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Modify 1 coil relay driver bridge to drive a 2 coil relay

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Zedman

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Hi all,

I have this bridge working with a 1 coil bistable relay, but for monetary reasons I have to use 2 coil relays involving minimal modifications on the current PCBs.

This bridge normaly puts 12V on both of relay's contacts and when Relay_1 or Relay_2 input gets high than the corresponding coil side gets pulled down to GND.

I tought I could use this feature of the bridge to pull down the sides of a 2 coil relay to GND while the common of the 2 coil is tied to 12V.
It works fine, but when no input provided I can measure around 3V on both of the coils. (between point A or B and 12V)

Why is this?
 

Re: Modify 1 coil relay driver bridge to drive a 2 coil rela

It's a rather wierd "bridge": on the drawing both PNPs (Q2 and Q6) are connected upside-down, so the polarities are reversed ..

IanP
:|
 

Re: Modify 1 coil relay driver bridge to drive a 2 coil rela

Apart from needing to correct the schematic, you didn't mention an important point: Do you want to keep the control waveforms
for the single-coil relay? The present circuit is a basic double push-pull driver. A two-coil driver circuit is much easier, just two transistors
with free-wheeling diodes, but it needs different control waveforms.

If you want to keep the present circuit and control waveforms, try to connect both coils in parallel, each with a series diode,
but different polarity.

P.S.: The control waveforms can be the same, I think. You can simply connect each coil between 12 V and the driver output, coil polarity as indicated.
 

    Z

    Zedman

    Points: 2
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Yes Q2 and Q6 is upside down, sorry.

Yes I want to use the same control signals and the same circuit if it's possible since there are a couple of PCBs already assembled.

The 2 coil relay has only 3 pins because + side of the coils are tied together so no way to connect them reverse parallel with serail diodes.
 

Re: Modify 1 coil relay driver bridge to drive a 2 coil rela

Connect the centre-tap to +Vcc, then each side of your "bridge" will drive one section of the coil ..
Dual-coil relays are latching relays, so all you need to control such a relay is to send a set or a re-set pulse to the relevant section of the coil ..

Have a look at the attached appnote - it explains how to control dual-coil relays ..

IanP
:|
 

    Z

    Zedman

    Points: 2
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