s. If the discharge circuit is run continually, it will need a minute or two to cool down before it meets the 5 second discharge requirement. I think that is allowed.

How does 2 minutes 5 seconds meet a 5 seconds discharge requirement?.

 

[Nigel Goodwin]

I thought that turning off a thyristor is difficult.

You just switch a capacitor across it with another thyristor, it's a common standard technique, used in fork lift trucks etc. for speed control.

 

[Galgso]

How does 2 minutes 5 seconds meet a 5 seconds discharge requirement?.

The discharge time can be exceeded in the event of a fault as long as it remains "reasonable" (the rules do not elaborate on what reasonable means but I think a couple minutes would be reasonable). Under normal circumstances the circuit should be able to do three consecutive five-second discharges.

 

[Galgso]

Get one and try it, see what happens - although as it will be permanently really hot, how fast will it cool down to discharge the capacitor? - it's purely a thermal effect. You might also try adding a second one in parallel (keep them spaced apart), and see what difference that makes - they will both run at the same high temperature, but should half the discharge time.

Yeah I'm filling out a purchase order right now. I will probably not use two unless I find that the discharge time is unacceptably high when hot because the discharge circuit will be in a small sealed box (probably a plastic junction box or similar) and I fear that it may get too hot for the plastic to handle without softening even with a single thermistor.

 

[Diver300]

How does 2 minutes 5 seconds meet a 5 seconds discharge requirement?.

The discharge resistor cannot ever discharge the capacitor if the supply is connected. It would be a completely different requirement to short out the supply as well as discharge the capacitor.

The requirement is to survive the fault that leaves the supply on permanently, and to recover from that after the fault has been fixed, which could take two minutes. Then it can meet the 5 second requirement.

 

[Tony Stewart]

That is pretty nice. I'm guessing this is what Nigel Goodwin meant by an active discharge. I did not think of using a current source for discharging. Thanks for sharing as this is pretty interesting but rjenkinsgb made me realise that the operating temperature range referred to the ambient temperature range rather than the temperature of the thermistor itself in the datasheet so my original thermistor solution would work I think as long as I adequately protect the surrounding components from the thermistor's heat. Also the caps need to be drained in less than 5s.

I wouldn't be able to use your suggested circuit without modifications as the circuit I am designing needs to be able to handle the maximum voltage permanently without being fried. A TO-220 MOSFET/BJT constant current sink would fail pretty quickly if exposed to continuous voltage hence my desire to use a PTC thermistor. Thanks for the idea though I'll keep it in mind if I need to design a similar circuit in the future with less stringent safety requirements.

They make transistors and FETs for >850V these days. Shall I find one for you? is the Polyfuse reliable enough for your job? Are you sure?

 

[Galgso]

They make transistors and FETs for >850V these days. Shall I find one for you? is the Polyfuse reliable enough for your job? Are you sure?

The drain to source (or collector to emitter) breakdown voltage is not the issue with using a current source for discharge. It's that the circuit is not capable of surviving without damage in the event of a fault where power remains on with the discharge circuit active, which is a requirement for my application. While it may be possible to modify the circuit to do so, the PTCs I am looking at cost a few £ and are as simple as it gets - just one component (apart from the contactor of course which I already have) so I believe such a circuit would fit my use case better than a transistor-based constant current discharge circuit. I nevertheless appreciate you sharing your circuit as I did not think of using such a circuit before for discharging DC bus capacitors and you have shown using falstad that it can indeed work.

 

[Tony Stewart]

Is there any indicator of HV when off?
If you wanted to add an LED,I would choose an ultrabright 5mm white 30 deg 16 Cd and insert between Re=470 and emitter.

I would not rely on a single Polyfuse for human safety but OK for equipment safety. There are lots of Hi_Rel solutions that won't protect from a blunder like, if one overlooks a cut in insulation on HV cables or equiv. which ought to be shielded.
My Professional advice is not a cheap and dirty fix. But if you want to improve reliability of the polyfuses with redundancy, use 4 with 2 in series and 2 in parallel.

By changing the requirements to a few minutes the average drain, iff left on is < 700 mW with a 160 s discharge time with a 1.2 mA CC rate. This is less than the average car leakage current when off.

Total BOM cost < $1 on perf-board with nylon standoffs using 2N6773 with hFE = 20

https://tinyurl.com/2a7s7d5p

 

[Galgso]

Is there any indicator of HV when off?
If you wanted to add an LED,I would choose an ultrabright 5mm white 30 deg 16 Cd and insert between Re=470 and emitter.

I would not rely on a single Polyfuse for human safety but OK for equipment safety. There are lots of Hi_Rel solutions that won't protect from a blunder like, if one overlooks a cut in insulation on HV cables or equiv. which ought to be shielded.
My Professional advice is not a cheap and dirty fix. But if you want to improve reliability of the polyfuses with redundancy, use 4 with 2 in series and 2 in parallel.

By changing the requirements to a few minutes the average drain, iff left on is < 700 mW with a 160 s discharge time with a 1.2 mA CC rate. This is less than the average car leakage current when off.

View attachment 149421

View attachment 149420
Total BOM cost < $1 on perf-board with nylon standoffs using 2N6773 with hFE = 20

https://tinyurl.com/2a7s7d5p

Yes there is indication as to whether HV is on/off and whether all the contactors are out or not. See my reply to rjenkinsgb in post #10. Also these thermistors are specifically designed to absorb energy for discharging capacitors and are designed to withstand the full rated voltage for over a thousand hours. There is an insulation monitoring device that wil cut power to the HV contactors in the event of an insulation fault. Note that an insulation fault in our case (on a car) is less severe anyways than it would be on equipment running off mains becuase there is no earth reference so you can't get shocked touching the chassis. I believe this solution would be safe enough.