low current limit

I don't get the dual redundant part. They want two current limits in series?

It probably needs work over a wide temperature range too.

George H.

Given independent, low-dropout, precise series current limiters and a requirement to recover, burp or foldback, things get interesting.

ly

How else would you get redundancy? It would seem to be used in a safety cr itical system.

My circuit has issues trying to maintain a tight current limit over all con ditions. If the load does not need to be grounded two single limit circuit s can be used which might meet the specifications. The way I have created mine it has a very low drop out, but the regulation of the two circuits is about 2 mA apart making it very hard to meet the tight operating vs. limit spec. They are using 100% safety margin which is likely in addition to wha tever safety margin is built into the device being powered, so perhaps over kill.

Rick C.

Rick C.

I played around with the circuit a bit more to come up with this. In the c ase of one pass transistor being uncontrolled on it is still within limits. It is hard to stay within 20 mA with a short of one of the current sense resistors. I put in three to limit the current to about 24 mA. To get it below 20 mA you will need to use several more so that the current doesn't c hange much with any one shorted.

From the sound of your posts something this crude isn't going to meet requi rements no matter what.

Rick C.

Version 4 SHEET 1 1292 680 WIRE -336 -304 -576 -304 WIRE -192 -304 -336 -304 WIRE -128 -304 -192 -304 WIRE 0 -304 -48 -304 WIRE 128 -304 80 -304 WIRE 272 -304 208 -304 WIRE 304 -304 272 -304 WIRE 448 -304 400 -304 WIRE 496 -304 448 -304 WIRE 656 -304 592 -304 WIRE 688 -304 656 -304 WIRE -576 -256 -576 -304 WIRE 688 -240 688 -304 WIRE -192 -144 -192 -304 WIRE -112 -144 -192 -144 WIRE 16 -144 -48 -144 WIRE 144 -144 80 -144 WIRE 304 -144 208 -144 WIRE 352 -144 352 -240 WIRE 352 -144 304 -144 WIRE -576 -128 -576 -176 WIRE 352 -128 352 -144 WIRE 688 -112 688 -160 WIRE 352 -16 352 -48 WIRE -192 32 -192 -144 WIRE -112 32 -192 32 WIRE 16 32 -48 32 WIRE 144 32 80 32 WIRE 480 32 208 32 WIRE 544 32 544 -240 WIRE 544 32 480 32 WIRE 544 64 544 32 WIRE 304 128 208 128 WIRE 384 128 304 128 WIRE 384 160 336 160 WIRE 208 176 208 128 WIRE 544 176 544 144 WIRE 208 304 208 256 WIRE 336 304 336 160 WIRE 336 304 208 304 WIRE 336 336 336 304 FLAG 544 176 0 FLAG -576 -128 0 FLAG 688 -112 0 FLAG -336 -304 V30 FLAG 272 -304 E1 FLAG 448 -304 C1E2 FLAG 304 -144 B1 FLAG 352 -16 0 FLAG 480 32 B2 FLAG 656 -304 C2 FLAG 304 128 Vres FLAG 336 336 0 SYMBOL pnp 400 -240 M270 WINDOW 0 58 62 VLeft 2 WINDOW 3 96 91 VLeft 2 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL res -144 -288 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R1 SYMATTR Value {Rsense} SYMBOL res 528 48 R0 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL diode 16 -128 R270 WINDOW 0 40 31 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL voltage -576 -272 R0 WINDOW 3 11 100 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 30 100ms 1s 1s 1.5s 5s) SYMBOL res 672 -256 R0 SYMATTR InstName R7 SYMATTR Value R=100*V(Vres)+0.01 SYMBOL diode -112 -128 R270 WINDOW 0 40 34 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL diode 144 -128 R270 WINDOW 0 40 31 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL diode -112 48 R270 WINDOW 0 40 34 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D4 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL res 336 -144 R0 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL pnp 592 -240 M270 WINDOW 0 58 62 VLeft 2 WINDOW 3 96 88 VLeft 2 SYMATTR InstName Q2 SYMATTR Value 2N4403 SYMBOL res -16 -288 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R2 SYMATTR Value {Rsense} SYMBOL res 112 -288 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R3 SYMATTR Value {Rsense} SYMBOL diode 16 48 R270 WINDOW 0 40 34 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D5 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL diode 144 48 R270 WINDOW 0 40 34 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D6 SYMATTR Value 1N914 SYMATTR Description Light Emitting Diode SYMATTR Type led SYMBOL voltage 208 160 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(16.9 0 1.6 500ms 100ms 1s) TEXT -562 112 Left 2 !.tran 4 TEXT -560 168 Left 2 !.param Rsense 19.1

how redundancy is assured. I have seen many circuits and devices with red undancy built in for safety, but without a means of verifying the redundanc y is working. For example, double insulated tools and appliances. If they are made with a fault, then the "double" part of the insulation is no long er effective.

correct

astic case together where a wire had been pinched between the cases so it c ould touch the screw. It was not actually shorting, but the thin insulatio n was likely to eventually give up allowing the conductor to touch the scre w and become dangerous.

the risk of both protection layers being faulty from new is way lower than with single layer protection. It's basic maths.

NT

is how redundancy is assured. I have seen many circuits and devices with r edundancy built in for safety, but without a means of verifying the redunda ncy is working. For example, double insulated tools and appliances. If th ey are made with a fault, then the "double" part of the insulation is no lo nger effective.

plastic case together where a wire had been pinched between the cases so it could touch the screw. It was not actually shorting, but the thin insulat ion was likely to eventually give up allowing the conductor to touch the sc rew and become dangerous.

n with single layer protection. It's basic maths.

If the product is made with a single defect, at that point it is no differe nt from a non-double insulated tool without the safety ground which no one considers safe. Faults can happen in the field. It's not math, it's logic .

The point is reliability through redundancy is only workable if both levels of protection can be independently verified.

Rick C.

Now that is a simple solution that is brillient!!!!! Same current limit circuit twice with different components in it.

I'll have to see if that passes the sniff test. I can even be rediculous enough to mae them from different manufacturers. :P

Thanks for all the input guys. This has really spread the joy of arbitrarily tight requirements.

Q2 dissipates 600 mW into a shorted load, too much for a TO92.

When Q2 melts, there is a direct path through D4 D3 Q2 to the load.

And in real life, the 20 mA limit will be very imprecise. This sim was tweaked to work.

y is how redundancy is assured. I have seen many circuits and devices with redundancy built in for safety, but without a means of verifying the redun dancy is working. For example, double insulated tools and appliances. If they are made with a fault, then the "double" part of the insulation is no longer effective.

e plastic case together where a wire had been pinched between the cases so it could touch the screw. It was not actually shorting, but the thin insul ation was likely to eventually give up allowing the conductor to touch the screw and become dangerous.

han with single layer protection. It's basic maths.

rent from a non-double insulated tool without the safety ground which no on e considers safe. Faults can happen in the field.

yes

ls of protection can be independently verified.

woosh

NT