6V Relays

siden

New Member
Hi,
I've been reading round my issue for a while now, I'm hoping you can confirm or argue my theory, and help with suggestions for resolution.

I have the PICAXE-20 Project Board and a couple of these relays on outputs from the Darlington driver: http://www.picaxestore.com/index.php/en_gb/electronic-components/relays-and-solenoids/rel002.html
The relays will be switched on independently (and only one at a time), to switch a separate simple circuit of 4.8V battery pack and motor.

Using the battery pack from the project board (3 * 1.5V) directly on the relays, they work fine.
But running through the project board I get a much quieter (barely perceivable) click, and it seems to disconnect one of the NC connections but not engage either NO connections. I'm not an electronics engineer, hope that makes sense.

Looking at the datasheet for the relay here: http://www.rapidonline.com/pdf/60-0405.pdf
It says the pull in voltage is 75%. I'm assuming this is 75% of the 6V rating, so my theory goes that they work fine directly with the 4.5V, but once hooked up to the picaxe, the voltage use from the rest of the system is reducing what's available to the relays. Does that make sense?
The datasheet also says nominal current is 75mA, I think I read somewhere that the Darlington driver is sinking 500mA, is that right? Could that be an issue?
Looking through the forum, most other folk seem to be working with 5V relays.

Any other thoughts on reasons the relays aren't working?

Ideally I'd like to run the picaxe on three rechargeable batteries, so 3.6V.

So, if my theory is correct, I guess there are two resolutions:
  1. Give more power, I could run the board off four rechargeable AA batteries to give 4.8V and hope
  2. Change the relays out some requiring less switching power, can anyone suggest a suitable DPDT relay?

I'm erring towards the second option. Any thoughts?
Can anyone suggest a better resolution?

Many thanks for your time, sorry for so many questions all in one post.
Cheers, siden.
 

JimPerry

Senior Member
The Darlington drivers will drop 1.2V so you will only get 3V .. consider a separate higher supply for the relays :confused:

PS: Welcome to the Forum :)
 
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Colinpc

New Member
Ther are 5v relays readily available. You should also use a driver that has a low voltage drop. Probably a logic level drive FET would be best. Do a search on this to find the threads discussing these FETs.

Fred
 

techElder

Well-known member
siden, you will avoid a lot of problems by simply adding another battery pack for the relay circuit. Four AA alkaline cells would be about 6V.

Also, something you said about your drivers implies that you might note quite understand what is going on with the electrons in a circuit. "... the Darlington driver is sinking 500mA ..."

Just because a transistor driver is capable of pulling 500mA through a load and from a power source by specification, doesn't mean that it will pull that much current.

The current draw is determined by the load and the power source. The load in your case is the combination of the relay and the driver itself.

Hope that explains it, and if not for you then for others reading.
 

Goeytex

Senior Member
With a supply of 4.5v and a Darlington voltage drop of 1.2 volts the relays will only be supplied about 3.3 volts which is insufficient. If you intend to eventually use a supply voltage of 3.6 then you will need to use 3V relays, ditch the darlingtons, and instead use either a single bipolar transistor or a logic level Mosfet.
 

eclectic

Moderator
Another thought for consideration ....

Rather than have the relays drain the batteries, use

latching relays.

One of several is the
HFD2 5v DPD.

They are available from a Brit supplier,
on next day delivery.
(I won't mention the name,
though I only have connection as a regular customer).

e
 

AllyCat

Senior Member
Hi,

Yes, welcome to the forum, and +1 to the above comments.

Also, bear in mind that only "fresh" alkaline cells deliver around 1.5 volts, which falls to ~1.2 volts about halfway to their (manufacturers' specified) "end of life". So IMHO, a "serious" design assumes 1.2 volts for any AA cells (unless they have "Lithium" in their description). Therefore, I'd design with 4 x AA cells, but if alkalines were lilely to be used, then the initial 6v might be a little high for a PICaxe, so put a forward diode (or a red LED to give some added "functionality") in series with the positive supply to the PICaxe (only).

By their nature, Darlingtons will always drop at least 700 mV (i,e, a forward-biassed silicon diode), and more if they're being pushed hard. So, for just a pair of relays, I'd use cheap NPN bipolar transistor drivers (e.g. BC548 or BC337), with about 1 kohm from each base to the corresponding PICaxe output pin. They should "lose" only a few hundred mV of the available supply voltage.

Of course you could use lower voltage relays, but their lower coil resistance will require more current, so the "law of diminishing returns" comes into play.

Cheers, Alan.
 

siden

New Member
That's great, thanks all.
Lots to read into.

A couple further questions based on the above...
I expect the relays to each be on for about 5 secs a day, and would worry about a latching relay sticking on if their power supply died. Is that a valid worry or another misunderstanding on my part? And does it make sense not to use latching relays for this application (based on this new information -- sorry)?
If I ditch the Darlington, source 3V relays, use three 1.2V batteries, then just follow this circuit from page 14 of the picaxe_manual3, all should be well, right?:
relays.PNG

That's seeming like the most straight forward approach. Or is there still some value in using a logic level fet?

Many thanks.
 

geoff07

Senior Member
Much depends on what you are switching with the relays. But there are such things as solid-state bistable relays, I even have some. The advantage is that they don't consume battery power to stay in either state, only to switch. They are logic level devices. I think mine are 3v, though I would have to root around in a box of old projects to be sure.
 

Circuit

Senior Member
Much depends on what you are switching with the relays. But there are such things as solid-state bistable relays, I even have some. The advantage is that they don't consume battery power to stay in either state, only to switch. They are logic level devices. I think mine are 3v, though I would have to root around in a box of old projects to be sure.
Okay, I give up! I have searched through most of my common suppliers for bistable solid-state relays and come up blank. A 3volt logic level bistable relay would be of immense value to me... Geoff, could you please provide a link to the device that you mention? If not to a supplier item then to the datasheet?
 

