2V / 2mA to drive 2A

It's likely that your 2V drive is not sufficient voltage for the darlington; easiest way out of this is to use a pseudo-darlington, an NPN transistor (emitter grounded) , NPN collector connecting to PNP base (PNP emitter to +V), and the PNP collector drives the positive terminal of the lamp. Various resistors complete the switch (I'd start with an emitter resistor for the NPN).

The IRF3708 is not happy with 2V drive. If it has to be TO220 this one isn't officially rated for that but could possibly do it:

But I would not do that on a commercial or safey/mission critical product. This one can, but is obsolete:

This one seems to be able to do it but you need to run sevrtal in parallel:

I would turn that 2V drive signal into a 12V drive signal first, then drive the FET with that.

--
Regards, Joerg 

http://www.analogconsultants.com/

your 2 volt drive signal is close to margin of not working.

The gate on the MOSFET has alot of capacitance, your output on your touch unit maybe shutting down due to over current during charging of the gate, also you have miller effects that can cause the gate to have a rippler effect because you don't have enough gate drive to snub it.

I doubt very much you are getting any ringing at the gate because you don't have enough drive and low enough Z to do it.

If you want to keep the same logic polarity you need to use a NPN in common emitter mode and the collector can pull the base of a PNP to common while the PNP collector drives the Gate and the PNP emitter goes to the +12 volt supply. You also need a pull down R on the gate to keep it off.

jamie

The idea was that the IRF3708 should have such low G-d threshold. Yet not.

I ended up using an IRF9510 (better 9520) P-channel, which I drive by a BC547 to pull it down - and a few resistors. This seems to work

I will take this to work where I can test it properly

The TIP120 has a minimum hfe of 1000, so 2 mA should be enough to control 2 A. However, the Vbe is at least 1.6 V, so there are only 0.4 V available below the 2 V source. With 390 ohms, the base current is 1 mA, so with hfe=1000 that would give 1 A of collector current, which seems to match your experiment.

Keep in mind that especially halogens have a very low cold resistance, thus drawing a huge current at startup, possibly exceeding the maximum current for TIP120. There might be power supply issues that cause the oscillation, for bipolars, Vce(sat) and also Vbe(sat) will increase, generating high dissipation as drawing a lot of base current (much more than the 2 mA available). In addition any resistance in any common ground wiring can cause osculations with high current gain devices.

Your 2 V 2 mA drive is clearly inadequate for driving directly a big Darlington or a FET, so you definitively need an intermediate stage in between.

Also watch up how the ground wires are physically connected to avoid common ground paths that might cause oscillations with a load which internal resistance varies greatly with filament temperature.

yes, the turn on surge current needs to be considered, it will be MANY times higher.

Either design the circuit to deal with it or include a slow start feature so the current ramps up gradually... or current limiting

I had this problem with a solid state turn signal flasher.... it is amazing how much current a cold incandescent bulb will pull.

And when you test it, the FIRST flash will be the real test, once the bulb is a bit warm, the subsequent surge current is less.

Let it cool for 30 sec before you test the inrush.

Mark

x 20W at 12V (1.7A), with a control signal of 2V and max 2mA (Q-Touch 42QT1

012)

rive a 10W halogene (0.8A) but not 20W.

ps, but they start to flash. Why?

Your circuit does not have the drive level to fully turn on the IRF3708 whi ch means it hangs at some voltage intermediate to 12V, and this keeps the l amps in the part of their operating region intermediate to cold and running temperatures resistance drawing current in excess of the capacity of the p ower supply, causing it to foldback, cool down, recover re-apply full volta ge, repeat.

ops a bit the 2V for this should not do it. Yes there is a capacitor there too.

tied to the top/12V. But I wonder.... so I ask here first.

Your load is outside the rated SOA of the TIP120, it is wrong part.

max 20W at 12V (1.7A), with a control signal of 2V and max 2mA (Q-Touch 42Q T1012)

drive a 10W halogene (0.8A) but not 20W.

amps, but they start to flash. Why?

hich means it hangs at some voltage intermediate to 12V, and this keeps the lamps in the part of their operating region intermediate to cold and runni ng temperatures resistance drawing current in excess of the capacity of the power supply, causing it to foldback, cool down, recover re-apply full vol tage, repeat.

drops a bit the 2V for this should not do it. Yes there is a capacitor ther e too.

on tied to the top/12V. But I wonder.... so I ask here first.

datasheet shows it's good for ~5A DC at up to ~15V, it'll only have a few volts CE when on so how is that outside the SOA?

