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Old 28th February 2007, 03:16 PM   #1
G is offline G  United States
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Default How to choose a thermistor to protect power supply?

Has anyone here ever calculated the value of a thermistor to limit current inrush in a power supply at turn on? If so I could use a hand.
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Old 28th February 2007, 03:24 PM   #2
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If you have a _toroid transformer_ less than 300 VA you won't need any inrush current limiter.
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Old 1st March 2007, 04:51 AM   #3
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I never considered it until just now, but will a thermistor work properly so little quiescent current?

I'm about to build an amp with a 600VA torroid. Currently I've got a CL60 thermistor on it. Would I be better off leaving it off?

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Old 1st March 2007, 06:26 AM   #4
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Quote:
Originally posted by peranders
If you have a _toroid transformer_ less than 300 VA you won't need any inrush current limiter.
Hi Peranders. I am thinking of building, actually I think assembling is more accurate, a UCD180 amplifier. After looking at the soft start board that DIY Cable sells I decided to go that route. I plan on using a 800VA toroidal transformer with two 50v windings with a couple of IXYS bridges and some Nichicon Gold Tunes to build the supply. I can build a much better supply than the one DIY Cable is selling for $195 for a lot less money so I will be building that. Still trying to convince myself to go ahead and build a (gasp!) SS amp. Are the UCD modules as good as all the reviews say?
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Old 1st March 2007, 06:36 AM   #5
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Gavin, I'll suggest you do a search about "inrush current limiter". We have already discussed it rather thouroughly.
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Old 1st March 2007, 07:54 AM   #6
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I had major problem with blowing 10A main fuses in my apartment when my tv and pc was on and i switched on my GC. Solution: Delay circuit for toroids

It works perfectly and now I never need to change my fuses
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Old 1st March 2007, 10:04 AM   #7
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A single CL-type thermistor seems possibly problematical, unless the load current will be constant and known in advance.

I stopped using them in my power supplies and switched to using a MOSFET-based soft-start circuit. That way, I get a known (and very, very low) resistance, after the soft start is complete. (And the total cost is not much higher, although it takes more room. However, it also doesn't get nearly as HOT as a CL thermistor would get.)

The circuit is very simple and uses only six components. It goes between the diode bridge and the main filter capacitor(s). There is a hefty P-MOSFET (e.g. 55-Volt/80-Amp STP80PF55, or IRF4905) in parallel with a low-value power resistor, which are placed in series with the power rail (i.e. between the bridge's +DC output and the filter caps' "+" side). The mosfet's Source is connected to the rectifier bridge and its Drain is connected to the filter caps. Then components are added so that the mosfet (normally "off"/open circuit) turns on "gradually", eventually becoming a 0.018-Ohm resistance in parallel with the power resistor, effectively removing the power resistor from the circuit.

One that I recently designed was configured like this: 10uF in series with 33k Ohms, from mosfet's Source pin to ground (e.g. to "-" DC pin of bridge/neg terminal of filter cap), with the mosfet's Gate connected to the point between the 10uF and 33k (which set the time-constant/delay), plus a 1 Ohm 5W resistor from Source to Drain (used only during the startup-delay time). I also add a 10 Meg resistor in parallel with a 15v Zener, from Gate to Source (with Zener's cathode toward Source).

Some or all of those components' values and ratings would almost-certainly need to be changed, for different power supplies. The example above was for a small power supply (56VA transformer, 18VAC secondary), with a 12000uF filter cap (followed immediately, in this case, by a boost-mode SMPS with 38vdc output). The mosfet turns on gradually, over about 100ms, which keeps the input current spikes below 30A or so. In this supply, after startup, the mosfet only dissipates about 0.7 Watt, max.

It was a bit difficult to determine what power rating was needed for the 1 Ohm "bypass" power resistor. In the case above, the little 1 Ohm 5 Watt resistor can experience short periods of 10 to 20 Amp currents, i.e. up to several hundred watts of dissipation. Its initial (and maximum) stress was shown (in simulations) to be having its current go from zero to eighteen amps (332 Watts) and back to zero, in about six milliseconds. Over the first 90 milliseconds, its worst-case average power dissipation was 85.38 Watts, with an average current of 7.03A (9.24A RMS). So, you would probably want to use a resistor for which the manufacturer's datasheet gives data for "pulse-handling", or "surge" ratings of some sort. I ended up using part number 594-AC05W1R000J from http://www.mouser.com , which has a link to the datasheet for that series, which includes pulse-handling graphs that seem to indicate that a 3W 1 Ohm resistor would not quite be able to handle the 90ms of such surges, forcing me to make room for the 5W version. (I hope that I interpeted the graphs correctly... But, no flames or explosions, so far.)

To redesign with this topology for another power supply, the easiest way would probably be to download the free LTSpice (aka SwitcherCAD) from http://www.linear.com , and get the STP80PF55 spice model from the st.com website (or a model for whatever P-MOSFET you choose, from wherever), and simulate your power supply's startup with it, with worst-case loading. Then you could simply TRY different values for the power resistor, and different RC time constants for the mosfet's gate drive, and find something that might work for your particular power supply.

The LTSpice simulator will give plots of any or all currents, voltages, power dissipations, etc, versus time, just by clicking the mouse "probe" wherever you want on the schematic. And when a simulation run has stopped, you can also get the average and/or integral of any plotted quantity, by holding down CTRL and clicking on the plot label. (Power dissipation for any device can be plotted by holding ALT and clicking on the device, on the schematic).

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Old 1st March 2007, 04:44 PM   #8
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Tom:

That circuit will do a good job slowly bringing up your caps, but it won't do anything about magnetizing current in a large transformer. Really, something line-side is needed for that, which a thermistor works very well for, at least in an amp with fairly constant current draw. Even some variation in load would be alright, as long as the continuous draw is high enough to keep the resistance near zero. I'm just expecting that these chips have a low enough idle current draw to allow the thermistor to cool. I guess I'll find out tonight when I hook that power supply up to my amp.

-Nick
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Old 1st March 2007, 05:41 PM   #9
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CJ900RR,

Can you tell us what mains voltage the Promitheus delay circuit is designed for?

If it is for something like 220 VAC mains voltage, what ocmponent changes would be needed to work with 120 VAC?
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Old 1st March 2007, 05:55 PM   #10
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Quote:
Originally posted by Dick West
CJ900RR,

Can you tell us what mains voltage the Promitheus delay circuit is designed for?

If it is for something like 220 VAC mains voltage, what ocmponent changes would be needed to work with 120 VAC?

Notice that this is an Elektor design which Promitheus of some peculiar reason has put his own copyright , anyway could take a peak at my softstart softstart and check at the bottom of the page. You'll to change the voltage dividing caps. They must be bigger in value. I have done a simulation (LTSpice) of this circuit. Drop me a message if you want it.
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