AllyCat

Senior Member
Hi,

I expect the relays to each be on for about 5 secs a day,
If I ditch the Darlington, source 3V relays, use three 1.2V batteries, then just follow this circuit from page 14 of the picaxe_manual3, all should be well, right?
IMHO you're on the right track; For a duty cycle of only 5 secconds a day, I don't see the point of the added complexity (and cost) of duplicated drivers (to operate the two coils), or an H-bridge to reverse the current in a single-coil type of bistable relay.

The 3 volt relays will have a coil resistance of about one quarter of the 6 volt type and draw about twice the current, but that shouldn't be an issue. With a 3.6 volt supply rail, you'll probably have enough "headroom" to return to the Darlingtons (if they happen to be already installed on the board you're using). But if you're ordering new components (relays) then it would be wise to ensure that you have suitable bipolar transistors, in case they're needed (or you want a minimal design). You might even drive the relays with an emitter-follower configuration and save a couple of 1k resistors. ;)

Cheers, Alan.
 

geoff07

Senior Member
Well I had a root around, and I had slightly mis-remembered what I had. In fact, though the control is solid state, and they are bistable, and they do operate off 3v, the actual switching element is mechanical, rated 8A at 250V.

This one: http://shop.ciseco.co.uk/3v-to-5v-bistable-latching-relay-kit/

is the closest I could find when searching briefly. But the pic shows what I have, marked CISECO bistable relay V5. It has four inputs: 3.3v, GND, Set, Reset, and the connections to the switch. I think it is DPST, it has four connections to the relay, A1, A2, B1, B2.
 

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John West

Senior Member
In a situation where the relays are on only 5 seconds a day and you want them to be off in the event of a power failure I'd stick with the relays you have, and turn them on with logic level MOSFET's. There is much info on them found on this site, and much more found elsewhere on the Web. Leave off the ULN2803 altogether. It's not appropriate for a low voltage application due to the substantial inherent voltage dropped by a Darlington transistor configuration. Darlingtons work best in applications where there is a need to rapidly switch a high voltage, high current load with a small control signal. (A PICAXE driving a 24 Volt relay, for example.) This ain't it.
 
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siden

New Member
Thanks again for your time,
I apologise for my last post, I hadn't realised at the time that the BCX38B/C was still a Darlington driver! So presumably is no better than sticking with the ULN2803 I have.

So, ditching the Darlington, and using MOSFETs, can someone please confirm these are the parts I'll need?:
Or suggest a better option?

And, another daft question, sorry, could I simply hook four of these FETs (for + and - in each direction) to the circuit with the motor and also ditch the relays? Or is there a good reason to keep the relays in there.

I really appreciate your help so far. Just want to make sure I'm ordering the right bits and building in a sensible way.
Thanks, Simon.
 

BeanieBots

Moderator
And, another daft question, sorry, could I simply hook four of these FETs (for + and - in each direction) to the circuit with the motor and also ditch the relays? Or is there a good reason to keep the relays in there.
.
Did I miss something? What motor? Current rating?
Transistors are not magic switches that can simply replace relays.
Sounds like you are trying to build your own H-Bridge.
It should be possible if you use two P-channel and two N-channel FETs.
(remember, the P-channel will be ON when the PICAXE drives its gate LOW).
Get the on's and off's mixed up and the FETs will be stuck permanently in one state (could be either on or off depending on how quick you kill the power!).

I would use a relay (could be a latching type if you want) to control direction and a N-channel logic level (IRL520)FET to turn it on & off.
Drive the gate via a 330R resistor. (same for both relay and motor)
 

siden

New Member
Sorry. Yes. I have two circuits, one is the picaxe project board with various switches and sensors as inputs, and currently two 6V relays as outputs. Then I have a separate 4.8V battery pack with a motor (I can't find the current rating) that I wanted the relays to switch on and off in both directions.
I had it all hooked up and working if I activated the relays directly. But when the picaxe tried to activate the relays it wouldn't work, not enough power.
 

AllyCat

Senior Member
Hi,

not enough power.
No, not enough voltage (because of the voltage "lost" (dropped) across the Darlington driver transistors). My advice is "Keep It SimpleS", relays are much easier to use if you need to reverse a motor's direction, their contacts drop negligible voltage and they're relatively easy to drive.

If you have a reasonably stable 4.8 volt battery pack (4 x NiMH cells? ) then I'd use that for the whole system. Provided the PICaxe board has its own supply decoupling capacitor and you use separate wires back to the battery pack (or at least do not connect the motor power via the PICaxe board) then it should work alright.

No need for fancy FETs, a BC548 (6p from the PICaxe Store) should be fine to drive most small relays (but don't forget the catching diode across the coil). You haven't told us what current the relays need (or their coil resistance) so we can't be sure of the base resistor needed for the transistors, but 1 kohm is probably fine (or 470 ohms to be safe). 4.5 or 5 volt relays might be better, but as you have 6 volt relays, they're certainly worth a try.

Cheers, Alan.
 

fernando_g

Senior Member
Discarded electronic thermostats are a good source to scavenge low power, low voltage relays. Since these thermostats run (mostly) on a pair of AA cells, the relays inside are rated for 2.4 volts.
They are also latcheable....see the photo for a sample I got.
IMG_0169.JPG
 

siden

New Member
Following up on this thread, I put in the IRL520s along with diodes and resistors and the existing 6V relays are running happily from the three AA batteries, all switching on and off from the picaxe.
Thank you very much for all your help and discussion.
Simon.
 
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