Electronics

ionfire · Joined: 06 Nov 2006 Posts: 2

Hi guys,
i wonder if you can help me. i'm building a circuit that needs to be controlled from a micro processor, that switchs a high current for a short time. the load for this circuit very low in resistance and < 1ohm and therefore will draw alot of current. What i need is a component that will be able to switch that current (@24v) for around 30-40ms but wont get damaged by the nearly short circuit output...

so basical something that switchs 24v from a TTL logic i/o line, max on time is around 30-40ms and something that wont be damaged by the high currents.. and preferably < £1.20 each.. it will be switched on then not used again for a long time..

i've been looking at some mosfets that have over current and over temperature cut outs which might do the trick.. but i'm really no good at analoge/power electronics.. (i'm a software engineer you see)..

any ideas?

thanks
chris

neon · Joined: 25 Feb 2006 Posts: 593

First of all switching devices do not get damaged by the short current trough it. The heat generated is the culprit keep it cool and you may switch away. At 24v and 1 ohm there is 24 amps min and is it AC/DC how fast does have to switch this load? answer that and we can help you. it will help to know the load parameters pure resistance or an impedance type of load.

ionfire · Joined: 06 Nov 2006 Posts: 2

hi,
sorry yes, this is DC current. i will explain a little further, the system is for igniting electric ignitor$ for firewrks displays. so basicaly they need a min of 200mA @12v to fire but because the resistance is so low (and you might have 10 or 20 in parallel) they can draw alot of current initialy.. currently the board i've designed uses darlington drivers, however at 24v there is too much current going through them which causes them to pop along with the i2c driver chips that run them..

the mosfets will only be on for around 30-40ms. i have seen other systems that don't use massive components to switch the current, so i assume this is because its on for such a short amount of time and doesn't get a chance to get hot?

i was thinking about a VNP10N06:
http://docs-europe.electrocomponents.com/webdocs/0029/0900766b80029a83.pdf
as it has over temperature shutdown and will switch from a TTL input.

thanks for the help so far..

rgds
chris

neon · Joined: 25 Feb 2006 Posts: 593

sorry this is late to respond but i missed somehow. my way to do it would be to use LM555 as one shot they are good for 16v 300ma output. fire one which fire 2 which fire 3 and so on to 30 if you want. you may tie a reset line to a switch to stop the sequence otherwise itm will continously keep going to the end [SAFETY] now you need a bunch of lm555 cheap $.69 each pluss parts may $1 per firing. how fast can it be as fast as they can fire your eyes are not that fast it will seems immidiate.A darlington is not good for switching anything is good for beta multiplier. Now all your switching will be 300 ma fast sequence.look into tjhat

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