-Lasse

On Thursday, June 26, 2014 7:03:18 PM UTC+1, Lasse Langwadt Christensen wro te:

om:

:

ome lights. As of now max 20W at 12V (1.7A), with a control signal of 2V an d max 2mA (Q-Touch 42QT1012)

0 ohm resistor. It can drive a 10W halogene (0.8A) but not 20W.

does work with both lamps, but they start to flash. Why?

RF3708 which means it hangs at some voltage intermediate to 12V, and this k eeps the lamps in the part of their operating region intermediate to cold a nd running temperatures resistance drawing current in excess of the capacit y of the power supply, causing it to foldback, cool down, recover re-apply full voltage, repeat.

but even that my battery drops a bit the 2V for this should not do it. Yes there is a capacitor there too.

urns on a PNP darlington tied to the top/12V. But I wonder.... so I ask her e first.

t.

12v 20w 12v 1.7A halogen will draw around 17A at cold switch on, far outsid e the tranny spec.

NT

On Thursday, June 26, 2014 2:03:18 PM UTC-4, Lasse Langwadt Christensen wro te:

om:

:

w max 20W at 12V (1.7A), with a control signal of 2V and max 2mA (Q-Touch 4

2QT1012)

an drive a 10W halogene (0.8A) but not 20W.

lamps, but they start to flash. Why?

which means it hangs at some voltage intermediate to 12V, and this keeps t he lamps in the part of their operating region intermediate to cold and run ning temperatures resistance drawing current in excess of the capacity of t he power supply, causing it to foldback, cool down, recover re-apply full v oltage, repeat.

y drops a bit the 2V for this should not do it. Yes there is a capacitor th ere too.

gton tied to the top/12V. But I wonder.... so I ask here first.

Those tungstens will draw over 10A for 10s if not 100 msec, that's over the SOA limit, which is 5A at DC for 1-12 Volt range VCE. The part is unworkab le without a kluge.

On Thursday, June 26, 2014 9:03:18 PM UTC+3, Lasse Langwadt Christensen wro te:

om:

:

w max 20W at 12V (1.7A), with a control signal of 2V and max 2mA (Q-Touch 4

2QT1012)

an drive a 10W halogene (0.8A) but not 20W.

lamps, but they start to flash. Why?

which means it hangs at some voltage intermediate to 12V, and this keeps t he lamps in the part of their operating region intermediate to cold and run ning temperatures resistance drawing current in excess of the capacity of t he power supply, causing it to foldback, cool down, recover re-apply full v oltage, repeat.

y drops a bit the 2V for this should not do it. Yes there is a capacitor th ere too.

gton tied to the top/12V. But I wonder.... so I ask here first.

What is SOA?

On a sunny day (Fri, 27 Jun 2014 00:27:19 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Silicon On Alu ;-)

Safe Operating Area.

There should be a graph on the device datasheet.

piglet

rote:

.com:

te:

some lights. As of now max 20W at 12V (1.7A), with a control signal of 2V and max 2mA (Q-Touch 42QT1012)

390 ohm resistor. It can drive a 10W halogene (0.8A) but not 20W.

m) does work with both lamps, but they start to flash. Why?

IRF3708 which means it hangs at some voltage intermediate to 12V, and this keeps the lamps in the part of their operating region intermediate to cold and running temperatures resistance drawing current in excess of the capac ity of the power supply, causing it to foldback, cool down, recover re-appl y full voltage, repeat.

e, but even that my battery drops a bit the 2V for this should not do it. Y es there is a capacitor there too.

turns on a PNP darlington tied to the top/12V. But I wonder.... so I ask h ere first.

art.

ide the tranny spec.

You could address this with current limiting or full SOA protection. If imp lementing basic current limiting just ensure its set so Pdiss doesnt go too high during limiting.

NT

Please don't purposely mislead people, Jan.

On a sunny day (Fri, 11 Jul 2014 09:34:33 -0500) it happened John S wrote in :

You fell for it? :-)

Oh. I did. Sorry.