Calculate the range. It's 1 to 5 Vbe's, which is class C most of the time. Only the last Vbe does anything.
Doesn't work when the current keeps rising 1A every minute.
I also don't have 1% resistors necessary to make such a selection.
First it was on the heatsink. It went into thermal runaway fairly quickly (heavy heatsink). Then it was glued to an output transistor. It went into thermal runaway overnight.
Tim
Assuming a sine wave, peaking at 20A, assuming decent sharing, each resistor sees 10A peak, half the time, about 5 watts into 0.22 ohms.
Bigger emitter resistors reduce the positive feedback thermal runaway loop gain, eventually enough to keep it from snapping. A bigger heat sink helps, too.
The simple Vbe multiplier could be replaced with something that has more orthogonal control over static drop and TC, so you could actually have the darlington quiescent currents drop at higher temps, which would also break (brake?) the runaway.
There's another trick, usable if you have overall feedback: use big emitter resistors, like an ohm maybe, and parallel them with diodes.
John
Yeah, fair enough, but all the more reason to make it fixed, or limit the adjustment range.
Well then something's wrong then, isn't it? Whip out the DMM and figure out why that's happening. Measure the Vbe generator and the darlington Vbe's hot, and you'll soon have your answer.
-- Cheers, James Arthur
Obviously, it's not falling at the same rate as the darlingtons, which is what Phil observed four posts ago.
The problem is overcompensating it so it's thermally stable. Maybe a CCS pulling a constant voltage into one of the Vbe divider resistors would work. That would increase gain required for a given Vbe offset = more tempco for the same voltage.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
That's not what Phil said. He hinted at it, referring to the difference the current density makes--we're trying to get you to solve it.
If you're already sure it's undercompensated, tweak the pot and fix it.
I thought the problem was keeping the thing from running away. If you want it *really* stable, you'll have to servo it. If you just want to keep it from blowing up, you might just increase the Vbe multiplier factor, e.g. 5Vbe instead of 4.
That leads to the problem John alluded to of separating the effects of d(Vbe)/dT from the Vbe multiplier's static Vbe, but it might be workable.
These assume that undercompensation of Vbe is really the problem, instead of thermal lag time between sensor and darlingtons making the thing unstable.
rk.
for
That doesn't account for the variable and uncertain Vbe characteristic of the output darlingtons. If you want to sense current, put a sense resistor somewhere and really sense it. You could use the ballasting resistors you've already got, if you really wanted to.
-- Cheers, James Arthur
Another reason that wasn't mention was the price. They are too expensive anyway. As relay replacement their advantage is the infinite number of on-off cycles. And most are made as zero crossing switch anyway. When we're talking about power, don't forget the rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.rene
For rather modest values of "infinite" in many cases.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
The photo-fet types switch in about a millisecond. They look purely resistive at modest currents and have zero offset, so can be used for things like thermocouple muxing, autozeroing, or gain switching in opamp circuits.
This is impressive:
The Ron * Coff figure of merit is about [1] 6 picoseconds.
John
[1] for certain values of "about"
I haven't looked at relays for a while but I don't remember any (at least in the reasonable price range) that had a 10moHm contact R. The other thing about relays is the Ron is for initial resistance; over use it will increase then there's contact bounce.
Both have pro's and cons it depends on the application and the budget.
Usually but not always. This is a very low qty app where having a block with four screw terminals and a big mounting bolt in the middle would have been much better that assembling a circuit board. Even if it cost $50-$100.
Not for the MOSFET versions. They have only their Rdson.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Cool. The datasheet doesn't want to download but that 80mA limit would cause a major phut ... *BANG* in my case.
Anyhow, we just ordered the parts for the usual good old discrete solution.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
[snip]
Yes, it is only the resistive part, but the mosfets are not of the most advanced sort. A standard SO8 cased 40V NFet is available with about 10mOhms. A 40V Photomos Relay in DIL8 is having in the order of 600mOhms. While the Fet is specified at above 10A, the relay is for 500mA. Similar the voltage drop at the rated current, the fet drops 100mV, the relay drops
300mV. This tells me, the FET technology within the relays is at least 5 year back. If it wasn't for the, related to the case, astonishing isolation voltage, the two spare pins on the LED side could have been used to supply some electronics on the secondary side.My comment about the 1.5V across were for a 25A part where we measured somewhat above 1V at rated current. A line unit in a beefy case.
Rene
Anyhow, after a perusal of this SSR market ended in utter disappointment and lots of yawning I just rolled my own. Like usual ;-)
I might check back in 10 years, maybe the SSR manufacturers have learned a thing or two by then :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
it's=20
=20
Something to remember, the typical SSR is designed to be a lower power replacement for a mechanical relay in a "low voltage" power control application. Where response in 1/2 60 Hz cycle (8.33 ms) is quite adequate. That is where the volume is.
--=20 Transmitted with recycled bits. Damnly my frank, I don't give a dear
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That's where they hope the volume is. Those things are too expensive for such mundane jobs. At one client I was called in to reduce the cost of one of their systems. Found over half a dozen SSRs. All of them were out of there, instantly.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
My !@#$%^&* pool equipment controller is full of SSR's :-(
The main pump SSR failed, so I bypassed it with a conventional clock mechanism for now. When it cools off ;-) I'll replace ALL the SSR's with small relays, and add a box full of contactors... everything is on there, main pump, cleaner, spa (and its heat pump), patio misters, lights :-(
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
What? Did a rookie design that? The thing must have cost a fortune. Either way the manufacturer has left money on the table. If it absolutely has to be electronic this kind of stuff is done with triacs. I mean, in qties you can get a BTB24-600 (600V, 25A) for well under a buck.
Yep, real men build it with Potter&Brumfield. Or I guess nowadays Tyco.
What always surprises me is why the sprinkler timer guys don't offer one with a program conducive to pools. Ok, you can program 4-6 zones to max, hang the filter pump on the master output and the sweep on zone 3, but it's a bit clumsy and also tricky to explain if one ever sells the house. All you'd have to do is hang contactors on it instead of valves. I always wanted to do that but the stone-age dial timers just run and run.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Maybe Joerg should redesign the controller for them. ;-)
it's
I'd absolutely love to!
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
=20
so=20
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...=20
IME contact bounce (and related EMI) is the biggest difference between SSR and mechanical relays.
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