Otmar
1 mW
For those of you adding jumpers to run equally off all cells, I have some concerns about it. I've posted on DIY about this but seeing there is a lot of interest here I figure I'll repost. I wrote it before this hacking thread taught me about the LM358 supply voltage tap issue.
I would suggest that people do not do only this mod and run 8 cells. I've been wondering why they did not connect this from the factory? So I reverse engineered the power supply circuit to find out.
The 3V loads on my non-logging model are 8.8mA peak and 4.4mA with the backlight off. This power is controlled by a B817-40W NPN transistor whose base is tied to the top of a 431 Zener. The Zener takes it's control voltage from a resistor divider on the emitter of the NPN. There is a 2K pullup resistor on the top of the 431 that provides base current to the NPN and keeps the 431 running. The trouble arises with the power dissipation of that resistor and or the transistor.
If you assume your max worst case cell voltage is 4.2V then I figure that resistor dissipation is .233W for 6 cells at 25.2V and .45W for 8 cells at 33.6V. This is a 1206 resistor which is commonly rated 1/4W and can also be found in 1/2W for a lot more money. Those ratings are on a cold day so normal practice is to halve the rating. I tested mine at 25V input and the resistor got hot, but not too hot. Putting in that jumper can double the heat produced and I think it would fail before long. It could also start a fire and that would not be good.
Overall I think the listed spec of 43V is quite optimistic!
I've been wondering how to work around this and I think that if we replace R37 with a 1/2W 4.7K ohm resistor it may survive and allow running off 8 cells. In that case I calculate that at a 8 cell max input voltage of 33.6V the resistor would dissipate .191W and the transistor would take .08W which should be safe. I will add 10 of the 541-4.70KUCT-ND on my next digikey order and report back when I've tested it.
Followed by:
Quote/
Originally Posted by swoozle
Then they mention changing another resistor (looks like the one you refer to) to reduce power usage:
"The red circle in attached pics-- 202 resistor is 2k, if you measure 7s pack, you can replace it to 10k, which would decrease the power consumption."
http://www.rcgroups.com/forums/showpost.php?p=13452879&postcount=143
Attachment 15088
Otmar, does this address the overheating danger sufficiently?
/Quote
Thanks for those links! Now I'll get nothing done today due to reading all the forums!
I'm concerned about the 10K for R37 and here is why:
At 33.6V input, R37 drops 30V for 3mA and .09W heat. That's fine.
But then the Q6 the B817-40W has to carry the rest of the heat load.
Max current draw is 8.8 mA, R37 provides 3 of those leaving 5.8 mA for Q6 to dissipate at 30V of drop.
30V * 5.8mA = .174 W. The internal silicon temperature is rated to rise 625 deg C above ambient per watt. So we multiply .174W * 625 and get 109 deg C of temp rise.
Theoretically if the air in the unit is under 40 degrees C (104F) then the transistor should not blow up right away, but in my book that's way too hot to be reliable!
My suggestion of using a 4.7K 1/2 Watt rated resistor moves more of the unavoidable heat into the resistor. 30V at 4.7K = .191W. and less into Q6 30V at 2.4 mA = .072W for a temp rise of 45 degrees C which is probably ok. There still is the possible problem that the heat from the resistor will heat up the transistor due to proximity and that's why I intend to test it on the bench. A good rule of thumb is that if you can hold your finger on it for ten seconds without it being so hot that you have to pull away then the part is not too hot. I'm not sure the resistor will pass that test, but we'll see.
These are coming from this thread: http://www.diyelectriccar.com/forums/showthread.php/needed-celllog8s-relay-circuit-56509p3.html
I hope this helps,
I would suggest that people do not do only this mod and run 8 cells. I've been wondering why they did not connect this from the factory? So I reverse engineered the power supply circuit to find out.
The 3V loads on my non-logging model are 8.8mA peak and 4.4mA with the backlight off. This power is controlled by a B817-40W NPN transistor whose base is tied to the top of a 431 Zener. The Zener takes it's control voltage from a resistor divider on the emitter of the NPN. There is a 2K pullup resistor on the top of the 431 that provides base current to the NPN and keeps the 431 running. The trouble arises with the power dissipation of that resistor and or the transistor.
If you assume your max worst case cell voltage is 4.2V then I figure that resistor dissipation is .233W for 6 cells at 25.2V and .45W for 8 cells at 33.6V. This is a 1206 resistor which is commonly rated 1/4W and can also be found in 1/2W for a lot more money. Those ratings are on a cold day so normal practice is to halve the rating. I tested mine at 25V input and the resistor got hot, but not too hot. Putting in that jumper can double the heat produced and I think it would fail before long. It could also start a fire and that would not be good.
Overall I think the listed spec of 43V is quite optimistic!
I've been wondering how to work around this and I think that if we replace R37 with a 1/2W 4.7K ohm resistor it may survive and allow running off 8 cells. In that case I calculate that at a 8 cell max input voltage of 33.6V the resistor would dissipate .191W and the transistor would take .08W which should be safe. I will add 10 of the 541-4.70KUCT-ND on my next digikey order and report back when I've tested it.
Followed by:
Quote/
Originally Posted by swoozle
Then they mention changing another resistor (looks like the one you refer to) to reduce power usage:
"The red circle in attached pics-- 202 resistor is 2k, if you measure 7s pack, you can replace it to 10k, which would decrease the power consumption."
http://www.rcgroups.com/forums/showpost.php?p=13452879&postcount=143
Attachment 15088
Otmar, does this address the overheating danger sufficiently?
/Quote
Thanks for those links! Now I'll get nothing done today due to reading all the forums!
I'm concerned about the 10K for R37 and here is why:
At 33.6V input, R37 drops 30V for 3mA and .09W heat. That's fine.
But then the Q6 the B817-40W has to carry the rest of the heat load.
Max current draw is 8.8 mA, R37 provides 3 of those leaving 5.8 mA for Q6 to dissipate at 30V of drop.
30V * 5.8mA = .174 W. The internal silicon temperature is rated to rise 625 deg C above ambient per watt. So we multiply .174W * 625 and get 109 deg C of temp rise.
Theoretically if the air in the unit is under 40 degrees C (104F) then the transistor should not blow up right away, but in my book that's way too hot to be reliable!
My suggestion of using a 4.7K 1/2 Watt rated resistor moves more of the unavoidable heat into the resistor. 30V at 4.7K = .191W. and less into Q6 30V at 2.4 mA = .072W for a temp rise of 45 degrees C which is probably ok. There still is the possible problem that the heat from the resistor will heat up the transistor due to proximity and that's why I intend to test it on the bench. A good rule of thumb is that if you can hold your finger on it for ten seconds without it being so hot that you have to pull away then the part is not too hot. I'm not sure the resistor will pass that test, but we'll see.
These are coming from this thread: http://www.diyelectriccar.com/forums/showthread.php/needed-celllog8s-relay-circuit-56509p3.html
I hope this helps,