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Are my calculations correct? — Parallax Forums

Are my calculations correct?

Nightrider7731Nightrider7731 Posts: 46
edited 2005-01-21 05:41 in BASIC Stamp
I will be driving a SSR via a transistor.· I would like to make sure my math is correct when picking the resistor between the pin and the base so I can limit the amount of juice the stamp·will be sourcing.· I'll be running about 8 instances of this setup at a time, so I want to be nice to my battery.

Data:
SSR:·[url=mailto:3.4mA@5]3.4mA@5[/url] Vdc
2N2222: .6 Vdc Base-Emitter sat. voltage
············approximating ~ 0.02 mA base current (from collector sat chart)


For the 2N2222, I believe the math should be
(5v-.6v)···· =··· .0002 A
···· R

4.4v····· = .0002 A
· R

R· =·22000 = 22K·ohms

Is·22K correct?·

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-01-20 22:04
    If the SSR only requires 3.4 mA, why bother with the transistor? If the input side is optically-isolated from the HV side there is no harm in running it directly.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Jon Williams
    Applications Engineer, Parallax
    Dallas, TX· USA
  • achilles03achilles03 Posts: 247
    edited 2005-01-20 22:09
    Yeah, I agree with Jon there. 3.4mA is chump change for the BS2. My project drove 4 SSR's at the same time at 5mA each. If all you need is 3.4mA, then use a resistor less than 1470 and greater than 1000 Ohms (3.4 to 5 mA), and just go directly to the SSR.

    Dave
  • Nightrider7731Nightrider7731 Posts: 46
    edited 2005-01-20 22:35
    Since I'll be running a max of 8 of them at one time (plus other items), and the stamp can only source 40 mA (or is it 50mA), I'd hate to blow almost 3/4 of the juice for just these components.· If the remaining components come in under 12 mA then·I'll be fine, but I just want to make sure I'm covered ahead of time.

    Thanks
  • achilles03achilles03 Posts: 247
    edited 2005-01-21 00:23
    What's the PN for the SSRs you're using? 3.4mA might be the "fast full on" current, but you might be able to get away with less current if need be. It might result in a slower turn on time, but if you're not going to miss a couple milliseconds, then it's no problem.

    Dave
  • Nightrider7731Nightrider7731 Posts: 46
    edited 2005-01-21 00:28
    It will probably be a mix from whatever I can get from eBay, but the one I quoted the specs on was a Crydom D1210.
  • JonbJonb Posts: 146
    edited 2005-01-21 02:43
    Might i suggest·trying a MAX335 IC?
    It is a Serial controlled 8 channel SPST switch and can operate continuously at ±4.5V to ±20V·with a sink of 30ma/pin. I have a few in my parts box and havent gotten around to trying them out. Perhaps a simpler option is a serial to parallel shift register? The ones I have on hand 74HC164 can sink 25ma/pin.

    Jon


    Post Edited (Jonb) : 1/21/2005 3:06:31 AM GMT
  • Nightrider7731Nightrider7731 Posts: 46
    edited 2005-01-21 05:41
    After a little more research, I realize that the BS2 can source 40 mA per 8 pin sets.· By dividing the load across the two sets I should have no issue.· Nonetheless, can someone let me know if my calculations were correct if I needed to go that route?



    Thanks!
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